Talk:Inertia/Archive 2

Archive 1

Archive 2

Page Title

To avoid confusion on what this page is about I propose that it is renamed to Principle of inertia (physics). This avoids ambiguity with the other meanings and also helps define it as a principle as opposed to a quantity. comments?--Light current 14:34, 14 January 2006 (UTC)

To avoid confusion, I think it is a good idea to make one article for each important meaning of the word 'inertia' and the expressions containing it, plus, after that articles are done, another article summarizing those meanings. After all that, if we see we can do it in only one page, or two, we just have to do it then.
For the archives, I don't know how it's working. --Aïki 15:18, 14 January 2006 (UTC)

Archiving: probably here: Wikipedia:Subpages --Aïki 16:20, 14 January 2006 (UTC)

By convention, new topics in a talk page really should go at the end, not at the beginning.. I didn't even notice this one because it was stuck in at the beginning. As for the proposal, please see my comments on page titles in response to your comment under the "lead para" topic (in summary, I don't think the page should be renamed, both because of my own views and based on Wikipedia conventions on page naming).

Regarding splitting it into multiple pages, I really don't think there's enough to say about most of the individual usages to make their pages anything more than stubs (really what is there to say about "inertia as used to mean mass" aside from "the term inertia is sometimes used to mean mass"?) I would suggest we work on one article which tries to represent clearly all of the different meanings. If there's only one meaning which has a lot of content (as is currently the case), then that we should have an article with that content as the bulk which also clearly explains the others. If it turns out that there's a lot of distinct material for two or more different meanings, then we can look at splitting different meanings off into their own articles, but I think it should be avoided unless there's clearly enough content to justify it. For one thing, I think it's actually harder to clearly distinguish between the different meanings if they're split into different articles (because somebody has to click through all of the multiple articles to get a full view of the subject, instead of just reading one). -- Foogod 23:12, 15 January 2006 (UTC)

The confusion seeming to disappear progressively, I think we can continue with the same title. We can always change it later, if we see it could be better to do so.

Regarding topics, I agree with Foogod. It's better for the others contributors, and for the contributor who create it, when putted at the end, because it is always there that we look at first. --Aïki 02:14, 16 January 2006 (UTC)

Inertia redefined

On 22 Dec 2005 I asked the following question:

The lead para says inertia has recently been redefined as resistance to change. Who redefined it and can I have a reference and a quotation of the new definition?--Light current 10:26, 22 December 2005 (UTC)

It has still not been answered satisfactorily. This means that the statement about inertia having been redefined is wrong and should be removed from the article. Any comment?--Light current 01:37, 9 January 2006 (UTC)

Copied from lower down as it seems to have relevance to above post:--Light current 03:18, 9 January 2006 (UTC)

The "broader second meaning" mentioned in this article is actually quite muddled and confusing, and leads to a lot of problems later on. I would suggest ignoring it completely. I think I see what the author was trying to say, but I think it tries to portray a colloquial altered usage as the primary definition of the term, which is wrong. I'll take a look and see whether I can clean things up a bit, but this looks like it might be a large task.. -- Foogod 03:13, 9 January 2006 (UTC)

How is inertia defined by quoting Newtons first law? Newtons first law doesnt mention the word inertia. Or is this a definition by omission?--Light current 00:04, 13 January 2006 (UTC)

OK, you have a point, but Newton does mention it elsewhere:

Materiæ vis insita est potentia resistendi, qua corpus unumquodq;, quantum in se est, perseverat in statu suo vel quiescendi vel movendi uniformiter in directum.

Hæc semper proportionalis est suo corpori, neq; differt quicquam ab inertia Massæ, nisi in modo concipiendi. Per inertiam materiæ fit ut corpus omne de statu suo vel quiescendi vel movendi difficulter deturbetur. Unde etiam vis insita nomine significantissimo vis inertiæ dici possit.

The "innate force" of matter is its power of resistence, through which every body, as much as possible, perseveres in its state of rest or motion in a uniform direction.

This is always proportional to its body, and does not differ at all from the inertia of mass, except in how we conceptualize it. It is because of the inertia of matter that it is difficult to disturb a body from its state of rest or motion. For this reasion, the innate force can also be called by the very meaningful name of "Inertia."

How's that? --Iustinus 19:18, 13 January 2006 (UTC)

OK . I think this quotation ought to be included in the article.--Light current 20:03, 13 January 2006 (UTC)

Actually, what I was aiming to say in my rewritten introduction (and possibly needs more clarification) is that the term inertia today is most commonly defined by referring to Newton's first law (the term itself has had several different meanings over the many years of scientific discovery and has not always meant the same thing as it does today (as I'm hoping to make clear in a rewritten history section, if I ever get it finished and posted). The fact that it is generally defined that way today does not necessarily mean that Newton defined it exactly that way when he first wrote his laws. Newton conceived of the property of inertia being implemented in terms of an innate force (which is not generally how the universe is considered to work by modern physicists). For this reason, Newton ascribed (in various contexts) the term "inertia" to mean both the general principle, and the specific "resistive force" which he perceived to be inherent in objects. This is one way of thinking about things, but it has some problems, so physics has largely moved away from this notion (particularly with the advent of General Relativity and quantum physics). -- Foogod 21:20, 13 January 2006 (UTC)

OK Foogod. Please wait whilst I formulate a response. --Light current 21:34, 13 January 2006 (UTC)

Lightcurrent, I just reverted a bunch of small edits you made to this page, because most of them were problematic, and are continuing changes which several people are already in the process of debating with you here in the talk page. Please don't continue making changes to further a given position while there's ongoing debate here about that position. Let's get the discussion sorted out first, please. -- Foogod 00:19, 14 January 2006 (UTC)

A. Bellamy: The recent redefinitions of inertia since my criticism of 9 Jan now in the Talk Archive are a great improvement. However the latest definition still makes the error of defining 'the principle of inertia' as Newton's first law of motion, which makes no mention of inertia, and which is highly confusing if a principle of X should explicitly explain X. Rather Newton's principle and definition of his notion of the force of inertia (vis inertiae) in the Principia, its Definition 3 of vis insita was as follows:

'Inherent force of matter is the power of resisting by which every body so far as it is able perseveres in its state of resting or of moving uniformly straight forward.' In the commentary that follows Newton then defines the force of inertia as being the same as this inherent force.

Newton's first law of motion is thus a consequence of his principle of inertia. A.Bellamy

Hi A.Bellamy.. could you please follow the Wikipedia convention of ending your comments using four tildes (~~~~)? This appends a username and datestamp to your comments so that people can tell who said what when.

As I explained earlier in this talk page when discussing this with Light current, it is important to understand that what Newton actually defined the term "inertia" to mean is actually not the same thing as what we use the term to mean today (it is close, but not the same). The modern definition of the term is generally accepted to refer to the principle described by Newton's First Law of Motion. Newton did not use the term inertia to mean this, however Newton's definition of inertia has significant problems in the face of modern physics and is basically not used anymore. (please also see the end of the recently-rewritten history section for coverage of this point as well) -- Foogod 23:26, 15 January 2006 (UTC)

Bellamy to Foogood: Thanks for your history of ideas lesson, but can you verify your claim about the modern definition of the term ? I can certainly produce counterexamples from modern Oxford dictionaries that define inertia as 'A PROPERTY OF MATTER that causes...', thus correctly using Newton's Definition 3 to define it rather than his first law of motion as you claim. In short, you are wrong. BUT MOREOVER note that the crucial logical point you are overlooking here is that if Wikipedia presents 'the principle of inertia' as Newton's first law of motion, then as Newton himself correctly pointed out in the quotation I gave in my initial contribution in the Talk Archive, this principle was essentially at least 2 millenia old by the 17th century and was essentially stated by Aristotle, and so the Wikipedia (pseudo)history section is profoundly mistaken in presenting it as something novel to the 17th century discovered by Galileo that somehow contradicted Aristotles' dynamics. BUT if you present the 'principle of inertia' as the Principia's Definiton 3 of the force of inertia (vis inertiae), then that notion of inertia as an inherent resistance to motion in all matter WAS apparently historically novel to the 17th century with Kepler, and Newton then introduced the further novelty of revising Kepler's notion of inertia to exclude its resistance to uniform straight motion, thus restoring Aristotle's principle of interminable rest or motion in a void that Kepler's theory of inertia denied. However, I note you have at least improved the end of the history section to which you refer. A.Bellamy--158.143.134.103 19:32, 17 January 2006 (UTC)

Sigh, well I guess I'll have to go dig through the textbooks and papers, but I'm fairly confident I can find some good physics references for you which show the current scientific interpretation of the term is the principle, rather than an innate quality. (I don't have much time to work on any of this right now, however, as I'm kinda swamped with work this week) This is mainly because if viewed scientifically as an innate aspect of matter, one either gets caught up (as Newton did) in imaginary forces which don't fit in modern classical mechanics equations, or alternately view it as a property rather than a force, in which case it becomes indistinguishable from mass, and therefore a useless definition. Defining inertia as the principle, however, remains meaningful and therefore potentially useful from a scientific standpoint. (It should also be noted that I admit I haven't checked the OED, but several other dictionaries do seem to define it this way as well, so if the OED defines it in the way you describe then it appears to be in the minority (though this is not sufficient reason to ignore it completely).)

Also, please note that Wikipedia does not (currently) present inertia as being Newton's First Law. What it does say is that that is the statement most commonly used to describe it (which I believe is a fairly true statement). It does not state that Newton invented it, or even that his First Law was particularly revolutionary (in fact, I think the history section makes it fairly clear that NFLM was only an incremental improvement on a concept which had been evolving over nearly 2 millenia). Nevertheless, I do think that it's appropriate for Wikipedia to reflect the strong association that exists in modern Physics between NFLM and the principle of inertia, including the fact that NFLM is often used as a standard description of what inertia is (and is even sometimes referred to as the "law of inertia"). -- Foogod 00:10, 19 January 2006 (UTC)

Factuality

I believe this article is now Factual. Any disagreements/comments before I remove dispute tag?--Light current 01:14, 28 October 2005 (UTC)

I don't agree with your statements that it's an unnecessary, incorrect way of thinking. Inertia is a perfectly valid way of thinking of the resistance of an object to a change in motion. I know of no other way of classifying mass and moment of inertia collectively, as resistance to linear and circular motion, other than using the word inertia. It's also a word that is commonly used in English, so the meaning is already somewhat known by most. Tossing it out doesn't help anything. More general, your wholesale deletion of material related to the topic, which others (not me) have spent time writing is not very nice. For instance, the train braking example. StuRat 01:48, 28 October 2005 (UTC)

You also keep adding the word "percieved" misspelled like that. I'm getting tired of fixing it. "I before E, EXCEPT AFTER C." StuRat 01:35, 28 October 2005 (UTC)

Only sometimes!!

Actually most of the time, such as in this case. Look "perceived" up if you don't believe me ! StuRat 02:28, 28 October 2005 (UTC)

Spelling is for nerds. Lets get the info correct!--Light current 02:33, 28 October 2005 (UTC)

Oh Dear. I thought I might upset someone. Now its happened. Inertia is a perfectly perverse way of thinking of an object changing in motion. Just refer to Newtons first law. Inertia doesnt exist. Its all in the mind.

What part of Newton's first law says "Inertia is imaginary" ? StuRat 02:26, 28 October 2005 (UTC)

No mention of inertia here When no force acts on an object (or when the forces acting on it cancel), it moves in a straight line at constant speed. --Light current 02:32, 28 October 2005 (UTC)

So by your logic, since Newton's laws don't explicitly mention China, then China must not exist either, LOL. StuRat 02:55, 28 October 2005 (UTC)

Exactly! China does not exist! Goodnight!--Light current 02:57, 28 October 2005 (UTC) Wholesale deletion of incorrect and misleading information is demanded in an entity such as WP. If you dont want it modified, dont write it. Anyones free to argue with my opinions on the talk page. I look forward to it.--Light current 02:19, 28 October 2005 (UTC)

I didn't write the following example, and don't see anything incorrect or misleading about it. Even with your absurd statement that inertia doesn't exist, you can just replace the word inertia with energy, as you did on your first edit. There is no need to delete all this valuable info:

"For example, in the braking of a railway train, arresting the linear motion requires that the substantial rotational inertia of the motors be converted to some other forms of energy, thus causing acoustic vibration of the wheels and frictional heating of the brakes on the railway carriage."

StuRat 02:35, 28 October 2005 (UTC)

This particular para says that rotational inertia (moment of inertia) is converted to energy. This is plainly wrong. The two entities are not equivalent. The bit about acoustic vibration and friction is totally irrelevant to the subject. Thats why I deleted it. Its not valuable info- its rubbish!--Light current 02:48, 28 October 2005 (UTC)

It's saying that the rotational kinetic energy (which can be quantified by muliplying the rotational velocity by the moment of inertia) is converted to energy in those other forms. That is entirely correct and relevant. StuRat 02:58, 28 October 2005 (UTC)

I'VE ARGUED ABOVE, PLEASE TAKE NOTE OF ALL ARGUMENTS-VOYAJER 12/1/05

Isn't inertia just mass, measured as F/a?

What's with this "inertia doesn't exist" idea? Is that supported in the literature?

I mean, objects have mass precisely insofar as they resist acceleration - that's inertia. The other measure is gravity - objects have mass insofar as they generate a gravitational field. The fact that inertial mass and gravitational mass agree is interesting, and all tied up with General Relativity. This fact cannot be expressed without the concept of inertia. What's wrong with that?

Inertia is the name given to the tendency of massive objects to resist changes in their state of motion. You want units of inertia? How about grams?

I'm not a physicist, so correct me if I'm wrong - that's just how it makes sense to me. -GTBacchus 19:42, 4 November 2005 (UTC)

You're quite correct. There is one person, Light current, who keeps putting this junk out there, and reverting any edits to fix it. If you will help us defend it, that would be appreciated. (Kenny56 seems to agree with us). StuRat 20:24, 4 November 2005 (UTC)

StuRat Do you mind not accusing me of putting junk in the article. I NEVER put junk into an article. Sometimes I take out other peoples useless or incorrect junk. I was trying to improve its accuacy.--Light current 10:36, 22 December 2005 (UTC)

Pretty much all concepts in physics are immaginary. Inertia is a intrinsic property of mass. Mass is also an immaginary concept. Gravity is another intrinsic property of mass. I don't see a "this entry is disputed" on gravity's page.

• • • INERTIA DOES EXIST in that it is a postulate, an assumption upon which the model for the theory of the Laws of Motion are based. (See Isaac Asimov "Understanding Physics") It does not have to be used in a scientific or mathematical formula as "assumptions" by definition cannot be proven, but are the basis for universal models and the mathematical formulas are derived therefrom. (See Wikipedia article on "theory") Einstein made the assumption that experimentation showed that the speed of light was a constant and the "aether" did not exist. All mathematical formulas in Relativity are extrapolations of this assumption. Without the "idea", the mental abstraction of "inertia", Newton could have never formulated the Laws of Motion. Without the "idea", the mental abstraction that the speed of light was constant for all reference frames in a seemingly "Galilean system" and that "the aether" did not exist, Einstein could not have formulated Relativity. Theoretical physics is the use of mental abstractions (models) to define physical laws.

Voyajer 30 November 2005

i agree with Voyajer's comments and would like to see the article reverted to some version that existed before Light Current imposed his POV. If he doesn't believe that inertia exists, then fine for him--that is his choice. But the article contents really should be written by those people who do believe it exists and is useful to explain the physical world. Kenny56 04:38, 30 November 2005 (UTC)

Do you believe in fairies as existing?--Light current 11:39, 22 December 2005 (UTC)?

A lot of Newton

In explaining Inertia the article looks at Newtonian Mechanics. Is that necessary? --LeonK 23:58, 21 November 2005 (UTC)

Considering the history of the Latin word meaning "inertia" and what it has come to mean in current etymology, it is impossible to explain it without reference to the modern usage in terms of Newtonian physics. --Voyajer 30 November 2005

Moment of Inertia?

Being a n00b Wikipedian, I'm reluctant to make changes, but the sentence in the "Rotational Inertia" section that reads "The somewhat inaccurate term moment of inertia is still used to describe the conservation of angular momentum for a rotating body" is poorly worded.

"Moment of inertia" is the rotational analogue of "mass"; this statement is akin to claiming that "Mass is used to describe the conservation of linear momentum for a translating body." Both properties describe resistence to acceleration; the current wording implies to me that a body with a greater moment of inertia somehow conserves angular momentum... more?

Good point. I suggest that you be bold and update the wording. Please sign posts on talk pages using "~~~~". Cutler 11:07, 24 November 2005 (UTC)

The Real Problem--where Voyajer maybe agrees with Light Current

Most physics textbooks describe inertia as: "The resistance to change in state of motion which all matter exhibits."

WRONG. Not what Newton said. Later erroneous construct.

Newton said that inertia was the tendency for bodies to remain at motion or at rest. No mention of resistance here. Very important. Newton says of Aristotle's view of inertia in the heavens as above: "because of the lack of resistance" bodies in the heavens maintain inertia. [Note: inertia means laziness NOT RESISTANCE--to resist, you have to actively exert yourself, but inertia is not exertion but laziness.]

A direct translation from the latin of Newton's first law with Newton's explanation is: "LAW I "Every body continues ever in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.

"Projectiles continue ever in their motions, so far as they are not retarded by the resistance of the air, or impelled downwards by the force of gravity. A top, whose parts by their cohesion are perpetually drawn aside from rectilinear motions, does not cease its rotation, otherwise than as it is retarded by the air. The greater bodies of the planets and comets, meeting with less resistance in more free spaces, continue ever their motions both progressive and circular for a much longer time.--end translation

There is a fundamental difference between Newton's definition of inertia and the textbook definition of "inertia as resistance":

Inertia cannot be quantified. It is a default state, the tendency to remain in motion despite any other variables i.e. mass, force, acceleration (which variables are Not part of the first law). There is no quantification of inertia in the first law. One body is NOT said to have more inertia than another body. All bodies have equal inertia and all bodies are therefore equally subject to "a force". How much force is at this point irrelevant.

The problem: To state that inertia is "a resistance" necessarily predicts a quantity. Resistance can be quantified. How much resistance is there between one object and another? This can be quantified. Newton didn't say in the first law that inertia was a concept that could be quantified and I believe for one that he meant it that way. I also believe that by quantifying the force in the second law, that Newton was not quantifying inertia. Inertia is the concept of the first law of a default state that can be changed by a force. How much force? Look at the second law. But to say how much anti-inertia or anti-resistance is an erroneous construct not present in Newton. Newton spoke of resistance only in terms of "the lack of resistance" in inertia so how can inertia be described as resistance? Contradictory.

Inertia is not resistance to anything. Inertia by Newton's first law is susceptible to force. It does not resist force. It bends to forces will. This is a main broad statement about all bodies from a star, to a planet, to a baseball. This has no quantification associated with it. If as some have said, "inertia is a force", then which of the four forces of nature is inertia? Inertia is not gravitational force, because gravitational force is an attraction to the massive body, not the momentum of a massive body which is what inertia describes. So it is not gravitational force that keeps a body's momentum ever continuous. It can't be any of the other four forces of nature. Therefore, it is not some new force, therefore, it is not a force at all.

Therefore, bring in the misleading statement that inertia is "a resistance" then you must quantify it. Wrong as I have said. This then nullifies the entire concept of the first law which is a generalization which makes it so important. Generalization in theories is what makes theories important, because generalization means "you may apply this theory to all things". The first law is written in the simplest, most general form, so to introduce the concept of "resistance" into the definition of inertia as described by the first law is introducing a quantification which is not present and I believe which is purposely not present. Newton not only felt that the terms of the first law were sufficient, but they were the most accurate way of stating the situation.

The second law in describing the amount of force necessary to change the momentum of lazy bodies as now described for individual bodies is going to a more specific description for each body. But not a more specific description of the word inertia. Rather a more specific description of the word "force". It is not inertia that varies with the mass of the object, but the amount of needed force to change the acceleration that is needed according to the mass of the object. Neither force, nor acceleration, nor mass are definitions of inertia nor do they define inertia. Inertia is the state without mass, force or acceleration. This is because mass cannot be defined without acceleration and force. So inertia is a state which exists without knowing the mass of the object and even maybe if we go the way of thinking of Heisenberg to abstract levels we can say: Inertia is a state which exists even when mass doesn't exist as long as a body exists i.e. Heisenberg would probably say when you aren't measuring the mass, mass doesn't exist but inertia still does. This is probably going farther than Newton wanted, but the idea is that inertia is independent of mass, force and acceleration. The idea of inertia is an abstract. Newton's laws of gravity show that all bodies are affected by the gravity of all other bodies because to drive gravitational force to zero, one would have to drive distance between the objects to infinity therefore all bodies subject to Newtonian mechanics can be defined in terms of mass that can be acted upon by force and acceleration.

Are "inertia" and "mass" the same thing? No. We don't measure and equate inertia and mass in our equations. Mass is not "the tendency to maintain momentum", but inertia is. So they are separate concepts. It is erroneous to say that inertia is equivalent to mass. How can the tendency to remain in a default state be equivalent to the mass of an object? Newton describes mass in Law 2, not in Law 1. MAIN POINT: Mass is a scalar. Inertia, if it could be measured has direction, so it must be a vector. A scalar and a vector cannot be the same thing.

The point of this philosophical oratory is that inertia is as stated in Newton's law and not as redefined by Physics textbooks. Inertia is the assumption that bodies tend to continue in their current state of motion or their current state of rest until acted upon by a force or as otherwise stated by Newton, they continue in a state of inertia because of a "lack of resistance" i.e. a lack of force. Since inertia exists where there is a "lack of resistance", then inertia is not "a resistance" but a default state, the basis for explaining mass, force and acceleration i.e. the basis for explaining gravity.

If physics textbooks have described inertia as a force or as equivalent to mass, then they violate the definition that inertia is a tendency to maintain momentum. Neither force nor mass has this tendency.

In fact, the article is still wrong in saying that inertia is equivalent to mass.--Voyajer 12/6/05

I agree with your excellent comment. Another way to see that inertia is not a resistance, is to say that when you applied a force F on an object, it will have an acceleration A, every time you do it, whatever the quantity of mass M of the object. Thus then, in any way, there is a resistance. To have a resistance, a counter-force is needed. The inertia is an absence of resistance instead of a resistance! The mass M is just there to explain why different objects have different accelerations when the same amount of force is applied to each one. Once at a new velocity v, or a new path, each object still has the tendancy to keep it, or inertia. We could say that, in between, the object loose is inertia but not his mass! This tendancy does not depend at all on the quantity M: it is there whatever the mass in presence.

Regarding inertia and mass, I would added that photons have no mass but they still have an inertia. So, inertia also exist even if there is no mass. 24.202.163.194 17:02, 31 December 2005 (UTC)

VOYAJER 12/7/05--I've given up and re-edited the article to include both definitions of inertia. It's original meaning in Newton's first law and its meaning in physics as a measurement.

I've removed the following paragraph not only because it doesn't belong in the "History" section, but because it tries to say that a "tendency" is a measurement of "mass". This is ambiguous. I don't think the two definitions of inertia are synonymous as I have stated.

This tendency is referred to as 'inertia' and when measured it is found to be equivalent to gravitational mass. [This is in dispute. Inertia is a tendency to maintain momentum, mass is a quantity. In fact, measuring a tendency to maintain momentum would be a vector because according to Newton it has direction. Mass is a scalar.] Inertia is defined as Force/acceleration. [This is in dispute. Inertia is a tendency to maintain momentum. Force over acceleration is the definition of mass.] That is, when an object is subjected to 1 Newton of force and it is seen to accelerate at 1 m/s faster every second, than it has an inertial mass of 1 kg. [The dispute is whether the term "inertial mass" describes "inertia" or describes "mass". It is disputed that "inertial mass" merely means "mass" not "inertia".] This term has real scientific meaning and the equivalence of inertial mass to gravitational mass helped lead Einstien to his General Theory of Relativity. [It is undisputed that inertia has real scientific meaning. It does. It is undisputed that Einstein used gravitational mass, but gravitational mass is not inertia.]

Voyajer, if you will put 4 tilde marks, "~" , [usually found on the top row of the keyboard next to the "1" key] at the end of your paragraph or section of comments, then your name, date and time will automatically be appended. It is a nice feature of the wiki that lets everyone know which comments were made by the different users.

Also if you click on your user name shown in color above, it will take you to your own user page which you can edit, and from there you can go to your own discussion/talk page to view messages that you have received. Feel free to delete this note once it no longer applies. Kenny56 05:47, 8 December 2005 (UTC)

Kenny56: Thank you. That was very helpful. After explaining the two different definitions of inertia in the main article, I've taken off the "disputed" code notation. Let me know if you think or anyone else reading thinks that the article is in some way "not up to par" yet. I can see that there are a great many good minds working on this.Voyajer 22:59, 8 December 2005 (UTC)

Moved from Talk:LC

On page Talk:Inertia someone claiming to be User:Light current asks "What is inertia and in what units is it measured? ... Hold out a stone and then release it. Acceleration of the stone toward Earth is immediate and at it highest rate upon release. No \"resistance\", no hesitation, no \"inertia\". ... I suggest that inertia ... has no units."

Yes I agree this sentence is a bit shaky. But I didnt write it, I found it on the web somewhere

Acceleration of the stone toward Earth is immediate and at it highest rate upon release. Specifically I dont agree that accn varies but is constant in this case.--Light current 10:45, 22 December 2005 (UTC)

I agree that many people make it sound complicated or nonsensical. I hope that you and other wikipedia editors will figure out ways of improving these explanations. I hope wikipedia will allow more people to better understand more stuff in less time than ever before.

After I fill a cup of water at the sink, I want it to be on the table. But I seem unable to instantly move it to desired location. I can't even instantly get it moving at high speed at the sink, then instantly stop it at the table. The property of the water (and the cup) that prevents (or "resists") me doing this is called "inertia".

No, I think its called mass! F= mdv/dt--Light current 10:22, 22 December 2005 (UTC)

I observe that there's a short time at the sink where I push sideways with my fingers (and the surface of the water is not level with the ground). Then I carry the cup at constant speed to the table (and the surface of the water is more-or-less level with the ground). Then there's a short time at the table where I push sideways with my fingers until the cup comes to a stop (and the surface of the water sloshes around).

I could measure (or estimate) how hard I push the cup sideways (in Newtons ), how long it takes me to get the cup up to speed (in seconds), and how fast that speed is (in meters/second). Combining all those values gives me a value for the inertia of the cup of water. That value is in units of N*s^2/m. (Some people measure inertia in units of lbf*s^2/ft, which is abbreviated "slug").

Well these are the units of mass are they not?--Light current 10:22, 22 December 2005 (UTC)

I could also combine all the values (force*time/speed) at the end of the trip, when I make the cup stop on the table.

We could apply the same calculation to dropping a stone.I don't see it instantly disappear from my hand and re-appear on the ground. I don't see it instantly start moving at some high speed, then maintain that constant speed until it hits the ground. (This is easier to see if I give it a slight upwards or sideways velocity at the instant I let go).(It certainly appears to instantly stop when it hits the ground, but let's not get into that just now).

I measure the force of gravity on the stone (with my fingertips, or with a spring scale if I want more accuracy), and how long it takes to get to a few different points (before it hits the ground). Then I combine those values (force*time/speed) to get the inertia of the stone.

Youre only measuring the mass again!--Light current 10:22, 22 December 2005 (UTC)

I can reduce the apparent inertia of an object by moving it indirectly via block and tackle or some other machine. The force and acceleration I feel (the inertia I feel) at my end of the rope is different from the force applied to that object, and its acceleration. (This is one way inertia is not exactly identical to mass. I know of no way to modify the mass of an object).

No, youre just changing the apparent force at your hand.--Light current 10:22, 22 December 2005 (UTC)

Feel free to move this section to the Talk:Inertia page. Perhaps bits of it could eventually be worked into a gentle introduction in the inertia article. I would suggest withdrawing the statements that "inertia ... has no units", since it has units of N*s^2/m or slugs.

--DavidCary 06:44, 22 December 2005 (UTC)

It looks like the statement has been removed already. I dont really have any quarrels with the page as it is now. I think it gives a fair representastion of the ideas.--Light current 12:00, 23 December 2005 (UTC)

Just for clarification, I think DavidCary (talk · contribs) misspoke when he said slugs. Slugs are a unit of weight, similar to newtons. A newton is the amount of force required to accelerate a mass of one kilogram at a rate of one metre per second squared. And a slug is the mass that accelerates by 1 ft/s² when a force of one pound-force (lbf) is exerted on it. Something can weigh one newton, similar to the way something can weight one slug. Neither slugs nor newtons are used to measure inertia. Otherwise, I just want to watch this from the sidelines. Thanks. -Scm83x 10:41, 22 December 2005 (UTC)

Inertia redefined

THe lead para says inertia has recently been redefined as resistance to change. Who redefined it and can I have a reference and a quotation of the new definition?--Light current 10:26, 22 December 2005 (UTC)

I meant to say the notion of inertia is incorrect. All the effects ascribed to so called 'inertia' are in fact due to mass, the whole mass, and nothing but the mass. Mass causes all the effects of inertia so inertia as such is not a concept that is distinct from mass. In fact it cannot be separated from mass. Inertia is mass so why do we need a separate word for it?--Light current 11:33, 22 December 2005 (UTC)

Could it be that a separate word is needed in order to show the different perspectives from which one could view an object? Namely where mass is a physical quantity that measures the amount of matter that an object contains, and inertia is a physical property of an object related to its response in the presence of external forces.

In the case of linear forces the inertia turns out to be proportional to the mass, and in the case of rotational forces or torques, the inertia is proportional to the mass and its distribution about the axis of rotation.

On the other hand maybe one word could be used for both of these concepts, but it doesn't seem to be taught that way in most textbooks. Sort of like electricity and magnetism are both manifestations of the same phenomena, yet we use two separate words for these concepts until the grand unification is discovered? Kenny56 03:00, 23 December 2005 (UTC)

Its a physical property that is indistinguishable from mass. If you doubt that, try to describe an experiment which could tell the difference! ;-)--Light current 03:10, 23 December 2005 (UTC)

According to relativistic physics, the property of inertia also applies for energy. For example, in particle accelerators that accelerate to near-lightspeed velocities, the magnetic fields that maintain the curvilinear trajectories of the particles need to be stronger than predicted by newtonian mechanics. Effectively, the circling particles have an additional inertia, and the determining factor for this additional inertia is the amount of kinetic energy. If we define 'mass' as the rest mass, which is the usual definition in physics, then we have that there is not a one-on-one correspondence between rest mass and inertia. --Cleonis | Talk 17:34, 31 December 2005 (UTC)

Correct! You should use the term relativistic mass. But youre not really saying anything here. The circling particles have additional mass due to the added KE (so if you say inertia is mass, they have additional inertia-- but why bother saying that?). Inertia is a concept- thats what the article should say "Its only a concept- its not real"--Light current 01:31, 9 January 2006 (UTC)

If we use the word inertia for the tendancy of an object to keep a constant direction and a constant speed, we cannot use it instead of the word mass.
If we use the word inertia and mass indistinctly, we cannot use the word inertia for the tendancy of an object to keep a constant direction and a constant speed  : we will have to find an other one, which could be simply the word tendancy!
But I think it is better to keep the word mass for mass, and the word inertia for the tendancy of an object to keep a constant direction and a constant speed, and the word energy in the sense that it is use in the energy-momentum 4-vector system, in which the mass is not the energy.
If we continue to say that inertia is mass and mass is inertia, and, that mass is energy and energy is mass, and, that energy is inertia and inertia is energy, then everybody on the planet earth will be in a state of total confusion! --24.202.163.194 18:48, 31 December 2005 (UTC)

This article appears to be rather confused about this whole subject, which I suspect is leading to some confusion here as well. In response to a few points made by Light current, I'll make an attempt to clear things up a bit (I hope):

• Inertia is not a quantity, it is a principle. It is simply a description of one of the fundamental ways that the universe has been determined to work. "inertia" is simply a name for the concept that "things tend to keep doing what they're doing unless acted upon by an outside force, and the amount things change under a given force depends in a particular way on their mass". You could put that sentence in any place where the term "inertia" is (correctly) used and it would mean basically the same thing, but "inertia" is just a much less wordy (and more precise) way to say it.
• Inertia has no units partly because it's a description of the way the world works, not something that you measure, or that exists to greater or lesser degrees. You can't say something has "more inertia" or "less inertia" than something else (people sometimes do, but this is erroneous. They usually either mean mass or momentum (see below)). Inertia is a behavior that all things exhibit equally, not something that's posessed by one thing or another.
• Inertia is not the same as mass. Inertia is a concept, mass is a measurable quantity. The concept of inertia says that mass affects the application of force in a particular way, that is all.
• Inertia is not the same as momentum. Again, inertia is a concept, and momentum is a measurable quantity. Inertia says that mass + force = a change in momentum.
• The concept of inertia is not "imaginary". It is quite clear that it this tendency of the universe does actually exist (as your own definition has suggested). Otherwise things would spontaneously go flying off in random ways all the time. Inertia is an abstract concept, which is not really the same thing as "imaginary".
• If it helps any, try to think of "mass", "force", and "momentum" as players in a game. "Inertia" is a name for one of the rules of how the game is played, it's not a player in the game itself.
• (it should be noted that in all of the above I'm speaking mainly from a classical point of view. The definition of inertia changes a bit in some relativistic contexts, but it still doesn't change the point that "inertia" is a principle of the way the universe works, not an actual quantity or a physical thing (it's a rule, not a player))

The "broader second meaning" mentioned in this article is actually quite muddled and confusing, and leads to a lot of problems later on. I would suggest ignoring it completely. I think I see what the author was trying to say, but I think it tries to portray a colloquial altered usage as the primary definition of the term, which is wrong. I'll take a look and see whether I can clean things up a bit, but this looks like it might be a large task.. -- Foogod 03:13, 9 January 2006 (UTC)

Inertia as arising from a field interaction

According to relativistic physics, energy also has the property of inertia. A particle at high speed does not only have the inertia that corresponds to the rest mass, there is additional inertia that correlates with the kinetic energy.

How would a particle with zero rest mass behave? The elementary particles called neutrino's give an indication. Neutrino's have exceedingly little rest mass, and they move with a velocity very close to the speed of light. (If photons are regarded as particles without rest mass then it appears that a particle without rest mass instantly jumps to lightspeed. )

It is intriguing to explore similarities between motion in space and electric current.
There appears to be an analogy between motion in space and current in a superconductor - no resistance. The electric-current analogon for inertia is inductance. A superconducting coil with self-induction will have zero resistance, but due to the self-induction there is opposition to any change of current strength. (In the case of a superconductor and no inductance, applying a voltage results in an instant jump to maximum current strength.)

In the case of electrical resistance, current strength is proportional to the applied voltage. In the case of inductance, the rate of change of current strength is proportional to the applied voltage.

It is possible to interpret inertia as arising from interaction with a field. Let's call that 'the inertia field'. The concept of a field is mostly known in the form of a vector field, such as the electrostatic field. The inertia field however, is a uniform scalar field. Both matter and energy couple to the inertia field. It is possible to re-interpret Newtons laws of motion as describing the properties of matter-field interaction of the inertia field. Newtons first law: if you are in a particular state of motion you remain so. Newtons second law: if a mechanical force is applied the rate of change of momentum is proportional to the applied force. Newtons third law can be seen als asserting uniformity of the inertia field: If object A exerts a force on object B, then the amount of change of momentum will be the same for A and B.

The advantage of interpreting inertia as arising from interaction with a field is that it facilitates a later transition from newtonian physics to general relativity. In general relativity the description of inertia and the description of gravitation are unified. General relativity describes a single field, and the properties of this field account for both the existence of inertia and the mediation of gravitational interaction. --Cleonis | Talk 19:59, 31 December 2005 (UTC)

This actually sounds very similar to the hypothesis that the phenomenon of inertia can be described as a field interaction between mass and the zero-point energy of empty space (the stochastic electrodynamics (SED) theory of inertia).. I'm not sure if that's what you're referring to, or if you're proposing something else? -- Foogod 03:28, 9 January 2006 (UTC)

Object at rest

When an object is at rest and stay at rest, is it due to inertia, or is it due to the mass of this object ? (We suppose that the force of friction is negligible.) --24.202.163.194 21:12, 31 December 2005 (UTC)

what is meant here by 'staying at rest'? The standard thought experiment here is that you are in a spacecraft, and you are way out in intergalactic space, so far away from any galaxy that any gravitational effects are negligable. In those circumstances, your only reference is what the onboard accelerometers measure. In the absence of gravitation, the only meaningfull distinction is between a state of 'not being accelerated by a mechanical force' and 'being accelerated by a mechanical force'. Let the spacecraft be equipped with thrusters. If the thrusters are exerting thrust, then the spacecraft is being accelerated by a mechanical force, and only then the inertia of objects inside the spacecraft can be measured. --Cleonis | Talk 06:11, 1 January 2006 (UTC)

Thank you for your excellent example. In your example, take the situation of 'not being accelerated by a mechanical force'. Then all the objects which are not fixed to the structure of the spacecraft will float in the inside space, because of the absence of gravitation or any other external force, and they will be at rest in respect of this structure, and too, each object will be at rest in respect of each other one. If they are at rest, is it due to inertia, or is it due to the mass of each of these objects ?

Supplementary questions: if the spacecraft accelerate, will those floating objects accelerate too? What will happen then, will it be due to inertia or will it be due to their mass? --24.202.163.194 17:50, 1 January 2006 (UTC)

If the thrusters of the spacecraft are not working, then macroscopic objects will not move relative to each other. However, individual molecules will have a non-zero kinetic energy. In fact, part of the quantum nature of particles is that they cannot have zero energy. No matter how close to the point of absolute zero the matter is cooled, molecules and atoms retain a minimum of quantum jitter. For macroscopic objects (assemblies of gazillions of atoms) the quantumphysical jitter averages out.

Macroscopic behavior of matter is a downstream consequence of the quantum behavior of the atoms involved. The quantum behavior aspect averages out completely. Even zo, it should be kept in mind that in quantum physics the very concept of 'being at rest' is far from straightforward.

Individual atoms moving relative to each other also have the property that they move in a straight line (on average) until they bounce against something. So the question becomes: why do quantum particles propagate in a straight line? The answer to that is not known. For quantum particles the full quantum particle/wave duality is valid. (There are interferometers that operate with atoms instead of with light. It is possible to elicit wave behavior with atoms.) What is known, and calculated with quantum mechanics, is that beams of atoms behave in similar ways to beams of light. The probability of finding the particle is by far the largest somewhere on a straight line. A downstream consequence of that is that macroscopic opbjects tend to move along a straight line.

In all, the concept of 'being at rest' is rather problematic. It can be a helpful tool in teaching newtonian dynamics, but it is unsuitable as a basis for discussing fundamentals of physics. Preferably, newtonian physics should be taught in such a way that a later transition to quantum physics is prepared for as much as possible. --Cleonis | Talk 20:17, 1 January 2006 (UTC)

'If the thrusters of the spacecraft are not working, then macroscopic objects will not move relative to each other.' and relative to the spacecraft, I presume.
Is it due to the inertia, the mass, or to a quantum phenomena? --24.202.163.194 02:25, 2 January 2006 (UTC)

Complement of information about the question.
This question was in relation with the section 'Inertial mass' of the article and with the edit that I add, and also with all the discussions in the talk page. I see that you seem to know a lot about physics, so if it possible for you to go take a look at them and give us your opinion on it, I think it will be helpful. Thank you.--24.202.163.194 02:40, 2 January 2006 (UTC)

My recommendation is not to worry about "the fundamental cause" of inertia, but to concentrate instead on describing the properties of inertia.

In physics, as in any science, there is no such thing as an exhaustive explanation. A new theory brings an innovation if it moves the level of description to a deeper level than before. For example, Kepler inferred from his observations that planetary orbits are ellipses in shape. Later Newtons showed that an inverse square law of gravitation will lead to elliptical orbits. Newton had moved the descripton of celestial mechanics to a deeper level.

I assume the general consensus is that the article should present inertia in terms of newtonian dynamics, for newtonian dynamics is the best way to begin teaching physics. At the same time, it must be kept in mind that newtonian dynamics has been superseded, by relativistic physics and by quantum physics. So the effort must be to apply the principles of newtonian dynamics as straightforward as possible; it's not a matter of 'getting at the true nature of inertia', for that is unknown.

In order to obtain a theory of celestial mechanics, Newton assumed his three laws of motion, and he assumed the inverse square law of gravitation. The resounding success of the theory jusitified the assumptions. Newton made no attempt to explain the inverse square law of gravitation, he assumed that law, and quite rightly so. Newton made no attempt to explain the cause of inertia, the properties of inertia are assumptions of newtonian dynamics, and quite rightly so. Scientists must avoid getting bogged down by unanswerable questions.

Inertia is so fundamental that relativistic physics and quantum physics must, like Newton had to, assume the properties of inertia (or something equivalent to it) in order to frame a theory at all.

Presumably you have read my contribution titled: 'Inertia as arising from a field interaction'. That describes my views.

As noted in many places, according to the theory of general relativity inertial mass and gravitational mass must be equivalent. That is: from the assumptions of general relativity it follows that the two must be equivalent. --Cleonis | Talk 14:11, 2 January 2006 (UTC)

Your answer is, again, very interesting. I agree with you that the article should present inertia in terms of newtonian dynamics. But the title of the article being Inertia, I think it should also gives the meanings that the word inertia has in others contexts. --24.202.163.194 02:56, 3 January 2006 (UTC)

I am pleased to read you find my considerations interesting. I think I have contributed all I can. I have been roaming around on wikipedia talk pages for some time now, and I don't think it is possible to obtain good wikipedia physics articles. Different people can be committed to totally different theories about physics, so different that there is no prospect of reaching a consensus. I am committed to a particular point of view, to me my point of view looks like the only logical one. Different belief systems about physics keep clashing; I don't think it's ever going to work. --Cleonis | Talk 09:38, 3 January 2006 (UTC)

In Wikipedia:Neutral point of view, it is said:«NPOV (Neutral Point Of View) is an official Wikipedia policy which states that articles should be written from a neutral point of view, representing all views fairly and without bias. According to Wikipedia founder Jimbo Wales, NPOV is "absolute and non-negotiable".»

Thus we have, we must!, represent all views i.e. here, all theories (wikipedia has criterias for this too). That means that we don't need at all to get a consensus, but made a fair place to each of the theories which respond to the criterias mentionned above. If we don't follow that, I agree with you that, surely and very certainly, 'it's ever going to work'. But if we follow the principles of wikipedia, it will work. It will take time, but it will work. So lets give it a try, at least. --24.202.163.194 16:34, 3 January 2006 (UTC)

A new article for the historical part?

The historical part taking more and more place in this discussion, I suggest that an new article should be created on this subject (any contributor who is registerd can do it), and that we should only make a short text on it in this article and refer the reader to the new one for more informations. Even the new one should be split in others one, one article for Aristotle, one for Kepler, etc. We should too transfer all the discussion on this matter to this new article. What do you think of that suggestion? --Aïki 17:57, 8 January 2006 (UTC)

You may be right. I would not be opposed to creating a new article (though I wouldn't be opposed to leaving things as they are either), but it seems to me that making a separate article for each philosopher/scientist would be going to far. --Iustinus 18:04, 8 January 2006 (UTC)

I would actually disagree with this. The purpose of having an article on the subject of "inertia" is to provide an understanding to the reader of what inertia is and how it relates to the world around us. Part of this understanding is a general (note: only general, no need for great detail) understanding of how the concept developed and who was involved in its development, and how it relates to other concepts which have been proposed as alternatives. I think a large portion of the useful content of this article is actually the history part, and removing that context (including removing the duplicated historical bits of the other sections) would leave this basically as a stub with only the basic definition of the term and not much else. I do think that the current history section is much too long and rambling, and unnecessarily duplicated in the rest of the article, which is why I'm actually about to post a rewrite of the history section as my next update (to be followed by some cleanup of the other parts of things as I have the chance), which should hopefully make things much better at least in that regard. As for the factual debates currently going on, I see no reason why it really matters whether they're done in this talk page or some other talk page; as long as they're present to the understanding of inertia and where the concept came from, why not have the discussions here? -- Foogod 20:07, 11 January 2006 (UTC)

Rewrite underway

I just wanted to put a note in to let people know that I'm looking at going through this article bit by bit and trying to clean up the form of it and make it a bit more readable, and that comments or suggestions on the changes I'm making are welcome. I've already rewritten the summary section a bit, and will be posting a rewrite of the "history" part of things shortly.

The main intent here is not to significantly alter the content of the article, but just to try to put it in a more readable (and in many cases more compact) form, and present things more in the format of an encyclopedia article. There are a few corrections here and there that I'm making along the way (such as trying to make the distinction more clear between the fundamental principle of inertia and some (technically incorrect) conventional uses of the term), but for the most part I'm just trying to rephrase what's already in the article in a more readable form, and possibly fill in some additional details.

One thing I would also like to do here, though, is try to improve the annotations and citations in the article a bit (there really don't seem to be much currently, and this shouldn't be hard for this topic). In this vein, however, I'm having some trouble with a couple of things:

• Can somebody point me to where Galileo stated "A body moving on a level surface will continue in the same direction at a constant speed unless disturbed."? It seems almost universally accepted that he did say this at some point, but I'm having a hard time tracking down exactly where it was written down, and would like to include this information, as it's a fairly significant event in the timeline.
• I can't seem to find any source anywhere for the quote attributed to Leonardo da Vinci: "Everything moveable thrown with fury through the air continues the motion of its mover; if, therefore, the latter move in a circle and release it in the course of this motion, its movement will be curved." It does not appear to be anywhere in his notebooks, and I can't find any reference to it readily in net searches. About the only place it seems to show up is in this article. Does anybody know where this came from or who attributed it to da Vinci?

Any help with these anybody can provide would be much appreciated... -- Foogod 20:22, 11 January 2006 (UTC)

I am guilty of all the unattributed quotes as I wrote the original History section a couple of years ago - one of my very earliest edits. I am sure that most of my content came from Catholic Encyclopedia or Stanford Encyclopedia of Philosophy but it doesn't look like the quotes are there. The other possibility is the papers by Clement and by McCloskey though I can't put my hands on my copies right now. I try not to look at the current state of this article as it is upsetting. I have a thin skin which is why I must give up this game and get a life. I will keep looking. Cutler 14:45, 13 January 2006 (UTC)

Update: I've finally finished putting together a rewritten history section. I have taken pretty much all the little tidbits floating around the previous history section and tried to put them together into a much more cohesive and readable whole.. I've also added a fair number of references for some of the statements.

A few notes on what I've done so far:

• I removed the da Vinci quote because I couldn't find anything anywhere to back up his having said it.. if we can find a reference for this, I'm all in favor of putting it back in, though.
• I removed the Descartes quote, because it seems to be problematic (it's rather taken out of context and subject to interpretation), and also because it seemed rather redundant. If people have objections to this I'll see what can be done to work it back in, but it may take some effort to do properly.
• I moved the Asimov stuff out of the middle of the history bits, because it really doesn't belong there.
• I'm still not really happy about a few of the references:
  • I'm still looking for a good citation for the Galileo quote. I've left it in because I'm pretty sure it's right, but I can't find a cite for it.
  • I'd like a better reference for Philoponus' position.. Ideally something he actually wrote..
  • I couldn't find any direct resources for Buridan's work, so I had to go with a quote found in another page on an academic site.. This one does need improvement too.

Anyway, comments on my changes are welcome.. -- Foogod 23:14, 14 January 2006 (UTC)

Oh, please also note that I haven't even looked at the rest of the article yet. I noted that the heading levels were screwy (everything in the article is a second-level heading, with no first-level ones), so I added an "Interpretations" heading to keep all the rest of the stuff from being part of the history section, but this heading may not really be appropriate for everything that's under it. I'm planning on looking at the heading levels in more detail when I look at those sections in more detail.. -- Foogod 23:18, 14 January 2006 (UTC)

LC, explain your recent revert!

Why did you revert my and Foogod's changes? Stop trying to introduce your own personal views into this article! The purpose of this article is to accurately reflect the views of the physics community. Pfalstad 00:54, 13 January 2006 (UTC)

My explanations have already been given in the first para on the talk page. Inertia is not a real phenomenon and must be explained as such! Its purely and simply an effect of mass- nothing more.--Light current 00:58, 13 January 2006 (UTC)

Sure inertia is an effect of mass, but it is still useful to keep separate the concepts "mass" and "inertia". For one thing, mass refers to a measurable value, and inertia refers to an effect that is related to (or caused by) an object's mass. Also, mass is related to gravity. It's useful to keep the law of inertia and the law of gravity separate. That is why we talk about "inertial mass" as separate from "gravitational mass", even though they are the same quantity, physically. We have separate words for "gravity" vs "mass" for the same reason. Pfalstad 01:55, 13 January 2006 (UTC)

What's your source for that assertion? Putting stuff like "inertia is an imaginary concept" in the article is crossing over into crank territory. Pfalstad 01:17, 13 January 2006 (UTC)

I don't have time to deal with this silliness right now, since I am leaving on a trip soon, but: inertia doesn't explain Newton's first law, it is Newton's first law! Pfalstad 01:18, 13 January 2006 (UTC)

Have a good trip. Ill look after the page for you!--Light current 01:27, 13 January 2006 (UTC)

Re "less controversial": that's better. But it's not a concept invented to explain the first law, it's equivalent to it. Newton's law is just a rigorous way to describe the behavior we call inertia. Inertia is the tendency of a body to obey Newton's first law. Unfortunately it's hard to argue about it since none of my physics books mention it except as part of the phrases "moment of inertia" and "inertial mass". Pfalstad 01:50, 13 January 2006 (UTC)

Its erroneous that the concept of inertia is taken as fundamental, when the fundamental causes of all the effects as described by Newton I is actually MASS and FORCE. Newton only implies inertia as a sort of negativc thing. ie if you dont have aforce, then things stay asthey are so this must be inertia. Can you not see the insanity of this topsy turvy argument? Thats why I say inertia is an invented phenomenon.--Light current 01:57, 13 January 2006 (UTC)

Do you have a source for this idea that inertia is an invented phenomenon? See WP:NOR.

Mass and force don't cause anything. Physical laws cause things. At least, in common parlance, which is what this encyclopedia is about. Would you say that Coulomb's law is imaginary, because charge causes electric forces? Would you say that the law of gravity is imaginary, because mass causes gravitational force? In a sense, it's true, but it's not how people talk. We talk about physical laws being real and causing physical effects. Do you agree that Newton's first law is known as "law of inertia"? Properly or not? Pfalstad 18:10, 13 January 2006 (UTC)

I would like to point out that contrary to the title of this section, Light current did not "revert" changes. His changes are obviously an attempt at refinement, not reversion, and there is nothing wrong with changing others' text in an attempt to improve it (that's what Wikipedia is all about). Having said that, I do have to agree that saying that inertia was "invented to explain" Newton's first law is ultimately wrong, for reasons I have gone into in the "inertia redefined" section above, which I think is the best place to continue this discussion for the moment. -- Foogod 23:27, 13 January 2006 (UTC)

He did revert; see [1]. Pfalstad 01:49, 14 January 2006 (UTC)

I was not aware that Newtons first law was called the 'Law of inertia'. The wording of the law as I was taught it did not include the word 'inertia'. However, if it can be shown that Newton did indeed formulate his first law using the term 'inertia'(whether in Latin of English) then we do have a definition of inertia and that is what we are writing about and so this defn should be included.

I must restate, however, that in today's modern thinking, the idea of inertia is a negative one (like suction which is a lack of pressure, or cold which is a lack of thermal energy-- these are not scientific terms).Neither should 'inertia' be considered a scientific term! Who can say honestly that (s)he was taught, in school physics, that inertia is a fundamental property (more fundamental than mass of force?). I dont remember any equations quoting inertia or any homework or exam questions on it. Is anyone else able to quote such emphasis on inertia in school (or university) physics lessons? --Light current 23:51, 13 January 2006 (UTC)

To make the point a little more obviously: Can anyone write down any equation involving inertia (symbol I , lets say). Not mass, not momentum or anything else. Just write an equation containing inertia!--Light current 00:18, 14 January 2006 (UTC)

No of course we can't write down an equation involving inertia, for the same reason we can't write down an equation involving law of gravity (symbol $L_{g}$). That doesn't make it "imaginary". Just not a quantity. Pfalstad 02:24, 14 January 2006 (UTC)

(Copied from above, since this discussion seems to be continuing in both places (can we please pick one place or other to discuss this stuff?):

Lightcurrent, I just reverted a bunch of small edits you made to this page, because most of them were problematic, and are continuing changes which several people are already in the process of debating with you here in the talk page. Please don't continue making changes to further a given position while there's ongoing debate here about that position. Let's get the discussion sorted out first, please. -- Foogod 00:19, 14 January 2006 (UTC)

I really cant see what is wrong with this para:

Inertia in physics may be described as the tendency of a body to remain at rest or in uniform motion unless acted upon by an external force. In this respect, it is a concept originally thought to be the underlying reason for Isaac Newton's first law of motion, which is often paraphrased as: --Light current 00:24, 14 January 2006 (UTC)

Well, first of all, changing "is defined" to "may be described" does nothing except add vagueness to imply there's some debate, when as far as I know there really isn't any (if you want to cite some sources I might be willing to change this position, but I still think it would be better to state clearly what the state of affairs is than use some wishy-washy language to gloss over the whole issue). That's minor compared to your other change, however, which says that inertia (as we use the term today) was thought to be the reason for the first law of motion, which is, frankly, wrong. "inertia" as we use the term today is Newton's first law of motion, and thus was never "thought to be the reason" for it at all. --- Foogod 00:36, 14 January 2006 (UTC)

Using the term 'is defined' imples somebody defined it in a definition. So

• a) who defined it and
• b) where is the actual definition?

If no answers to these, then 'may be described' I feel is better phrasing.--Light current 00:42, 14 January 2006 (UTC)

And regarding your argument made earlier, just because Newton didn't call it inertia does not mean that that's not what we call inertia today. That's like saying that a sports car is not a car, because back in old-times "car" meant "carriage", and all carriages had horses in front of them, so a sports car can't be a car unless it's got horses in front of it. -- Foogod 00:36, 14 January 2006 (UTC)

I refer you to my question of an equation quoting inertia as one of the terms. Can you show one? ie Inertia is not scientifically defined in a definition to my knowledge. ( a defn using the actual word INERTIA)--Light current 00:42, 14 January 2006 (UTC)

That question only shows that you really haven't understood what everybody here has been trying to tell you inertia is. I challenge you to explain to me how Newton's First Law of Motion can be put into an equation, because that's what you're asking somebody to do. (I repeat: inertia is Newton's First Law of Motion.) As for a definition using the word INERTIA, I suggest you take a look at a dictionary [2] [3]. -- Foogod 00:49, 14 January 2006 (UTC)

Newtons first law of motion can be written as:

P= mv = constant where m is mass, v is velocity, P is momoentum. --Light current 01:00, 14 January 2006 (UTC)

P = mv is not Newton's first law, but the definition of momentum. p = constant is not true in general. The momentum of a body is only constant when it is not acted on by a (unbalanced) force. Pfalstad 02:15, 14 January 2006 (UTC)

So your definition of inertia is:

Inertia is Newtons first law of motion

I hardly regard that as a satisfactory scientific definition!--Light current 00:57, 14 January 2006 (UTC)

Umm, regarding your first comment.. that equation isn't anything like Newton's First Law of Motion (have you even read the law?). Newton's First Law cannot be represented in mathematical terms, because it's not a mathematical concept, it's a physical principle arrived at by empirical evidence (that's why it's called a principle, which means something that can't be derived from anything else). One thing you don't seem to be understanding is that Physics and Math are not the same thing. Something can be a physical definition without being a mathematical one.

On the contrary, the equation I quote is exactly /Newtons fisrst law of motion because it states in mathematical terms that momentum is constant which is exactly what Newton said. A body persisting in a state of constant motion has a constant momentum. No force = no accn = no change of velocity = contant motion (or rest). Newton I is synonymous with principle of conservation of momentum. No need for inertia here! Now you write me a similar equation with inertia in it!--Light current 01:34, 14 January 2006 (UTC)

Regarding "a satisfactory scientific definition", I'm really sorry to hear that you don't regard it that way, but fortunately you are not the authority on the subject. The fact is that most scientists regard that to be a satisfactory scientific definition, which is really what counts (to be more accurate, the more technically accurate definition is "inertia is the principle which is described by Newton's First Law of Motion", but in this context the distinction isn't that important, so I've been simplifying for clarity). -- Foogod 01:10, 14 January 2006 (UTC)

I dont think scientists regard it as a satisfactory definition. In the first place, where did they learn this definition, where did you learn this definition. In what books is this definition quoted. Are there any references that you can point to that have this definition. Regarding your 'technically accurate' definition, a definition that does not include the word being defined is no definition at all. That is patently OBVIOUS to anyone (scientist or not).--Light current 01:34, 14 January 2006 (UTC)

So here we may have it! Inertia is conservation of momentum! nothing more nothing less!--Light current 01:40, 14 January 2006 (UTC)

I'd be very interested to have you cite some sources where reputable scientists state they don't consider that to be a satisfactory definition. (if you can't cite sources, it doesn't belong in Wikipedia anyway). Contrarily, there are numerous Physics textbooks out there which define it in exactly that way (and I can provide references for as soon as I can get to a decent library to find them, if you're really going to insist on all that extra work (sigh).. I was really rather hoping I didn't have to go to that kind of effort to prove something that should be pretty obvious from all of the dictionary definitions). In any case regarding your other assertion, my definition does include the word being defined. Please read it again. It is, in fact, the very first word in the definition, so I don't really know how I can make it more prominent: "inertia is the principle which is described by Newton's First Law of Motion". That is a definition. It uses the word "inertia". It is very clear, and is basically consistent with current rigorous usage of the term in all (classical physics) materials I'm aware of.

Well if it was obvious to me i wouldnt be arguing the case. It just appears to be a pretty sloppy definition to me. But there again- Im not a physicist! However, if that is how the physics community defines this (to me) less than useful concept, so be it. I suppose we'll have to perpetuate the fallacy of inertia.--Light current 02:17, 14 January 2006 (UTC)

Please note: "inertia is conservation of momentum" is close, but not quite correct, because momentum is a scalar quantity, and thus doesn't describe direction. Conservation of momentum is a consequence of inertia, but inertia is actually a more comprehensive concept which also includes direction. -- Foogod 02:04, 14 January 2006 (UTC)

Actually momentum is a vector quantity. So my defn. does work. If not - why not? --Light current 02:34, 14 January 2006 (UTC)

I just explained why, in the text right below this: Pfalstad 02:37, 14 January 2006 (UTC)

Inertia is not conservation of momentum; "conservation of momentum" usually refers to the total momentum of a system. Inertia refers to the motion of an object. Momentum of an object is not, in general, conserved. If you act on it with a force, its momentum changes. Anyway, all this is your original research, so it shouldnt' be in the article. Pfalstad 02:19, 14 January 2006 (UTC)

Well If you reread my post on this, youll see that I was in fact talking about an isolated body with no external forces acting. Anyway, regarding your reference to original research, Im not trying to put anything in the article that is not generally accepted, but I am trying to ensure that the article is

• a) Correct and
• b) clear in its explanations

Only by questions will the truth be found!--Light current 02:27, 14 January 2006 (UTC)

I'm just saying that the terms "inertia" and the term "conservation of momentum" have different meanings in general, so they're not equivalent. But sure, for an isolated body, the law of inertia and the law of conservation of momentum make the same prediction about its motion. So does Newton's second law. So you can use any term you want in that case. Pfalstad 02:35, 14 January 2006 (UTC)

Ill choose to quote law of conservation of momentum - its far clearer to me and can be written down mathematically. But it should be made clear on the page that inertia is just a concept and not a real thing like mass, force. etc. I feel the page is ambiguous in that respect at the moment. Maybe inertia could be explained by referring to the idea of conservation of momentum? Thats all Im suggesting!--Light current 02:45, 14 January 2006 (UTC)

Yes that could be clarified (as long as you don't say it's "imaginary"). I would call it a "principle" as the article does. Part of the problem is that the term is used in different ways; in common usage (sometimes by physicists, I'm sure) it is a quantity, like momentum. We say a large object has "more inertia". Pfalstad 02:52, 14 January 2006 (UTC)

Inertia cannot be defined as a quantity or as anything- But it can be described as a concept. The word defined in the lead para is wrong and should be changed to describe(d). The concept of inertia is that for an isolated body 'momentum tends to be conserved'. Simple! Why cant we put that statement in the lead para?

Momentum tends to be conserved, unless the object acted on by a force. The quote from Newton explains this without using the terms "momentum" and "conserved" which might be confusing to beginners. It also gives us some historical context. Inertia came first, then momentum. Pfalstad 03:59, 14 January 2006 (UTC)

No, of course a concept is a concept and its real; even if it's describing an imaginary thing that cannot be defined by an equation. The concept of inertia is real. I think its a bad concept that tries to describe something that has another cause. But thats only my view!--Light current 03:04, 14 January 2006 (UTC)

Umm, everything in physics is "just a concept". Including conservation of momentum, and even (believe it or not) mass and force. They don't really exist either, they're just concepts to describe our observations of the physical world. (Arguably, "mass" is more abstract a concept than inertia, actually, because according to relativity there's no real distinction between mass and energy anyway.) There's nothing anywhere which makes mass, or conservation of momentum, more "real" than inertia, and the fact that you've somehow decided it must be so does not actually make it true. Regardless, there are some other problems here.. I'll try to tackle them all together, since this discussion is getting very fragmented (on a side note, could you please put all your replies at the end of a given topic? I didn't even see your comment regarding equations above until just now because it was put in the middle of previous text)

First, your equation above does not say the same thing as NFLM (I'm just going to start abbreviating Newton's First Law of Motion, if you don't mind, it's getting to be a pain to type):

• NFLM does not say anything about momentum at all. It doesn't even use the word! (Isn't it rather hypocritical to attack everybody else's definitions because the relevant words aren't included and then turn around and try to define your own things with words that aren't there?)
• NFLM talks about force and velocity. Your equation doesn't even mention force, so how can it be equivalent?
• NFLM does not talk about mass at all. Your equation includes mass, therefore it cannot be equivalent to NFLM.

Secondly, conservation of momentum is not the same thing as inertia for several reasons. First, inertia does not say anything about mass. It therefore does not say anything about momentum. Inertia only applies to force and velocity. Secondly, conservation of momentum (if taken by itself) implies that mass and velocity are interchangeable. Thus, an object could (spontaneously) convert mass into velocity or vice-versa. Inertia, on the other hand, explicitly says this is not possible (therefore the two principles actually say two different things about the situation: they cannot be the same thing).

Thirdly, your assertion that concepts can't be defined, only described, is, well, silly. All words can be defined, regardless of what they refer to. Inertia has a definition in dictionaries, just like any other word. Defining something just means "describing what it means", so if it can be described, then you've automatically defined it as well. Moreover, there's nothing inherently more "sloppy" in the definition of inertia than the definition of conservation of momentum. If you absolutely must have a mathematical description of inertia, try this:

$F=0\to {\vec {v}}={\vec {k}}$ where ${\vec {k}}$ is some constant.

(Please note that this still isn't actually equivalent to NFLM, because the important point about NFLM is the fact that it states this is a principle which applies to the physical world, and there's no way to state that with mathematics (this is why math and physics are not the same thing). The above, however, is a reasonable mathematical definiton of the principle of inertia, so you should be happy now, right?

Fourth, you keep talking about causes, which is wrong. Inertia has nothing to do with causes of anything. The principle of inertia describes behavior, it does not cause behavior (neither does conservation of momentum, actually). If you can't understand this, you don't understand inertia, and trying to make claims about something you don't understand is, again, silly. I suggest you try to actually learn what the concept is before declaring it "imaginary". -- Foogod 03:58, 14 January 2006 (UTC)

Well I agree that the concept of inertia describes behaviour (in a roundabout way) but so does the concept of mass even better. My argument is that the article does not seem to be saying that. It seems to be ascribing some fundamental magical essence to inertia. AS I said before, the concept is real, the subject of the concept is not real in the sense that inertia depends on mass and only mass. BTW I know what the concept is and cheap shots are no way to resolve arguments.--Light current 04:17, 14 January 2006 (UTC)

No, that is exactly my point. You do not understand these concepts. If you did, you would not be saying things like "mass describes behavior", because it doesn't. Mass doesn't describe anything: it is a quantity, not a description. I don't know how to get it across to you more clearly. The reason these terms are imaginary to you is because you have made up your own definitions for them which aren't the same as everybody else's.

Let me just ask you this: What makes conservation of momentum any more real than inertia? When you really get down to it, inertia is basically just "conservation of velocity". Is momentum somehow more "real" than velocity? -- Foogod 10:27, 14 January 2006 (UTC)

Just a name

' ... if you dont have a force, then things stay as they are, so this must be inertia.' must be replaced by: ' ... if you dont have a net force on an object, then this object keeps its state of motion: this is what is called the 'principle of inertia', or simply, 'inertia'.

Inertia, here, is just a name that is gived to a situation. It is not at all a explanation of this situation. --Aïki 19:15, 13 January 2006 (UTC)

Some equations which use Inertia, I, submitted for LightCurrent,

L = I * omega , and KE = .5*I*omega^2

where L is angular momentum, I is inertia, and omega is angular velocity, KE kinetic energy.Kenny56 08:37, 14 January 2006 (UTC)

That's moment of inertia; different thing. Pfalstad 09:32, 14 January 2006 (UTC)

Principle of inertia: quotations

1) 'Galileo's principle of inertia .... Newton adopted this principle of Galileo's for the 'First Law' of motion as presented in The Principia: 'Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.' .' Source: Physics - A new introduction course, Parts I & II - Particles and Newtonian Mechanics, by A. P. French and A. M. Hudson, Massachussets Institute of Technology, Science Teaching Center. --Aïki 02:07, 14 January 2006 (UTC)

2) 'LAW I. Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.' Newton. [4]
--Aïki 02:41, 14 January 2006 (UTC)

Usage of the word 'inertia'

The word 'inertia' is often used instead of the expression 'principle of inertia': it is then a shorthand.
It is also used in the expression 'inertia of a body', which expression is used instead of mass.
These two usage depend on the context.
Also, when someone says that 'this object has more inertia than this other object', he then means that one has more mass than the other.
It is always the context which determines its meaning. But I agree that it could be much confusing for someone who is not use to all those contexts. --Aïki 04:02, 14 January 2006 (UTC)

I totally agree with the above, but trying to get User:Foogod to agree is something eles altogether!--Light current 04:38, 14 January 2006 (UTC)

I actually agree completely with Aïki, but I suspect you don't actually realize what he/she(?) just said. Aïki really just restated what I had written when I edited the article summary a while back:

In common usage, the term "inertia" is sometimes also used to refer to an object's momentum, or to describe its "amount of resistance to change" (which is technically the same as its mass). It is important to understand that these uses of the term are not the same as Newton's more fundamental description of "inertia" as a principle of the way the universe works (which is not a measurable thing).

In other words, some people sometimes use the term "inertia" to refer to other things than the principle of inertia. However, the principle of inertia is still an important and valid principle, and is the primary meaning of the term. In many cases, one needs to use context to determine whether somebody is referring to the principle or to something else, but I think we've pretty well established from the conversation above that we're talking about the principle of inertia here, not the (conventional but inaccurate) other uses of the term. (Aïki, please tell me if I have misinterpreted your statement) -- Foogod 10:46, 14 January 2006 (UTC)

Please do not underestimate my understanding. I know what Aiki is saying and I agree with it. Its just a pity that the article is not as clear as he is in stating the facts!--Light current 14:02, 14 January 2006 (UTC)

Not at all Foogod. It seems to me that we have to be aware of the multiple uses, good or bad, of the word 'inertia'. When we see this word use in a text, we have to determine in which sense the author is using it by looking at the context. If he using it, for example, in reference to Aristotle or somebody else in history, it is not the modern meaning for sure. But we have to know too the modern meaning to make the difference. The worst thing that could happen then, is to take an old meaning as the modern one! That could happen easily with Newton who was on the border line, if I can say, between the old ideas on physics and the new ones, beeing himself one of the founders of the 'new physics' with Galileo, Kepler and some others. So, we have to be very careful with this word 'inertia' and its multiple usages, particulary in vulgarisation and amateur's texts. The same is true with the word 'inertial'. Thus let us be 'awake'! --Aïki 14:34, 14 January 2006 (UTC)

I agree that the article does need to explain this, particularly since it can be a source of confusion, and I've got no problem with expanding on the summary later in the article to explain this aspect of things (I haven't gotten to trying to work on that part of the article yet). Feel free to give it a go if you're so inclined. (I don't think we should get into too much detail in the summary, because it is, after all, a summary. This really deserves its own section, probably.) I do think, however, that it's also very important to emphasize that these uses are vulgarizations, and not the generally accepted scientific meaning of the term. Moreover, in my opinion, assuming we've got enough of an explanation to prevent people being confused by such usages, the bulk of this article really should focus more on explaining the principle of inertia, because that's the concept which doesn't have articles elsewhere to explain it (if people want to know about mass or momentum, there are articles on mass and momentum already). -- Foogod 20:48, 14 January 2006 (UTC)

Oh, one small correction I would make to your statements above: In my experience, usually when people talk about something having "more inertia" than something else, what they really mean is momentum, not necessarily mass (for example, people sometimes say that a faster moving object has "more inertia"). That's why I mentioned momentum in the summary paragraph also.. -- Foogod 20:52, 14 January 2006 (UTC)

At last I tend to find myself agreeing (rather surprisingly) with User:Foogod and not surprisingly User:Aiki and I hope we can all come to an amicable settlement of what is best for this page. I have stated my opinions; it is now for others to voice their opinions. I will go with the majority viewpoint! --Light current 21:11, 14 January 2006 (UTC)

I wanted to mention, that I just reverted the change to split the current introduction section into introduction vs. other interpretations. As I mentioned above, I'm not at all opposed to having another section detailing the differences in usage (in fact I think it would be a good idea), but I do also think that we need to at least briefly cover the distinctions between all usages in the article summary, otherwise it's not a good representation of the whole (which the summary should be: a brief restatement of the whole rest of the article). We need to mention this both in the summary and elsewhere in the article, not just one or the other. -- Foogod 23:21, 14 January 2006 (UTC)

Inertia and "quantifiable"

Lightcurrent, please leave in the parenthetical phrase about the principle of inertia being non-quantifiable, because this is an important distinction (and contrary to your assertions, does make sense). If you don't understand what is meant by this, please ask, or better yet, go read up on the distinction between "qualitative" and "quantitative" statements. -- Foogod 23:29, 14 January 2006 (UTC)

I removed the phrase because it was not grammatically clear as to its meaning. If you would like to explain the meaning I'm sure we could put it back in. Until then --REVERT--Light current 23:32, 14 January 2006 (UTC)

I believe it was perfectly grammatically clear: Principles in physics are not quantifiable things. They are qualitative aspects of the science. This is one of the things which distinguishes the principle of inertia from quantitative things like mass and momentum (and why using the term inertia to refer to those is fundamentally not correct). -- Foogod 23:38, 14 January 2006 (UTC)

Look, if I say its grammatically not clear, I mean its grammatically not clear. Can you please make it make sense without the brackets? Thanks!--Light current 23:46, 14 January 2006 (UTC)

Fine, there. I've re-added it without parentheses. Is this more grammatically clear to you? Honestly, I don't see why the grammatical meaning of parenthetical phrases should be unclear to somebody with a decent grasp of the english language.. as far as I know they're taught in pretty much all english grammar courses. -- Foogod 23:53, 14 January 2006 (UTC)

Look if I say I cant understand something or I think its ambiguous, its not for the want of trying to understand. I could make an educated guess, but were not here to puzzle the readers and keep them guessing are we??. WP is not a puzzle book altho it sometimes seems like it! A sentence may seem perfectly clear to you, but then it would becuase you wrote it. If WP is going to be better than all the rest it will only be because we have a number of editors (hopefully) looking at each article and seeing if it makes sense to them.--Light current 23:59, 14 January 2006 (UTC)

Ok, look, I'll admit that I responded a bit testily to this thing, and for that I apologize. I do want to say that I do believe that you are a reasonably intelligent person who does appear to make an effort (most of the time) to try to understand what you're talking about, and I acknowledge that you found that construction to be confusing. My point, however, was that you make edits as if you're somehow the ultimate authority on everything, and you assume that if it's confusing to you then it must be confusing to everyone else. This is, quite frankly, arrogant.

I do not believe that that the grammatical construction I used was confusing to most english speakers. The fact that it is confusing to you does not automatically make it bad, it just makes it confusing for you. Part of the responsibility of good editing is to attempt to find out whether a problem you're having is a problem with the article, or just a problem with your reading of it. The responsible course of action here would not be to summarily edit out the text, but to ask for opinions from other editors to determine whether it needs to be changed or not.

At the very least, a responsible editor would not simply remove part of an article for grammatical reasons, but would instead attempt to determine what the original author was trying to say and then restate it in a more readable way. Simply removing it changes the content of the article (which shouldn't be done for presentational reasons if it can be helped). -- Foogod 00:26, 15 January 2006 (UTC)

Obviously I cant agree here. If anything, the action of simplifying something so that I can understand it should imply not arrogance but ignorance/low intellegence. I dont admit to those either, but your argument here is not logical. I can also admit that when reading WP pages I make an effort all of the time to check the facts and search out ambiguity. (Hence our discussion on inertia). Im not particularly interested in inertia. It just so happens that when I came across the page, the inadequacy of the explanation stuck out like a sore thumb. You must also realise that sometimes it is necessary to provoke other editors into action by somewhat radical means in order to get down to the nitty gritty of the subject and create great articles that everyone can understand. Im not the ultimate authority on everything (or anything); but conducting a robust argument (whether I beleive in it or not) with others is one way or the other to flush out the real truth behind a subject. This technique has worked fairly well over the short time I have been editing science and engineering articles. I have not made many friends doing this, but hey-- Im here to help write a pedia not make friends! Hope this explains my attitude.--Light current 00:57, 15 January 2006 (UTC)

I would also like to point out that I personally believe it to not have been unclear for most english speakers, but I also recognize that I may be wrong. If other editors also believe that it should be changed, then I'm perfectly willing to work with others to try to make it more readable. (My objection is to the implication that you're somehow more of an authority on what's good grammar than I am, just because you say so.) -- Foogod 00:32, 15 January 2006 (UTC)

I go by what I was taught at school regarding grammar and I did point out the problem with this section a few times without response!. No offence intended to those who wrote the material.--Light current 00:57, 15 January 2006 (UTC)

I do note that you still haven't ever bothered to explain why you find that construction to be confusing. What's confusing about having the phrase in parentheses? You really can't expect people to fix things if you don't ever bother to tell them what's wrong with them.

As for your argument above (again, could we please keep discussions linear (add text at the bottom)? Otherwise it's easy to miss bits and it becomes really difficult for people reading later to figure out who said what in what order): My argument is quite logical (actually, I made more than one, so I'm not sure which one you're referring to). First argument: You believe that your version is better than my version. I (clearly) do not. In response to this, instead of trying to get other people's opinions to decide who is really right, or looking for citable references to determine which is more correct, you just decided to keep removing my edits on the presumption that you must be right and I must be wrong. That is arrogant. Second argument: You decided that the grammar was inadequate. In response to that you removed the whole thing. Removing the whole thing alters the content of the article, not just the grammar. Bad grammar does not imply bad content, therefore removing content because of grammar problems is a bad practice. A better approach would have been to attempt to reword, not remove. (if you were unable to understand it well enough to attempt to reword it, then you should have asked somebody to explain what was intended, or asked somebody else to attempt to reword it, neither of which have I ever seen you do here).

As for not making a lot of friends, well, that's up to you, but making edits without any consideration for or attempt to work with other editors is also a good way to get a lot of your stuff summarily reverted by other people (and indeed, I think most of your edits to this page have been reverted (mostly not by me, I would like to point out)), which ultimately is not productive and doesn't help Wikipedia in any way, so your argument that this is a good way to make changes is dubious, in my opinion, and you might want to reconsider exactly how much of your effort is actually resulting in anything productive. -- Foogod 01:33, 15 January 2006 (UTC)

OK. Ill spell it out for you. In this sentence,:

It is important to understand that these (erroneous) uses of the term are not the same as Newton's more fundamental description of "inertia" as a principle of the way the universe works, which is not a quantifiable thing.

It is not clear what the phrase 'which is not a quantifiable thing' refers to. Does it refer to:

• description
• inertia
• principle
• the way the universe works

And please dont say its obvious. I would not be asking if it was.--Light current 01:42, 15 January 2006 (UTC)

See, if you'd actually said you were having a problem with unclear antecedents, then it would have been extremely easy for me to modify things to make this clearer for you, instead of going through this revert battle and long argument on the subject, and we could have gotten on to doing something more productive with our lives...

As it is, frankly, considering that the whole point of the statement is that all of the potential antecedents you listed are supposed to be roughly equivalent to one another, it really doesn't seem to me that it changes the meaning that significantly regardless of which one you apply it to. In any case, using the standard rules of nearest antecedent, the logical conclusion is that the phrase applies either to "a principle of the way the universe works", or "the way the universe works". In both cases it's correct, and in both cases it says the same thing about inertia.

Anyway, it looks like we've moved past this issue, so let's just drop it and move on with more useful discussions. -- Foogod 22:02, 15 January 2006 (UTC)

Eggs and omlettes

AS I said before, you gotta break a few eggs if you like to eat omlettes!--Light current 01:44, 15 January 2006 (UTC)

Sequential posting

I happen to be in favor of sequential posting but have fallen into the trap of placing interrupting comments in others posts to deal with a specific point there and then. Many other users do this as well (maybe cos its easier).

I agree with you that, especially if you have more than 2 respondents in a discussion, it makes the whole thing very difficult to follow. When I first came to WP, I attempted to reorder others' posts to make the flow chronological but was quickly and severly chastised for doing so. However, I have been thinking of proposing this sequential method as a WP wide recommendation. Now that I have found someone of like mind, I may do it. Would you be prepared to support me on this proposal--Light current 01:53, 15 January 2006 (UTC)?

I'm not sure whether that's really warranted, particularly since we're talking about discussion pages here, and most WP policies/recommendations/etc are intended to be about article content, not talk pages. In my opinion it depends a bit on the type of discussion going on too. For the sorts of things we were doing where there were large discussions with lots of text, sequential posting is a good idea, IMHO, because otherwise things get lost and arguments get fragmented. In other discussions, however, where people may just be leaving one or two sentence responses to individual issues, interleaving comments might be appropriate (I've seen it work in some places just fine) and might actually make it easier to tell who's responding to what. If you're so inclined, however, feel free to propose it, and depending on how the proposal is worded and whether it fits with the rest of WP's guidelines (which I haven't really read up on in this department), I might be willing to support it.. I don't know whether you'd be able to get enough other WP folks to support it for it to actually be successful, though, just for the reasons I mentioned here..

Personally, if we're discussing talk-page style, I would much prefer a guideline that the name/date of the responder should be at the beginning of the response instead of tacked on the end the way everyone does now, but that seems to be something that a lot of other Wikipedians are against, so I go with convention.. -- Foogod 22:51, 15 January 2006 (UTC)

Already three ways

I'm not sure of what you mean exactly by this 'sequential posting', but actually there is already three ways for a better organisation of the talk page: 1- the 'indent way'; 2- making sections (we can make links to it) [5] ; 3- making sub-sections (like the one above) [6] and sub-sub-sections, and even more [7]. 2 and 3 make a research easier when, ulteriorly, we want to find a specific edit or an information inside it. --Aïki 00:55, 16 January 2006 (UTC)

Interesting observation

I find it interesting that the following paragraph exists in the article:

Rotational inertia

Another form of inertia is rotational inertia, which refers to the fact that a rotating body maintains its state of uniform rotational motion. Its angular momentum is unchanged, unless an external torque is applied; this is called conservation of angular momentum.

my bolding

This seems to say about rotating systems exactly what I was saying about linear systems. So is the statement correct, or am I correct? Or am I not understanding things again?--Light current 04:01, 15 January 2006 (UTC)

What are you saying about linear systems? Are you saying that you can describe the motion of a body using conservation of linear momentum instead of inertia? Sure you can. Are you saying that inertia is more properly (or more fundamentally) called "conservation of linear momentum"? That's not true historically, since Galileo first described it using "inertia", before momentum was ever defined. Also, "inertia" is a shorter simpler and more familiar term, that is typically applied to single bodies, whereas conservation of momentum is usually applied to interactions. And also the standard physics terms "inertial mass" and "momentum of inertia" refer to inertia, not momentum. Pfalstad 17:09, 15 January 2006 (UTC)

As for the section you quoted, I'm not sure that "rotational inertia" is a proper scientific term. I don't know if there's a "law of rotational inertia" that is commonly used. A google search reveals that "rotational inertia" is sometimes used as a synonym for moment of inertia. Also the statement about angular momentum seems misplaced, since angular momentum can be conserved even if the speed of rotation changes (e.g. the figure skater example). That's what conservation of angular momentum often refers to, in fact. So this whole section may have problems. Pfalstad 18:31, 15 January 2006 (UTC)

I was saying that you can describe the motion of a body using conservation of linear momentum instead of inertia. I am saying that the principle of inertia is really the same thing as "conservation of linear momentum" as it turns out, and that the similarity could be pointed up in the article. Im not seeking to redefine the old explanation, but it does no harm to point out its problems. (as I think you have now done in the lead para.)

They're not the same thing; conservation of linear momentum implies the principle of inertia (if you assume mass doesn't change), but not the other way around. A mention of conservation of momentum could be made somewhere, if it would help shed some light on the subject, but not in the lead para, and not stating that it is equivalent. Pfalstad 20:05, 15 January 2006 (UTC)

Strictly speaking you are correct if you are talking relativistically, but I thought we were talking non relativistically about a single bodies.--Light current 20:10, 15 January 2006 (UTC)

A law that applies only for single bodies is not the same as a law that applies to multiple bodies. Yes, the principle of inertia is the same as a weakened version of the conservation of linear momentum that is restricted to single bodies of constant mass. But saying that seems confusing and unenlightening, and physicists generally don't talk that way. It is more typical and makes more sense to say that one law implies another as I said above.

I also agree that the section on rotation inertia is sloppy, and needs tightening.(deleting- with ref to angular momentum or something similar?) --Light current 18:45, 15 January 2006 (UTC)

Anyway, so what youre saying is that the para on rotational inertis is not correct and needs modification??--Light current 20:31, 15 January 2006 (UTC)

That paragraph does need to be changed, yes. The main problem with it is that it doesn't make it clear that what is often called "rotational inertia" is not inertia. Inertia does not have anything to do with things which are rotating (as you can see by reading NFLM, inertia only says that things will keep moving in a straight line absent of forces. In order to rotate, there must be forces, therefore inertia does not actually apply to that situation). People sometimes use the term rotational inertia to refer to conservation of angular momentum (the same way that people sometimes use inertia to refer to momentum or mass), but this does not actually imply anything about the principle of inertia, because it's completely unrelated. -- Foogod 22:09, 15 January 2006 (UTC)

Oh, and I just wanted to point out that the principle of inertia is not actually the same as "a weakened version of conservation of linear momentum", for the reasons I've already laid out in the earlier discussion above (specifically, the two cannot be the same because conservation of momentum includes mass and inertia doesn't, and thus they say different things in terms of what one is allowed to do with mass and velocity). If anything, inertia is a stronger statement than conservation of momentum (because it isn't all wishy-washy about the relationship between mass and velocity). If you really need a simplification of inertia to these terms, I suggest you think of inertia more as "conservation of velocity", rather than "conservation of momentum". Yes, in most cases, they result in similar results (which is good, because otherwise we'd have some real problems with the universe), but that doesn't mean they're always the same. -- Foogod 22:16, 15 January 2006 (UTC)

Lead para

THanks Paul. I think the lead para is now a great deal clearer regarding inertia being a concept or principal rather than a real quantitty. Of course, if the page had originally been called Principle of inertia (physics), than I think many of the problems regarding its definition/description could have been avoided.--Light current 18:36, 15 January 2006 (UTC)

I'm not quite as happy with the text as I might be from a readability standpoint, but for the most part that's minor stuff, and as the actual content seems reasonably correct, I'm willing to live with that if it makes other people happy. I don't think the page title should really have mattered that much.. I can, however, understand the confusion resulting from the text which was previously here (some of which still is) which was itself rather confused. (As far as naming of articles, I'm personally of the opinion that an article should be named according to the simplest, most common term used to refer to the concept, unless this conflicts with other articles. As most people simply say "inertia" instead of "the principle of inertia", and this meaning is the primary definition of "inertia" in all of the dictionaries I can find, I think the title of this article is reasonable. The real issue is what the text of the article says. (on a side note, things like "(physics)" are only added to the end of articles if there's more than one article in existence with the same name which need to be distinguished)) -- Foogod 22:35, 15 January 2006 (UTC)

The confusion seeming to disappear progressively, I think we can continue with the same title. We can always change it later, if we see it could be better to do so. --Aïki 02:00, 16 January 2006 (UTC)

I still think we should now move the article to my newly created page called Principle of inertia (physics) and make a clean start. --Light current 02:17, 16 January 2006 (UTC)

I did not known you had created it. If we maintain it, it seems to me that this new article will have to be only on the principle of inertia, and not on the others meanings of the word inertia which should be left to the article 'Inertia'. Probably, too, we will have to change this last one in consequence. I hope all this will be for the better. --Aïki 02:48, 16 January 2006 (UTC)

I agree it should be on the principle of inetia. The other meanings can be left on the current page! --Light current 02:50, 16 January 2006 (UTC)

No, don't just create a copy of this page. Move it properly, so the history is retained! Pfalstad 03:52, 16 January 2006 (UTC)

I dont know how to do it properly- any way it will require agreement from all parties?--Light current 17:10, 16 January 2006 (UTC)

Well agreement would be nice, but you can use the "move" link at the top of the page to move it. I'm not sure of the details since I have never done it. I think a move is probably a good idea, considering all the disambiguation stuff at the top, plus the confusion about what inertia means in physics vs. common usage. Pfalstad 21:45, 17 January 2006 (UTC)

Since you've already created Principle of inertia (physics), you'll need to get an admin to help you move the page, since what's there now will have to be deleted first, to make way for this page moving to that title. I'd be more than happy to help out in that capacity, but why don't we get at least a rough consensus first? Let me know, or if another admin wants to go for it, there you go. -GTBacchus^(talk) 22:13, 17 January 2006 (UTC)

Support, I vote yes to moving material to Principle of inertia (physics)--Light current 23:25, 17 January 2006 (UTC)

For me, moving the article is equivalent to change the name of the page and only that. The text being the same, it will change almost nothing. The confusion will not diminish only because the article has changed name. It is of no use. Instead we just have to split the present article in two parts, one for the principle of inertia, and one for the other meanings. These two sections representing two articles in one. Or we make two articles with each its name, the new one having to be create entirely, and the present having to change its text in consequence, which I don't see to be a good thing in the present state of affairs. So, I'll go for to split the present article in two major sections well definite, with an introduction stating that fact. --Aïki 02:01, 18 January 2006 (UTC)

Oppose. Aïki raised some good points. Furthermore, it seems unlikely that we will be writing separate articles on the various meanings of the word "inertia." I suggest instead that if we really think the disambiguation text is too long, then it should be moved to Inertia (disambiguation), with an {{template:otheruses}} template replacing it. But Inertia is the right place for the "Principle of Inertia" to be described, complexities notwithstanding. --Iustinus 07:26, 18 January 2006 (UTC)

Iustinus, I don't know how those things work. If a reader search for the word inertia in wikipedia, what will he find first? If another one search for the expression principle of inertia, what will he find first? --Aïki 15:49, 18 January 2006 (UTC)

Under my plan it would go like this: Inertia would be the page it is now. At the top of the page would be a text that woudl say something like This page is about the Principle of Inertia in physics. For other uses of the word, see Inertia (disambiguation). At Inertia (disambiguation) there would be a text very similar to what is at the top of the page now. Principle of Inertia would reditect to Inertia.--Iustinus 17:57, 18 January 2006 (UTC)

Oppose. I'm not going to say that moving/renaming/splitting in some form shouldn't happen eventually, because I will freely admit that I can't tell for sure. What I do believe, however, is that all of this is premature at this point and we shouldn't be doing anything like this yet. I don't think anyone here would disagree that this article is currently in heavy flux. I think what we should be focusing on trying to finish cleaning up the current article, and get it stabilized, comprehensive and reasonably accurate about all (physics-related) definitions of the term inertia. Once that's done, then we should take a look at what we have, and evaluate whether there's enough distinct material to split things into different pages, whether the material we have would make more sense under a different name, etc. Not now. Fix the current article now, then decide the relatively minor aspects of organization and page naming after. -- Foogod 00:34, 19 January 2006 (UTC)

I might be inclined to support something along the lines of what Iustinus proposed, but I'm not really sure whether even that's warranted at this point. It should also be emphasized that this page is not currently exclusively about the principle of inertia, but also explains several related topics such as inertial reference frames and inertial mass (which have their own pages, but it is useful to present the basic concepts in the context of the whole here as well). Some of these sections probably need cleanup, but I think they are potentially good aspects of the article; however, they wouldn't fit if we defined the article to be only about the principle. Personally, I think it could be quite valuable to have an article which gives the reader a comprehensive overview of the wide range of topics which the term "inertia" can actually cover, which is part of the reason I'm opposed to trying to split or narrow things down unless we have to, because it inherently destroys any ability we would have to do this useful thing. -- Foogod 00:34, 19 January 2006 (UTC)

Oh, and I'd also like to reiterate my objection to any page name with "(physics)" on the end of it which doesn't need it. Wikipedia convention is that this sort of parenthetical classifier is only used when there's already another article with the same name which needs to be distinguished. There is currently no other "Principle of inertia" in Wikipedia, therefore the "(physics)" part is unwarranted and should not be used, regardless of any other discussions about whether to move or not. -- Foogod 00:34, 19 January 2006 (UTC)

Oppose. In my opinion, up to now, the best plan is the one propose by Foogod. I think that it is better that all the discussions we had in the talk page be keep with one article, this one here, instead of being splitted between many pages, at least till everything becomes clear for everybody, or a consensus be made on the content of the article.--Aïki 01:54, 19 January 2006 (UTC)

Early understanding of motion the same as Newton's ?

The current 'Early understanding of motion' section starts with the following claim, into which I have inserted comments in square brackets and capitals to indicate its fundamental mistake in claiming a difference between Aristotle's and Newton's physics in this respect:

"Prior to the Renaissance in the 15th century, the generally accepted theory of motion in western philosophy was that proposed by Aristotle (around 335 BC to 322 BC), which stated that in the absence of a force, all objects (on earth) [WHICH THEREFORE HAVE GRAVITY BY VIRTUE OF BEING ON EARTH] would naturally come to rest in a state of no movement, and that moving objects [ON EARTH] only continue to move so long as there is a force inducing them to do so[1] "

This is true, BUT the same is also true of Newton's physics and also of 'classical' physics, in which such as a missile on earth would also require a countervailing force as powerful as its gravity to sustain a vertically upward uniform motion in the absence of air resistance. This is hardly rocket science, more O-level physics! Thus Newton's and Aristotle's dynamics are essentially the same on this issue.

Re this passage, in the first instance Aristotle did not die aged 13, as it implies. And secondly, its footnoted reference is to Aristotle's 'Physics', but no particular passage from this text is cited, suggesting its essential point may be UNVERIFIABLE. Indeed, it is clear from the classically cited passage 'Physics' 4.8.215a14-18 that the object/missile in question is a missile ON EARTH, and that it therefore has GRAVITY because it has a natural place. So the reason why it would not move in a vacuum is because there would be no force to overcome its gravitational resistance to motion. All continuing motion against resistance requires a force both in Aristotle's dynamics and also in Newton's 17th century development of it.

Can any Wikipedian verify that any passage of Aristotle's 'Physics' says that the motion of terrestrial bodies would require a force IF THEY DID NOT HAVE GRAVITY that resists motion nor any other resistance to motion, whereby it differs from Newton's dynamics ? In such GRAVITY-FREE circumstances, Aristotle's principle, like Newton's, is that 'Either a body will remain at rest or continue moving indefinitely, unless externally impeded.' ('Physics' 4.8.215a19-22). Newton himself correctly cited this principle as essentially an early statement of his Principia's first law of motion. [See my original Newton quotation in 'The historical misunderstanding of Inertia', now archived to the Talk Archive weblinked at the beginning of this page.] The relative novelty of Newton's dynamics re Aristotle's was that, unlike Aristotle, he attributed the cause of this behaviour to the inherent FORCE of inertia.

82.35.65.192 03:08, 16 January 2006 (UTC)A.Bellamy 16 Jan '06

I had a nice long, point-by-point response to this all typed up with lots of nice quotes and citations and everything, and then my browser burped and I lost it all, and I don't have time to re-write it now, so please forgive my bad mood and the shortness of this reply.

First, the text above does not say Aristotle died at 13. It says that's the period in which he proposed his theory.

Second, regarding the citations, I just added those recently and I figured that having something was better than the nothing that was there previously. I'm still working on improving several of them, including that one. Feel free to do it yourself if you think I'm moving too slowly. Otherwise shut up and go away.

Third, regarding the assertion that Aristotelian and Newtonian ideas of motion were the same thing, this is blatantly wrong in too many ways for me to get into now. To start with, your reasoning is completely flawed because you're trying to interpret what Aristotle said using Newton's ideas of how the universe works, many of which nobody even conceived of as possibilities at the time Aristotle was writing. Aristotle didn't believe there was anyplace where gravity didn't exist, so (a) asking for a quote saying how he believed earthly motion behaved in such a situation is stupid, because he didn't even conceive of it as a possibility, so of course he never said anything on the subject and (b) trying to infer what Aristotle might have thought on the subject is meaningless, and amounts to trying to stick Newtonian words into Aristotle's mouth. Aristotle said quite clearly that the natural state of matter was at rest. He did not say "moving until gravity stops it" or anything of the kind. Anyway, even Aristotle didn't think gravity impeded horizontal motion, so that's obviously not an adequate explanation for anything. For a good example of what Aristotle did think, go look at his explanation of why an arrow keeps flying after it's left the bow (he maintained it was because it was pushed by the air around it (note, not resisted, but pushed forward). It is clear from this that Aristotle believed that the arrow needed a continual application of horizontal force throughout its entire flight to keep it moving, or it would simply stop. This is not in any way a Newtonian idea of motion.)

Fourth, as far as your quote attributed to "'Physics' 4.8.215a19-22", I have just read through all of chapter 8 of book 4 of Physics, and I can't find anything resembling that statement anywhere. In fact, the entirety of that chapter appears to be devoted to refuting the existence of the void (including an argument for why movement is impossible in a vacuum. Are you claiming this is consistent with Newton also?).

Aristotle's concept of motion was a fundamental leap forward which influenced everyone who came after him, but claiming that Newton was just restating Aristotle is wrong. Aristotle's ideas had to undergo centuries of evolution by a large number of people (including Newton, but also including many others) to come to the modern idea of how motion works. Nobody claimed Newton invented it all by himself (in fact I think the history section shows that pretty clearly), but claiming that it's the same as Aristotle ignores the very significant contributions of Philoponus, Buridan, Galileo, Newton, and lots of others, and shows a complete lack of understanding of the difference in worldview which has happened over the past 2 millenia or so. -- Foogod 03:03, 19 January 2006 (UTC)

It would be nice if we could get our hands on the Newton quote given by Bellamy at Talk:Inertia/archive#The_historical_misunderstanding_of_inertia, which seems to imply that Newton read Aristotle that way. I should probably check the TLG text of Aristotle as well: it is frequently the case that different editions will divide the text differently. I would really like to know if and/or to what degree Mr. Bellamy's characterization of Aristotle is correct. --Iustinus 07:27, 19 January 2006 (UTC)

Thanks Iustinus, but surely you have already got the Newton quote in your hands ! I gave you it and the reference in Hall & Hall [1962]. Just look at the 'Fragment...' paper listed in the Contents. My analyses suggest Newton interpreted the Physics passage correctly, but the Heavens passage incorrectly, although the latter does confirm Aristotle believed bodies would have no inherent resistance to motion without their gravity, and thus nothing to terminate their otherwise unresisted motion in a void without gravity, as attested in the Physics passage. I give you the Newton quote yet again:

“All those ancients knew the first law [of motion] who attributed to atoms in an infinite vacuum a motion which was rectilinear, extremely swift and perpetual because of the lack of resistance...ARISTOTLE was of the same mind, since he expresses his opinion thus [in On The Heavens, 3.2.301b]: 'If a body, destitute of gravity and levity, be moved, it is necessary that it be moved by an external force. And when it is once moved by a force, it will conserve its motion indefinitely'. And again in [Physics 4.8.215a19] speaking of motion in the void where there is no impediment he writes: 'Why a body once moved should come to rest anywhere no one can say. For why should it rest here rather than there ? Hence EITHER it will not be moved, OR it must be moved indefinitely, UNLESS something stronger impedes it [My caps to illustrate it has the same logical structure as Newton’s first law].' " [From one of Newton's Scientific Papers in The Portsmouth Collection, first published in Hall & Hall's 1962 Unpublished Scientific Papers of Isaac Newton]

Iustinus, why do you put the burden of proof on the thesis that Aristotle affirmed ‘the principle of inertia’, rather than on Wikipedia to verify he denied it, for which no satisfactory evidence has been produced to date.

See 'Original Languages' for my further efforts to help you. --80.6.94.131 18:54, 19 January 2006 (UTC)A.Bellamy

Mr. Bellamy, I wouldn't say I'm putting the burden of proof on either thesis, I just want it proved one way or another! I do not know Aristotle's physics very well, and am increasingly coming to realize that if I want to know the truth of this matter, I will have to read it myself.

If I sounded like I was placing the burden expressly on you, it is probably because of Foogod's difficulty in finding the passages you cited. I have yet to try to track those down myself.

On a similar note, I did indeed notice your comment at Talk:Inertia/archive#Original Languages. Thanks for the help. It's really frustrating that that ended up on the archive page, but probably not worth it to move it to the current page unless I have something specific to add to it. And at this rate the current discussion is bound to be archived again soon anyway!

By the way, I reiterate my request that you get a Wikipedia account. While it is by no means necessary, it would facilitate our dealings with each other. --Iustinus 20:06, 19 January 2006 (UTC)

If you get a Wikipedia account, you can use the name you used till now (A.Bellamy). It would be easier to follow your numerous edits. A name is easier to retain than a number. Too, creating subsections, make it easier to follow your thinking, as well for you than for the other contributors. You can use too /br between < and >
for a return at the beginning of the next line (like I just did). --Aïki 01:26, 23 January 2006 (UTC)

Mr Iustinus, Foogood and you should have no difficulty in finding the Aristotle passage given I have provided the exact line by line location as Physics 4.8.215a19-22, especially if you are using a line numbered edition such as the 1929 Loeb Wicksteed & Cornford, or Ross, or Hussey's 1993 Bks 1-IV translation, for example. But if you have never grappled with the problem of arriving at a coherent logically joined up analysis of all Aristotle's crucial passages in Physics and Heavens, you will have much homework to do.

My comment on Original Languages was intended to refer to further suggestions I was about to add, but then just as I finished them the computer wiped themn all out, so they didn't make it. Here they are now, as follows. Meanwhile, will take a look at this Wikipedia account business when time. --80.6.94.131 17:16, 20 January 2006 (UTC)A.Bellamy

Mr. Bellamy (or do I take your last comment to mean that you prefer just "Bellamy"?), thank you very much for your tireless work. I myself have been to swamped with other things to track what is going on in this subject, but when I have more time your suggestions under "ORIGINAL LANGUAGES" will be invaluable. I also look forward to looking up these Aristotle passages myself, but again I can't take care of this right now. I just wanted to make sure to thank you promptly, as you have been very helpful.

As for getting a Wikipedia account, that can be done quickly. If you are interested, the relevant page is here. I hope you decide to sign up. --Iustinus 19:35, 22 January 2006 (UTC)

Foogood versus Newton on Aristotle, the law of inertia and gravity: A.Bellamy Replies

[NB This section should be read in conjunction with Part 5 The historical misunderstanding of inertia in Talk Archive 1 @ Talk:Inertia/archive#1 ]

In a logically invalid attempt to refute Newton's anti-modernity thesis that Aristotle espoused his first law of motion, Foogood claims "Aristotle said quite clearly that the natural state of matter was at rest.", as though this somehow logically contradicts Newton's 'law of inertia'. But it does not, simply because Newton's law is not about "the NATURAL state of matter" nor about “the NORMAL motion of matter” as Foogood and Wikipedia claim, but rather about the NON-NATURAL and ABNORMAL state of bodies when unperturbed by natural forces such as gravity. Newton apparently regarded this state of affairs as so UNNATURAL and ABNORMAL that it never exists in nature, in which all bodies are perturbed by the multiple gravitational forces exerted by massive celestial bodies that accelerate them and cause such as the planets to move in curvilinear motions. Thus Foogood's claim about Aristotle is logically irrelevant to refuting Newton's thesis that he espoused his first law of motion.

Moreover, unlike Newton, Foogood mistakenly overlooks the generally accepted convention since scholastic physics that Aristotle's theory of the nature of sublunar bodies was his theory of gravity (or levity), whereby 'nature' = 'gravity'. For example, in this same Aristotelian usage Galileo's theory of "NATURALLY accelerated motion" in his Discorsi meant a theory of ‘gravitationally accelerated motion’ (i.e. gravitational free-fall). And thus Foogood fails to understand the crucial logical point of Newton's observation that the NON-NATURAL state of bodies in Aristotle's dynamics, that is, of bodies WITHOUT their nature/gravity, is either rest or interminable motion if unperturbed by any external resistance. For to make a logically valid comparison between Aristotle's dynamics and Newton's first law which says how bodies would behave if unperturbed by forces such as gravity, unlike modern commentators Newton realised one must compare what the first law says just with those passages where Aristotle also considers how bodies would behave WITHOUT THEIR GRAVITY/NATURE, and not with those passages in which bodies have their gravity/nature, as in the traditional logically invalid comparison that Foogood makes of how bodies WITH natures/gravity behave. And Newton identifies these logically relevant passages as Physics 4.8.215a19-22 and On The Heavens 3.2.301b.

According to Aristotle's theory of the nature/gravity of sublunar heavy bodies, it is an internal principle of both motion and of rest, that is, it causes natural motion directly towards their natural place at the centre of the world and earth when they are displaced from it and are otherwise unconstrained, and where they seek to be at rest, and it causes natural rest in their natural place or at the nearest place they can get to it, such as on a horizontal surface, where they have a tendency to rest and to resist motion in any direction other than straight downwards. So a body's nature/gravity is an internal principle that causes both motion and also resistance to motion. For on the one hand it causes natural motion, and on the other hand it resists its contrary, namely 'violent' motion, that is, any motion not directly to the centre of the earth, which includes horizontal motion and even downward motion just one degree or less away from the vertical. In scholastic physics this virtually omnidirectional gravitational resistance to motion came to be resolved into two different vertical and horizontal component tendencies, technically called 'the tendency to a contrary motion' and 'the tendency towards rest'. The former was a gravitational tendency to move straight downward that resists any upward motion and the latter was a gravitational tendency to be at rest on the horizontal that resists any horizontal motion wherever it happens to be on the horizontal and whether at rest or in motion along the horizontal. As Oresme expressed this Aristotelian theory of gravity in his 14th century De Caelo et Mundo:

"For the reason why such things as men or animals experience work or effort in moving themselves or other heavy things is that their WEIGHT inclines them towards rest or to be moved with some other contrary motion"

But without their inherent gravity, bodies would have no internal resistance to motion in Aristotle's dynamics, neither the tendency to rest nor the tendency to a contrary motion.

A traditional logical error made by commentators is to presume that in addition to his theory of gravitational/natural motion and resistance to virtually all except downward motion by the inherent nature/gravity of bodies, Aristotle also posited a second internal inherent property of bodies that resists all motion whatever, namely by its tendency towards rest, and which they call 'inertia'. Thus on this analysis Aristotle posited two inherent properties of matter that resist motion, namely the virtually omnidirectional resistance of gravity and the wholly omnidirectional resistance of inertia. But this is apparently a blunder of illogical analysis that fails to see that the inherent tendency to rest in Aristotle's dynamics is solely due to a component of gravitational resistance to motion rather than to some additional non-gravitational property of 'inertia', and that Aristotle's dynamics does not posit any inherent tendency to resist all motion whatever. For if it did, it would not have predicted that gravitational fall in a vacuum (i.e. 'free-fall') would be infinitely fast as in Physics 4.8.215a.25f, because then this would be prevented by the internal resistance of any such omnidirectional 'inertia', just as it is in Kepler's and Newton's dynamics. Those like Foogood who apparently believe Aristotle believed in both gravity and also in some additional nature of bodies to be at rest fail to see the latter is just part of the former whereby it must be discounted when comparing the predictions of Aristotle's dynamics with Newton's first law of the non-gravitational/non-natural behaviour of bodies.

Butterfield's Blunder

One of the most striking examples of this illogical blunder in accounts of Aristotle's dynamics of mistakenly attributing a second inherent resistance to motion in bodies called 'inertia' in addition to that of gravity is to be found in the account of Aristotle's theory of motion given by the Cambridge historian Herbert Butterfield in his by now well outdated 1949 The Origins of Modern Science, cited in the Wikipedia list of book references on inertia. In the following extract Butterfield first gives an account of Aristotle's theory of nature/gravity according to which nature/gravity opposes 'violent' motion, which Butterfield commendably correctly describes as 'motion in ANY other direction than straight to the centre of the earth', and which must logically therefore include horizontal motion, contra Foogood ("even Aristotle didn't think gravity impeded horizontal motion"). He then also correctly tells us that such violent motion requires a mover, but crucially fails to note this could logically be because such violent motion is resisted by the nature/gravity of the body which it contradicts, as he has himself just indicated. And then suddenly out of the blue Butterfield illogically conjures up "the Aristotelian doctrine of inertia" as a "doctrine of rest" that supposedly explains why violent motion needs a mover, namely to overcome resistance from some non-gravitational "inertial" tendency to rest, but which resistance is in fact entirely Butterfield's illogical concoction. For it is just the gravitational tendency to rest and resist motion contrary to gravity that Butterfield has already depicted, albeit omitting mention of its tendency to rest aspect, rather than some additional second force of inertia inherent in bodies. This traditional error was further promoted by Annaliese Maier's 1950s attempts to prove that the 'tendency to rest' in scholastic physics was due to inertia rather than to gravity, but whose documentation in the Oresme extract she quoted, as shown above, proved the very opposite to the logically astute reader.

BUTTERFIELD: "On the Aristotelian theory [of motion] all heavy terrestrial bodies have a natural tendency towards the centre of the universe, which...was at or near the centre of the earth; but motion IN ANY OTHER DIRECTION was violent motion, because it contradicted the ordinary tendency of a body to move to what was regarded as its natural place [and to be at rest there]. Such motion depended upon the operation of a mover, and the Aristotelian DOCTRINE OF INERTIA was a doctrine of rest - it was motion, not rest that always required to be explained." [p.3 Butterfield 1957 edition. My insertions in square brackets]

Thus like Foogood and Wikipedia, Butterfield failed to see that the main reason why sublunar violent motion required a mover in Aristotle's dynamics was to overcome gravitational resistance to motion, just as it does in modern physics, thus promoting the myth of a fundamental discontinuity and post-medieval modernity in physics.

Foogood also mistakenly claims "Aristotle didn't believe there was anyplace where gravity didn't exist, so (a) asking for a quote saying how he believed earthly motion behaved in such a situation is stupid, because he didn't even conceive of it as a possibility, so of course he never said anything on the subject". But of course in fact Aristotle believed gravity did not exist in most of the universe that is filled by quintessential celestial matter that has neither gravity nor levity. The domain of gravity is entirely restricted to the relatively small sublunar terrestrial region. And he also denied gravity would exist in a pure void without natural places, because by definition nature/gravity presupposes there are natural places that bodies gravitate towards. Unlike Foogood, Isaac Newton correctly believed Aristotle crucially devoted two passages to saying how earthly bodies without gravity would behave, namely Physics 4.8.215a19-22 and On the Heavens 3.2.301b.

Foogood also wrongly claims "Anyway, even Aristotle didn't think gravity impeded horizontal motion, so that's obviously not an adequate explanation for anything." For it is at least logically deducible from the conjunction of Physics 7.5 and Heavens 3.2.301b, that the resistance to horizontal violent motion measured by weight in Physics 7.5, as in its traditional algebraic representation as v α F/W, must be purely gravitational, since by Heavens 3.2.301b weightless bodies have no such resistance to violent (i.e. externally enforced) motion, and so would be moved with infinite speed, unlike the violent motion of heavy bodies in Physics 7.5. And in the quotation already provided above, Wikipedia's academic authority Butterfield also described violent motion opposed by nature/gravity as being in ANY other direction than straight downwards, which must therefore include horizontal motion.

As for Foogood's claim that gravitational resistance to horizontal motion in Aristotle's dynamics cannot explain anything, to the contrary as already pointed out elsewhere, the compounding of both vertical and horizontal gravitational uniform decelerations in projectile motion can mathematically explain the non-parabolic three stage projectile trajectories depicted by 16th century scholastic physics such as described in paragraph 4 of the following McTutor article and depicted in Hall's 'The Science of Ballistics in the 17th century'. This arguably provides independent evidence that scholastic physics believed in horizontal weight, that is, a gravitational tendency to rest along the horizontal. This gravitational resistance to horizontal motion was subsequently rejected by such as Cusa and Galileo's theory of gravity, which replaced it by a principle of perpetual uniform circular motion of heavy bodies around the surface of a gravitationally concentric sphere (i.e. around the horizon), and a principle of indifference to rest or uniform motion of a heavy body along such a 'horizontal' surface, which thus denied the Aristotelian gravitational tendency to rest on the horizontal. Galileo attributed the well-known resistance to motion along the horizontal that Aristotle had attributed to gravity, and which Kepler newly attributed to non-gravitational 'inertia', solely to inter-surface friction, notably also proportional to weight just as Aristotle's gravitational horizontal resistance had been. Newton's physics subsequently also added the resistance of the force of inertia to that of friction, notably again also proportional to weight, and which resists a change from rest to motion and any non-uniform motion. Thus the gravitational tendency to rest and resist horizontal motion of Aristotelian and scholastic physics that explained the well-known phenomenon of resistance to motion along the horizontal was eventually abolished and replaced by the resistances of friction and of inertia. This was primarily because of the replacement of the Aristotelian 'natural place' theory of gravity by the Platonic cognate theory of gravity as a mutual attraction between like bodies, which both Galileo and Kepler subscribed to, and which does not predict any gravitational attraction along the spherical gravitational 'horizontal'.

It is perhaps interesting and ironical to note that just as Kepler's concept of inertia had originally split-off the gravitational tendency to rest component from the scholastic theory of gravitational resistance and universalised, omni-directionalised and de-gravitationalised it, so in the positivistic physics of the 19th century that, like Descartes, abhorred Aristotelian inherent forces such as Newton's 'force of inertia', Mach sought to reduce the force of inertia back into the force of gravity by attributing a body's inertial resistance to motion to the combined gravitational attractions of all the fixed stars.

But contrary to Foogood's claim that 'Aristotle explained the flight of an arrow as needing a continual application of horizontal force throughout its entire flight to keep it moving, or it would simply stop', there is no such discussion of the flight of an arrow in Aristotle's works. Nor do any of Aristotle's discussions of projectile motion specify its direction, whereby it could be straight upwards, for example. The key example of horizontal motion requiring a continual horizontal force in Aristotle's dynamics is traditionally regarded as to be found in his Physics 7.5 in its example of a gang of hauliers hauling a ship, and whose average speed is said to be inversely proportional to its weight.

Nevertheless, Foogood is right in claiming that in Aristotle's dynamics 'the horizontal flight of an arrow would require a continual application of horizontal force throughout its entire flight to keep it moving, or it would simply stop', but obviously wrong in thinking Newton or elementary 'modern' physics would disagree with this, albeit they agree for different dynamical reasons, such as attributing the horizontal resistance to motion to air resistance rather than to any horizontal gravity, but which resistance still requires a continual force acting against it to sustain motion. Impetus dynamics also agreed with Aristotle's on this point, but just identified the continual moving force required to sustain motion as an internal force of impetus impressed directly within the missile itself, rather than a motive force only impressed in the air behind it as Aristotle had suggested. Thus Foogood's attempt to portray a fundamental disagreement between Aristotle's and 'modern' physics over whether continuing sublunar motion requires a mover by misrepresenting both of them fails. For in both, continuing motion against resistance requires a mover, and continuing motion without any resistance does not.

For an online academic account of the history of the theory of gravity that, unlike Foogood's Wikipedia, at least recognises the standard convention that Aristotle's theory of the inherent 'nature' of sublunar bodies was his theory of gravity, see the History Topics article Theories of Gravitation to be found on the St Andrews University McTutor History of Maths website @ [8] which at its very beginning quotes Aristotle's theory of nature/gravity from Physics Book 2 as follows:

Aristotle put forward his ideas on why objects fall to Earth, and also on motion in general, in works written around 330 BC. Aristotle writes in Book II of Physics:-

Some existing things are natural, while others are due to other causes. Those that are natural are ... the simple bodies such as earth, fire, air and water; for we say that these things and things of this sort are natural. All these things evidently differ from those that are not naturally constituted, since each of them has within itself a principle of motion and stability in place ... A nature is a type of principle and cause of motion and stability within these things to which it primarily belongs ... A nature, then, is what we have said; and the things that have a nature are those that have this sort of principle. All things are substances, for a substance is a sort of subject, and a nature is invariably in a subject. The things that are in accordance with nature include both these and whatever belongs to them in their own right, as travelling upward belongs to fire ...

So Aristotle argues that the stone falls because it has a "nature within it" which causes its motion to its natural place which is the centre of the Earth.

--80.6.94.131 19:16, 31 January 2006 (UTC)A.Bellamy ________________________________________

ORIGINAL LANGUAGES

IUSTINUS WROTE OF THE HISTORY SECTION OF THE ARTICLE: "Now can anyone tell me where the Galilieo, and da Vinci quotes come from? There are also quotes from Kepler and Descartes: these have the source listed, but not the chapter or any other information, so that would be nice to have as well. Can anyone help? -Iustinus 21:53, 4 January 2006 (UTC)"

The Descartes principle I have already suggested is as follows:

DESCARTES: PP Pt 2, Pr 37: "The First Law of Nature: that each thing as far as in it lies, continues always in the same state; and that which is once moved always continues so to move." (Haldane & Ross translation)

But as stated this principle is blathering nonsense. For obviously, a body in a state of circular motion or moving in a figure of eight, for example, may not always continue in it if perturbed otherwise or unforced, and nor do bodies once moved always continue to move if perturbed. The Koyre-Cohen view that Descartes first formulated the principle of inertia and that Newton modelled his first law of motion on it both seem untenable just from comparing the two statements, and it appears far more likely that Newton modelled his first law on Aristotle's Physics statement: 'EITHER a body [without gravity in an infinite pure void] will not be moved OR it must be moved indefinitely, UNLESS something stronger impedes it.' [Paraphrase of Line 20 of 4.8.215a19-22] But you might enjoy Cohen's 1964 Royal Society 'Quantum in se est...' paper on the Lucretian origin of Descartes and Newton's phrase 'in so far as in it lies'.

KEPLER: Wikipedia does not give any quote from Kepler, neither from the Epitome nor elsewhere, so what quote do you refer to ? Moreover, I had previously understood Kepler first introduced his theory of inertia in his 1607 Astronomia Nova. There are lots of Latin quotes from Kepler both in Koyre's 'Galilean Studies' and 'The Astronomical Revolution'.

DA VINCI: I cannot find the statement given in the passages on impetus dynamics in Irma Richter's Oxford selections from the Notebooks. I wonder if it is to be found with a reference somewhere in Duhem's pioneering work on Leonardo and the Parisian impetus theory ? But it is plausible at least in respect of the fact that before Benedetti's critique of it from the example of sling-shots, the Parisian impetus theory maintained impetus acted directionally in the manner in which it was created. So that of a spinning mill-wheel, derived from rotating it, was thought to be circular, for example, like the impetus imparted to the celestial spheres by God. I note with humour that on this pre-Benedettian impetus theory, rather than having died of being hit by a slingshot stone, instead Goliath must have died of amazement at the sight of David's stone continuing its circular rotation on the spot even after release from his sling, rather than flying off rectilinearly at a tangent to its previous circular path.

GALILEO: Wikipedia says: "The principle of inertia first formulated by Galileo, is one of the fundamental principles of classical physics used to describe the normal motion of matter, and how it is affected by applied forces. A body moving on a level surface will continue in the same direction at constant speed unless disturbed."

I suggest this first 'Galilean' paragraph of the explanation of inertia be removed because on the one hand its adds little or nothing to understanding what inertia is but on the other it introduces falsehoods and historical problems for various reasons.

For the principle that this apparently concocted and misunderstood statement may be intended to refer to, see Galileo's Discorsi, such as p243-5 and p268 of the Discorsi to be found on pages 196f and p217 of the 1974 Stillman Drake English translation. Galileo posited a principle of the conservation of 'horizontal' motion as a component of his theory of projectile motion that commences on Discorsi p.268. HOWEVER, note that it is clear from these passages that the 'horizontal plane' is not 'geometrically' but rather 'gravitationally' horizontal, that is, one on which "there is no cause of acceleration or retardation", so it has neither a gravitational uphill nor downhill. But thereby it must be a literally horizon-tal surface, (NB the horizon of the sea is an arc of the circle of the Earth's circumference) or in other words, the surface of a sphere concentric with the Earth's centre of gravity. Consequently, and especially also given that Galileo's cosmogony apparently posited the planets move in circles of their own accord after falling into their orbits from their point of creation beyond Saturn, 20th century scholars have claimed Galileo held a principle of circular inertia rather than Newton's first law, because a gravitationally level surface is the surface of a sphere concentric with the Earth's centre of gravity. (However, also note Newton's dynamics predicts just the same as Galileo in these circumstances, but people do not conclude Newton held a principle of circular inertia.)

My own view is that Galileo did not state any 'principle of inertia' just like Newton's first law. Rather, like Benedetti, it seems he held a principle of rectilinearly acting impetus, and that impetus is conserved in the absence of all resistance and causes uniform motion. [See 'Galileo's law of straight uniform impetus' below.]

Galileo's famous elaboration of a Cusan thought-experiment - his principle of the endless continuation of the motion of a perfectly smooth ball around the surface of a perfectly smooth gravitationally concentric sphere because the ball never moves uphill against vertical gravitational resistance - has been misinterpreted as a 'refutation' of some Aristotelian and Keplerian principle that bodies have some inherent 'inertial' resistance to motion. But in fact it was a 'refutation' of the NON-INERTIAL scholastic Aristotelian GRAVITATIONAL horizontal 'tendency to rest' i.e. according to Cusa and Galileo, the horizontal component of gravitational resistance to motion posited in scholastic dynamics does not exist and that empirical resistance it explains is rather explained as due to friction, not gravity.

OCKHAM: His work was 14th century, not 12th century as Wikipedia claims.

--80.6.94.131 17:18, 20 January 2006 (UTC)A.Bellamy

In your Galileo part of the text just above, you write: HOWEVER, note that it is clear from these passages that the 'horizontal plane' is not 'geometrically' but rather 'gravitationally' horizontal ....

Can you post these passages here or are they too long to do so? We have to think that there are a lot of readers who are coming in wikipedia to search for informations. It is the job of the contributors to furnish them the quotations that we introduce, not only the coordinates of the book from which these quotations are taken. Thank you for your comprehension. --Aïki 02:36, 23 January 2006 (UTC)

Galileo's principle of perpetual rest or uniform circular motion

In response to the above request, the following passages from the Stillman Drake English translations of the 1638 Discorsi and the 1632 Dialogo demonstrate that Galileo's horizontal or level surface is not a geometrically level plane, but rather a 'gravitationally level' surface, and hence in fact the spherical surface of a sphere around the earth, such as the surface of a perfectly calm sea, for example. Hence perpetual motion around such a spherical surface is circular.

Thus the alleged 'Galilean principle of inertia' formulated by Wikipedia that "A body moving on a level surface will continue in the same direction at constant speed unless disturbed.", and said to be 'THE principle of inertia', is not a principle of uniform straight motion if unperturbed such as Newton's first law is. In fact some scholars refer to it as a principle of circular inertia. But on the other hand, nor did Galileo deny the principle of inertia in the sense of Newton's first law of motion. Rather, unlike Newton and Aristotle, he simply never says how bodies would move if unperturbed by forces such as gravity. The perpetual circular motion he posits is that of a body with gravity that keeps it in contact with the spherical surface that is concentric with the earth's centre of gravity, and of course Newton's dynamics predicts just the same provided the speed is not fast enough to create a centrifugal force that overcomes centripetal gravity and lifts the body off the spherical surface. Because he never considers how a body without gravity would behave, thus Galileo neither asserts nor denies the standard principle of inertia, which is not contradicted by asserting that a body with gravity could have a perpetual circular motion. As Koyre put it in his Galilean Studies to explain why Galileo did not formulate the principle of inertia, 'Galileo was unable to abstract from gravity', whereas according to Newton, Aristotle did abstract from the gravity or 'nature' of sublunar bodies in the two passages he cited as evidence that he espoused the Principia's first law of motion (On The Heavens 3.2.301b and Physics 4.8.215a19-22.

Salviati is traditionally regarded as Galileo's spokesman in his Dialogo and Discorsi, whereby he may be interpreted as expressing Galileo's own principle of perpetual rest or uniform circular motion in his dialogues with Simplicius, as follows.

"DISCORSI p217: ACADEMICIAN'S TREATISE: I mentally conceive of some moveable being projected on a horizontal plane, all impediments being put aside. Now it is evident from what has been said elsewhere at greater length that equable motion on this plane would be perpetual if the plane were of infinite extent; ...

DISCORSI p223 SIMP. "To these difficulties I add some more. One is that we assume the plane to be horizontal, which would be neither rising nor falling, and to be a straight line - as if every part of such a line could be at the same distance from the centre [of the earth and so of gravity], which is not true. For as we move away from its midpoint towards it extremities, this [line] departs ever farther from the centre [of the earth], and hence it is always rising. One consequence of this is that it is impossible that the motion is perpetuated, or even remains equable through any distance; rather, it would be always growing weaker.

SALV. I admit that the conclusions demonstrated in the abstract are altered in the concrete, and are so falsified that horizontal [motion] in not equable;..."

.....

DIALOGO p148: "SALV. Then in order for a surface to be neither downward nor upward, all its parts must be equally distant from the centre. Are there any such surfaces in the world ?

SIMP. Plenty of them, such would be the surface of our terrestrial globe if it were not rough and mountainous as it is. But there is that of the water when it is placid and tranquil.

SALV. Then a ship, when it moves over a calm sea, is one of these moveables which course over a surface that is tilted neither up nor down, and if all external and accidental obstacles were removed, it would thus be disposed to move incessantly and uniformly from an impulse once received."

.....

DIALOGO p32 "SALV. I therefore conclude that only circular motion can naturally suit bodies which are integral parts of the universe as constituted in the best arrangement, and that the most that can be said for straight motion is that it is assigned by nature to its bodies (and their parts) whenever these are to be found outside their proper places, arranged badly, and are therefore in need of being restored to their natural state by the shortest path. From which it seems to me one may reasonably conclude that for the maintenance of perfect order among the parts of the universe, it is necessary to say that moveable bodies are moveable only circularly; if there are any that do not move circularly, these are necessarily immoveable, nothing but rest and circular motion being suitable to the preservation of order."

--80.6.94.131 17:16, 27 January 2006 (UTC)A.Bellamy

What I understand from these passages, is that equable motion on this plane would be perpetual in the mentally conceived experiment, in the abstract; and not perpetual in the concrete, because the abstract horizontal plane does not exist, as is, in the concrete.

In the concrete, the horizontal, as defined here in respect with the center of the Earth, is always rising. Thus, the experiment mentally conceived, could not be done in this concrete hrizontal plane, i.e. the 'perpetual equable motion' could not happened in this 'earthly' situation.

Do you understand it in the same way?
--Aïki 18:30, 27 January 2006 (UTC)

Galileo's law of straight uniform impetus

The nearest Galileo comes to stating a law of straight uniform motion like Newton's first law that abstracts from gravitational perturbation (i.e. from the weight of the body) is in the discussion of the straight tangential uniform motion of slingshots in the Dialogo p189-94 (Drake's English translation), which expresses the law of impetus that the motive force of impetus causes and conserves straight uniform motion in the absence of all resistance, and in which Salviati summarises Simplicius's acceptance of it as follows:

SALVIATI: "Up to this point you knew all by yourself that the circular motion of the projector impresses an impetus upon the projectile to move, when they separate, along the straight line tangent to the circle of motion at the point of separation, and that continuing with this motion, it travels farther from the thrower. And you have said that the projectile would continue to move along that line if it were not inclined downwards by its own WEIGHT, from which fact the line of motion derives its curvature. It seems to me that you also know by yourself that this bending always bends toward the centre of the earth, for all heavy bodies tend that way. ...the moving body in continuing its straight motion goes UNIFORMLY farther from the centre of that circle of which its previous motion was a part. ...a body which leaves from the point of tangency and moves along the tangent goes UNIFORMLY away from the point of contact and from the circumference of the circle." p193-4 [My caps.]

But note this is a law of impetus rather than a law of inertia, since Galileo had no concept of inertia as an inherent resistance to motion like that of Kepler and Newton's Principia Definition 3. And nor do such passages establish Galileo was the first to formulate the so called 'law of inertia' in the sense of Newton's first law, as Wikipedia claims, following Mach's claim in the early editions of his 'The Science of Mechanics' which still dominates traditional positivist history of science. It should also be noted that Galileo did not deploy this principle in his theory of projectile motion, but rather his principle of perpetual uniform circular motion of a heavy body on a spherical surface. This partly explains why he got into such a muddle over whether the projectile motion of a heavy body falling from a tower is parabolic or semi-circular before Fermat pointed out such free-fall would be neither, but a spiral on Galileo's assumptions. --80.6.94.131 15:45, 18 February 2006 (UTC)A.Bellamy

Aristotle's arrow

If it is true that Aristotle give the explanation you said about the arrow, here above: (Foogod 03:03, 19 January 2006 (UTC)), then I agree with you that It is clear from this that Aristotle believed that the arrow needed a continual application of horizontal force throughout its entire flight to keep it moving, or it would simply stop..

This continual application of horizontal force etc..., can be translate in modern physics by the equation, or formula, F = mv, wich is very different, if not very contradictory, from the Newton's formula, or equation, F = ma.

The equation F = mv means that it takes a force if we want to have velocity, and too, that if we have no force we cannot have any velocity at all, which is absolutely contrary to the modern principle of inertia which state that any object can have a velocity whitout any force at all, and which modern principle of inertia is part of the modern view of motion, and too, is the first law of motion state by Newton.

Therefore, this alone, is suffisant to say that it is absolutely impossible that the Aristotle's view of motion is, or even could be, the same as the Newton's view of motion, which is too the modern view of motion in classicals physics.
--Aïki 04:02, 19 January 2006 (UTC)

FOOGOOD, where can we find this discussion you cite, as below, of the flight of an arrow in Aristotle's texts ? Nowhere ?:

Foogood said:"For a good example of what Aristotle did think, go look at his explanation of why an arrow keeps flying after it's left the bow (he maintained it was because it was pushed by the air around it (note, not resisted, but pushed forward)"

Again note that in Newton's physics, as in Aristotle's, the continuing flight of an arrow would also require the continuing application of a force, for otherwise it would eventually fall to earth, because its momentum would be destroyed by gravity and air resistance. All CONTINUING motion against resistance requires a countervailing force in both Newton's and Aristotle's physics. That is all I am pointing out against your statements to the contrary.

--80.6.94.131 17:16, 20 January 2006 (UTC)A.Bellamy

Continuing force on the arrow

In Newton's physics, an arrow does not need a continuing force applied to it to continue to fly.

[AB: But your following comments show it does.]

Not at all. (Note: Usually, a conclusion is put at the end of a demonstration, not at the beginning.)

If it goes down, it is only because of the gravitational force between the arrow and the Earth.

[AB: So it would need a continuing force to make it continue upwards ?]

The real conclusion: If it were no gravitational force, it would not go down, so it would continue in a straight line, whitout any force pushing the arrow ahead.

The resistance of the air just diminish the speed (velocity) of the arrow, meaning that if it were no gravitational force at all, the arrow would continue its fly, always in the same direction, but with a speed continually diminishing, this because the friction with the air is a continual force applied on the arrow.

[AB: And so it would stop. So it would need a continuing force to continue moving ?]

The resistance of the air would finally stop it (if, before it does so, the arrow don't crash on Earth because of the gravitational force), because it is a force, and a force, in Newton's physics, produce an acceleration which, here, is a slowing one i.e. the velocity v of the arrow will diminish till v = 0.

During that laps of time, the arrow continue to fly whitout any force pushing it continually. It is only the velocity that change, and it is because of a force which is in a opposited direction. The only force which is acting on the arrow, horizontally, is the resistance of the air, and it is pushing bacward continually.

If it were another force of the same magnitude of the resistance of air and opposite to it, the horizontal velocity v of the arrow would not change at all, because the net force, horizontally, on the arrow would then be zero, meaning no force (net). Having no force (net) on it, the arrow would thus fly at the same speed v (horizontally) whitout stopping.

This is the correct way to see it, in the newtonian's system and modern physics.

This it what the Newton's physics says.

[AB: Namely that a continuing motion against resistance would require a continuing force, just as in Aristotle's dynamics ?]

This is not correctly stated and it is not what it is said above, because, in the case of the arrow, there is a laps of time where the fly continue with only one force i.e. the resistance of the air, no other (in the horizontal direction; the gravitational force being in the vertical direction).
Too, the word 'motion' is too loose: it has to be changed for a more precise one, or a more precise expression.
For Aristotle, I don't know (actually).

For Aristotle, I don't know, I did not read his physics till now.
--Aïki 01:50, 23 January 2006 (UTC)

Aiki, see my quick inserted edits above in square brackets, marked 'AB'. --80.6.94.131 19:15, 24 January 2006 (UTC)A.Bellamy

Destruction of the momentum

In modern physics, the momentum is equal to the mass time the velocity i.e. = mv. The gravitation force gives to the arrow a downward acceleration i.e. increase its velocity in the vertical direction. Therefore the 'gravity', as you call it, increase the momentum of the arrow in the vertical direction. We are very far from a 'destruction' of the momentum!
--Aïki 15:20, 24 January 2006 (UTC)

CHALLENGE FOR VERIFICATION: Surely Ockham rejected impetus theory ?

In its (pseudo-?)history of the development of 'the principle of inertia', Wikipedia currently claims:

'In the 12th century William of Ockham argued in favor of Philoponus's theory that motion was maintained by some property of the object, imparted when it was set in motion[2, but also included in his view the idea that the inherent property which maintained an object's motion also dissipated as it moved, thus believing (as Aristotle did) that the natural state of matter was one of non-motion, and motion was only a temporary condition[citation needed].'

But Wikipedia does not verify this claim that Ockham believed in the impetus theory of projectile motion in any Ockham text. Moreover, the textual evidence from Ockham's critical discussion of the impetus theory of projecile motion in his Reportatio II Q. xxvi is to the contrary. For as he says in disbelief "For it would be astonishing if my hand caused a power in a stone through coming into contact with the stone by local motion" [p141 Ockham: Philosophical Writings ed. P.Boehner, 1957]. As Boehner comments in his Introduction to Ockham's writings: "...Ockham refuses to accept the theory that explains projectile motion by some quality given to a stone thrown into the air. It is sufficient to assume that the motion imparted by the hand to the stone remains until it is impeded." [pxlviii Boehner op cit]

Hence on this basis it seems Ockham rejected impetus dynamics, whether Philoponan self-decaying or Avicennan permanent impetus, and believed 'the principle of inertia' that motion would continue of its own accord if unimpeded, as it also seems did Aristotle. So unless Ockham subsequently changed his Reportatio position, Wikipedia's unverified claim that Ockham affirmed Philoponan impetus theory and its suggestion that he denied 'the principle of inertia' seems as mistaken as its unverified claim that Aristotle denied 'the principle of inertia'. After Aristotle's affirmation of 'the principle of inertia' that 'if there were an unforced and unresisted motion in a void then it would be interminable', the only people I am aware of whose dynamics logically denied it by apparently predicting that an unforced and unresisted motion in a void would terminate were Philoponus and followers, Galileo's 1590 De Motu Pisan dynamics and Kepler.

I conclude Wikipedia's unverified and apparently mistaken claim about Ockham should be removed until a revised verifiable alternative is found. --80.6.94.131 18:26, 19 January 2006 (UTC)A.Bellamy

Why dont you remove it then?--Light current 18:33, 19 January 2006 (UTC)

Because I thought somebody may post up verifying evidence of a later position Ockham adopted. Also Ockham's work was 14th century, not 12th century as claimed. --80.6.94.131 17:16, 20 January 2006 (UTC)A.Bellamy

Archive or sub-page?

THIS PAGE IS AN ARCHIVE: PLEASE DO NOT ADD POSTS HERE
If we cannot post in an archive, I think we can in a sub-page and thus be able to continue the discussions which were still active, but I'm not familiar with those things. Somebody knows about it? (There is some informations in Wikipedia:Subpages, if somebody wants to work on that.) --Aïki 01:48, 16 January 2006 (UTC)

No, I think the correct protocol is to add new comments to the current talk page. Please correct me if I'm wrong.--Light current 02:12, 16 January 2006 (UTC)

Maybe what you should do is copy the relevant post from the archive onto the current talk page then comment on it underneath?--Light current 03:22, 16 January 2006 (UTC)

I don't know nothing actually about that stuff, so I cannot pronounce myself about it. But thank you anyway for your suggestion, it could be usefulf. --Aïki 03:37, 16 January 2006 (UTC)

I believe Light current is correct. If a relevant part of an active discussion was moved to an archive (which shouldn't happen ideally, but things sometimes happen), the correct procedure as far as I'm aware is to continue the discussion on the main discussion page, referencing the archive page for previous discussion (if necessary, I think copying a little bit of the archived discussion to the current talk page is acceptable if needed to directly address something, but this should be kept to a minimum and only used when other options (like just pointing to the archive page) aren't adequate for some reason). The reason discussion shouldn't happen on the archive page is also the same reason you shouldn't do it on a subpage, which is that most people won't know it's going on and won't see it. This is why you should always do current discussions on the main talk page. -- Foogod 03:08, 19 January 2006 (UTC)

There are two main methods for archiving a talk page, detailed below. Regardless of which method you choose, you should leave current, ongoing discussions on the existing talk page. Wikipedia:How to archive a talk page --Aïki 01:57, 23 January 2006 (UTC)

Qualitative/quantitative

I'm not aware of anyone using inertia quantitatively to describe momentum/mass. That would require them to say "this object has an inertia of 5 kg", right? Is that what you are saying, Foogod? Qualitatively would be "this object has a lot of inertia". Pfalstad 03:44, 16 January 2006 (UTC)

Well, it depends on how you look at it, I suppose. You're right that the statement "x has a lot of inertia" is a qualitative statement. On the other hand, the statement "x has more inertia than y" could be viewed as a quantitative comparison (even though no actual numbers are specified). I do think I was misunderstanding the original intent when I made my correction.. I also think, however, that the use of this term here is probably not necessary and runs the risk of confusing things, because we use the term "qualitative" for one definition, and then emphasize that the other definition is "not quantifiable". The two terms will likely be seen as comparing/contrasting to each other (as I originally thought the intent was) due to their similarity, and since both basically say the same thing, if this is what a reader focuses on they may be left confused about what the distinction is.

Given that it's really used both qualitatively and quantitatively depending on context, maybe we should just remove the specifier completely? -- Foogod 23:39, 18 January 2006 (UTC)

Ok.. Pfalstad 03:44, 19 January 2006 (UTC)

Inertia is a property of matter

Am I right?

About what?--Light current 00:46, 9 February 2006 (UTC)

About Inertia, being a property of matter. Narker 03:04, 9 February 2006 (UTC)

Yes and no. Read the article--Light current 03:05, 9 February 2006 (UTC)

Narker is right because inertia is only a property of matter and not a principle, and Light Current saying "and No" and Wikipedia are wrong in claiming "It should be emphasised that 'inertia' is a scientific principle, and thus not quantifiable", because inertia is a quantifiable property of matter in Newton's physics, quantified by mass, and is not a scientific principle. More later. --80.6.94.131 15:46, 19 February 2006 (UTC)A.Bellamy

Inertia is only a principle! It is not measurable. Read the article.--Light current 20:36, 19 February 2006 (UTC)

I did, but apparently you did not. Read the section entitled Mass as a measure of inertia. But of course the article is logically incoherent, presumably a problem of Wikipedia's brave new idea of 'encyclopedia entries by democratic committee' ? --80.6.94.131 19:12, 27 February 2006 (UTC)A.Bellamy

Indeed.. That section seems wrong and should be fixed up. Pfalstad 21:10, 27 February 2006 (UTC)

Keep Inertia is a property of matter 19:55, 29 April 2007 (UTC)

Sec. 2.1.1. "Theory of impetus": Momentum being caused by motion? WRONG!

I think that it must not be stated in the encyclopedia such a description that diverge from the modern physical knowledge. In classical physics there is no mention of cause at all: it is said that momentum and motion are just connected to eash other, such that they always come together. But in quantum mechanics the operator of momentum is gradient of the wave function; and according to the Schroedinger equation, the spatial motion is caused by the gradient. So actually it comes that momentum is causing motion; and Buridan said almost the same: that impetus is causing motion. I don't know any branch of physics where it would be stated that momentum is caused by motion, except the case of poorly understood beginner's textbook. Fir-tree 00:21, 30 March 2006 (UTC)

Aristotle affirms the 'law of inertia' in refuting motion in a void

From checking out Isaac Newton's claim that Aristotle affirmed his first law of motion in Aristotle's Physics 4.8.215a19-22, on 19 January above Foogood concluded: "As far as your quote attributed to "'Physics' 4.8.215a19-22", I have just read through all of chapter 8 of book 4 of Physics, and I can't find anything resembling that statement anywhere. In fact, the entirety of that chapter appears to be devoted to refuting the existence of the void (including an argument for why movement is impossible in a vacuum. Are you claiming this is consistent with Newton also?)."

Physics 4.8 is indeed entirely concerned with refuting the existence of the void. But the logical issue here is not whether Aristotle or Newton denied or affirmed the void, which is a red herring, but whether Aristotle affirmed the 'law of inertia' as Newton claimed. Descartes, for example, asserted the law of inertia but denied the void, and Newton also denied the void because he denied mutual gravitational attraction at a distance without any intervening medium. The fallacy that asserting the law of inertia first requires asserting the existence of an infinite void stems from the error of thinking the law asserts the actuality of an endless straight motion in the world, whereby there must be an infinite space without any obstacles to make room for it. This major error in the field apparently stems from the logically challenged Koyre (in his Galilean Studies and From the Closed World to the Infinite Universe) and such as his American devotee Edward Grant (in Much Ado About Nothing), and has unfortunately become a standard feature of histories of the emergence of the law of inertia, thus illustrating the historiographical adage ‘Without elementary logic, the history of science is blind’.

Aristotle's reductio ad absurdum argument against the void in Physics 215a19-22 is traditionally interpreted by commentators to be that 'IF there were an externally unforced and externally unresisted motion of a body without gravity in an infinite void, THEN it would be interminable', which is the principle of the continuation of motion implied by the 'law of inertia'. This principle is in fact the major premise of this refutation of the possibility of motion in a void. And because Aristotle regarded interminable locomotion as a contradiction in terms and thus impossible (i.e. the tacit minor premise of his enthymemetic argument), then by the logical principle of modus tollens he therefore logically concluded any such motion in a void was also impossible.

But Aristotle clearly did not deny the law of inertia. For that would be to assert that such unforced and unresisted motion in a void would terminate, and if he had done so he would then have been unable to logically infer an interminable locomotion, the absurdity upon which this reductio ad absurdum refutation of motion in a void crucially rested. Thus the law of inertia was asserted as the crucial major premise of Aristotle's refutation of motion in a void on the ground of its interminability. But unlike the more logically astute Newton, contemporary commentators who claim Aristotle denied the law of inertia overlook this consideration of elementary logic to the contrary. And certainly Aristotle never denied Newton's first law of motion, which logically would be to assert there is some body neither at rest nor in uniform straight motion that is not perturbed by impressed forces.

The other two arguments against the possibility of motion in a void in Physics 4.8 are for the two dynamically different cases of the violent projectile motion of a body with gravity in 215a14-18 and the natural motion of a body with gravity in 215b12-22. In the first of these the body's own weight resists the motion, but in a vacuum there is no medium to provide an external countervailing propelling force to overcome it. And so Aristotle's law of motion v α F/R becomes v α 0/W here and thus there is no motion i.e. v = 0.

The second case is that of gravitational free-fall, where the body's own weight is now the propelling force and there is no resistance from any medium. Thus v α F/R becomes v α W/0, and hence its speed would be infinite, and thus such motion is an impossibility. This second case also reveals bodies have no inherent resistance to motion in addition to their gravity, because if they did then R > 0 even when there is no resistant medium and thus their speed could not be infinite. Thus without their gravity they would have no inherent resistance to motion whatever, which is why Aristotle's dynamics, like Newton's, predicts an externally unforced and externally unresisted motion in an infinite void would continue forever, simply because there would be no resistance to terminate it.

But although it affirmed the law of inertia in respect of the continuation of externally unforced and externally unresisted motion, the leading problem of Aristotle's dynamics was that it therefore also predicted that a powered but unresisted motion would be infinitely fast, as it did for gravitational free-fall in Physics 215b12-22 as above and also for the enforced motion of a weightless body in On The Heavens 301b. And yet as such as Philoponus and Avempace pointed out, it also posited that the motion of the celestial spheres was powered and unresisted, but it is observably not infinitely fast, the stellar sphere taking 24 hours to revolve. In a typical logical pattern of creative responses to refutations in science that posit some new entity in order to square the predictions of its formulas with 'reality', like that of Zwecky's positing of dark matter to explain why rotating galaxies do not fly apart, the eventual solution apparently due to Averroes and Kepler was to posit inertia, an inherent resistance to motion in bodies proportional to their mass. Thus there would be a resistance to motion even when there is no resistant medium nor gravitational resistance, so that v α F/R became Kepler's v α F/m in such circumstances. This was a crucial step towards Newton's second law a α (F-R)/m, but Kepler's inertial dynamics denies the law of inertia because an externally unforced and externally unresisted motion would be terminated by the body's inherent inertia, a resistance to all motion. It was Newton's subsequent revision of Kepler's notion of inertial resistance to all motion to exclude any resistance to uniform motion that restored Aristotle's original 'law of inertia' as expressed in Newton's first law within the new 'inertial' form of Aristotelian dynamics introduced by Kepler. --80.6.94.131 18:07, 9 May 2006 (UTC)Logicus

Aristotle inconsistent ?

Proposed edit: In the article's current passage: "Aristotle explained the continued motion of projectiles, which are separated from their projector, by the action of the surrounding medium which continues to move the projectile in some way.[1]. As a consequence, Aristotle concluded that motion in a void was impossible for there would be nothing there to keep it in motion.[2] Somewhat inconsistently, Aristotle continued by asserting that if one could imagine such a void, an object in a void would act according to inertial laws:

[N]o one could say why a thing once set in motion should stop anywhere; for why should it stop here rather than here? So that a thing will either be at rest or must be moved ad infinitum, unless something more powerful get in its way. [3]"

I propose the clause "Somewhat inconsistently,..." be deleted until this alleged inconsistency is established, if at all. Presumably unlike Newton, the editor is assuming, without justification, that the motion in a void in Physics 215a19-22 is also a case of violent motion against gravity like that in Physics 215a14-19. This is understandable given the Loeb apparent mistranslation of both motions as projectile motions, that is, as both being violent motions against gravity, but not in the Hardie & Gaye translation cited by the article, in which it need not be presumed the motion in the second passage is a violent motion against gravity.

I also propose associated edits to eliminate the unwarranted presumption of an inconsistency here, as follows:

'As a consequence, Aristotle concluded that such violent motion was impossible because there would be nothing to keep the body in motion against the resistance of its own gravity. Then in a statement Newton regarded as expressing his first law of motion, Aristotle continued by asserting that a body in (non-violent) motion in a void would continue moving forever:'

Logicus 18:54, 20 November 2006 (UTC)

Merge

There is a POV-fork by banned User:Light current at Principle of inertia (physics). Can pls somebody check whether there's anything to merge there? I'm inclinded to just delete and re-created as redirect. --Pjacobi 22:27, 21 April 2007 (UTC)

Looks like it's more or less just a copy of Inertia as it looked at the time, with a few sentences moved around and some text removed. I'm not even sure I understand what POV User:Light current was trying to push, but I see no material in the article worth including in this one. Delete and redirect seems appropriate. Tengfred 11:38, 22 April 2007 (UTC)

Done. --Pjacobi 12:03, 22 April 2007 (UTC)

Great. I didn't know Light current had been banned, or I would have done it myself. Pfalstad 17:29, 22 April 2007 (UTC)

Quick and dirty. Admit it was confusing with twin articles on the subject, "as it looked at the time", i e before April 3, when Tengfred made the difference. I noted that the deleted article still contained a sub-title on Novel interpretations a few days ago. What is the idea of having categories like Fringe subjects without critical scientific evaluation or even Pseudoscience, when a long article on established physics does not even have any hint as to what further work is going on to find out what is behind the mysterious property (inertia)?7 Kurtan 15:30, 24 April 2007 (UTC)

While getting rid of the section "Novel interpretations" was not the reason for my support of the merge, I'm not sad to see it go. It was nowhere near appropriate for this article. WP:UNDUE clearly states that tiny minority views should not be included in articles, and a theory with one published article and no citations seems pretty minor to me. Tengfred 16:37, 24 April 2007 (UTC)

Amen to that. --RE 17:55, 24 April 2007 (UTC)

Inertia is not always "dependent upon the mass and shape of the object."

I've deleted the sentence "Inertia is dependent upon the mass and shape of the object." from the introduction because shape only influences rotational inertia; it is irrelevant to the more common rectilinear inertia. The former version was confusing and I had thought of changing it to read "Inertia is dependent upon the mass of the object," but felt that would cause further confusion regarding rotational inertia. --SteveMcCluskey 13:53, 22 April 2007 (UTC)

Novel interpretations?

I see Kurtan re-introduced this section. I don't think it is appropriate for the article, but I figure we should have a discussion about it. As it stands now, the section doesn't really have any contents, and I don't see the need for a stub. I don't think this is a section that is obviously needed in the article, nor do I see that it has any hope of ever being more than a stub, without introducing pseudoscience or other material that is inappropriate for the article. Tengfred 09:03, 26 April 2007 (UTC)

Again and again?

Lack of consensus as to the true inherent nature of everything is the normal driving force of any scientific progress. Don't see why this should noted here, unless specific notable novel interpretations are widely discussed by scientists.

Pjacobi 10:08, 26 April 2007 (UTC)

Avicenna's projectile trajectory unsourced

I have put a 'citation needed' marker on the diagram of what is alleged to be Avicenna's projectile trajectory in the section on 'Islamic theories' since it is both unsourced and possibly false i.e. he never proposed any such trajectory. I further propose to remove this diagram if no evidence is forthcoming that Avicenna ever proposed such. --Logicus (talk) 16:25, 22 January 2008 (UTC)

Projectile trajectory diagrams of Philoponus and Albert Saxony flagged as unsourced. Probably only Galileo ever represented projectile motion as zero elevation horizontal projection, as part of his celestial dynamics and cosmogony in which the planets are put into orbit by a 90 degree deflection into the horizontal by God after a period of free-fall from their point of creation.--Logicus (talk) 19:18, 28 January 2008 (UTC)

The error here may well be that what were intended as vertically upward projectile trajectories in their textual discussions implicitly referred to here may have been misinterpreted as horizontal trajectories by reading history backwards through Galilean spectacles. For whilst it is certainly the case that it can be mathematically demonstrated that the resultant of a downward uniform acceleration due to the vertical gravitational inclinatio ad contraria and a horizontal uniform deceleration to a state of rest due to the horizontal gravitational inclinatio ad quietem in some scholastic dynamics can produce the two and three stage projectile scholastic projectile trajectories for oblique projections depicted in Rupert Hall's PhD thesis - published as Ballistics in the 17th century - which are more realistic than Galileo's mistaken parabolic trajectories, nevertheless these would not produce any more than two-stage trajectories at most for horizontal projections, namely a curvilinear stage and also a second vertical stage for those special cases when the horizontal impetus is wholly destroyed by gravity before the projectile reaches the ground.

The current diagrams of Philoponus and Albert of Saxony projectile trajectories should be deleted as false and unsourced in order to avoid the misleading impression that scholastic dynamicists were silly fools and that Galileo was not grossly mistaken about projectile trajectories.--Logicus (talk) 19:32, 1 February 2008 (UTC)

Equation without reference

After the equation P=mv, each aspect of the equation is given an explanation; P= momentum, m = mass, v = velocity.

However the next equation, F = ma, doesn't explain what F or a equals. I'm not a scientist, so I'm sure it's obvious to others however I think it should be made clearer. Master z0b (talk) 06:09, 19 February 2008 (UTC)

I read this almost a year later, and it still hadn't been explained, so I explained it in the article (F is force, m is mass, and a is acceleration). -- Another Stickler (talk) 09:53, 8 January 2009 (UTC)

islamic inertia reaxmined

i slapped a tag on that inertia part of islamic concepts. I cant find anything to back up these claims, and what worse i ve already looked up one of the sources ref. and looked it up and it doesnt saying anything like that, in fact the book has absolutely nothing to do with the hisotry of science whatsoever soooo, the book being by Abdus Salam, and investigating the others, plus some of the claims here also justy dont add up from a scientific point of view. —Preceding unsigned comment added by 24.36.181.171 (talk) 03:24, 1 April 2008 (UTC)

changes to history of inertia

i have completely removed the fact that somehow Alhacen discovered the law of inertia, since it simply not true but merely reflects original research done by the editor of this article. I could not find a single source, not even the self-promoting, biased, and distortionist islamic science pages making this claim. The ref. that is used to back this up does not a have a quote like that at all, and in fact the book has nothing do with the history of islamic science or science for that matter. The claim is that experimentation was used to back this up, okay what experiments then. This whole claim is essentialy bogus, not to mention it also demonstrates the editors total lack of understanding of the concept of inertia. On the very beginning of the page inertia is defined not just as objects moving continuosly, but also ones staying static if thats their original form of motion. This whole thing represents the worst type of revisionist history, not only is it false, the its the editor makeing their own assumptions, with limited knowledge of the topic. I was always suspicious of this claim and it looks as though my intution was correct.

"Abū Rayhān al-Bīrūnī (973-1048) was the first physicist to realize that acceleration is connected with non-uniform motion.[11] The first scientist to reject Aristotle's idea that a constant force produces uniform motion was the Arabic Muslim physicist and philosopher Hibat Allah Abu'l-Barakat al-Baghdaadi in the early 12th century. He was the first to argue that a force applied continuously produces acceleration, which is considered "the fundamental law of classical mechanics", and vaguely foreshadows Newton's second law of motion." This whole thing has been removed since it has nothing to do with the idea of inertia, and it makes some bold claims. The bold claims that "Hibat Allah Abu'l-Barakat al-Baghdaadi in the early 12th century. He was the first to argue that a force applied continuously produces acceleration, which is considered "the fundamental law of classical mechanics", thats not a fundemental law of classical mechanics, the fundemental laws of classical mechaincs are Newton's laws of motion, the idea is just merely conceptual in nature and one was so basic and trivial it was assumed to be true by Gallileo and Newton soooooo. —Preceding unsigned comment added by Tomasz Prochownik (talk • contribs) 04:31, 7 April 2008 (UTC)

I don't know much about the history of science in the Islamic world, but the idea that a continuous force produces acceleration is certainly a fundamental law of classical mechanics, and not at all basic or trivial. This idea was alien to the ancient Greek philosophers, and it took a long time for the early European natural philosophers to get past the Greek conception of a constant force producing uniform motion.

As you say, the fundamental laws of classical mechanics are Newton's laws of motion. The fact that a constant force produces acceleration is Newton's Second Law.--Srleffler (talk) 04:12, 9 April 2008 (UTC)

Contrary to what you claim, the book cited in the article does indeed claim that Alhacen "enunciated the law of inertia". It's in the fourth line down from the top of page 181. You also claim that the book has nothing to do with the history of Islamic science. The essay cited is entitled "Islam and Science". I am restoring this cited material to the article.--Srleffler (talk) 04:23, 9 April 2008 (UTC)

Note that page numbering varies between editions and between hard and softcover. It's on page 181 in the second-edition searchable copy on Amazon. Just search the book for "law of inertia".--Srleffler (talk) 05:05, 9 April 2008 (UTC)

found the quotation but enucating the law of inertia is not the same as discovering it since that would have to be verfied by some type of experimentation, which is of course made here, and if thats the case what experimentation????? am dying to know. Thats of course the key since without any experiments there isnt much to distinguish it from philosophy, and of course there is no claim at least in this book that shows experimentation occured. Secondly, the law of inertia is not just about objects moving preptually if their motion remains undesturbed but also remaining at rest. so in reality all that exists here is well speculative philosophy. Hence this is definately going to be revised. If someone can find any source other then this making the calim of the discovery of inertia am more then willing to see it, but until then it remains purely philosophy, which is not science. As for acceleration and "Hibat Allah Abu'l-Barakat al-Baghdaadi in the early 12th century. He was the first to argue that a force applied continuously produces acceleration, which is considered "the fundamental law of classical mechanics",[13] and vaguely foreshadows Newton's second law of motion." is not Newton's second law as claimed, Newtons second law deals with the concepts of force, acceleration, mass, and their relation to one another soooo. Secondly, what does Newton;s law really state in words, well what it says is that any change in motion is proportional to the force applied on the object and in the same direction. All thats said by Hibat Allah Abu'l-Barakat al-Baghdaadi that application of force produces motion but as with any law of nature the devil is in the details as they say, no information provided how motion changes proportionally and direction and thats is they key to Newton's Second law, the detials which allow one to make qualitative predictions. This is related as much to Newton;s second law as someone saying there is always a reaction for every action, thats doesnt really provide one with much insight. So on essence it is trivial since it has no predictive power and offers no insight whatsoever into how mass, force, and acceleration are related. Lastly, to cap of this little rabble this is the page that deals with inertia, not acceleration, so why is it listed here???Tomasz Prochownik (talk) 04:51, 10 April 2008 (UTC)

It's possible the text in the article overstates the case or covers more ground than is relevant to the topic of inertia. If so, some editing is in order. I restored the text because you deleted cited material based on an argument with obvious flaws.

I question the distinction you are drawing between philosophy and science. When discussing the early history of science it is important to cover natural philosophy. Science grew out of philosophy, and the thoughts of ancient philosophers were an important influence on early scientists. Many of the earliest scientists did work in both fields (Descartes comes to mind, but there are many other examples). I don't offhand know to what extent early western scientists were aware of and influenced by the work of the Islamic philosophers mentioned here, but certainly there was some influence of science and philosophy in the Islamic world on early Western science. Additionally, an early recognition of the concept of inertia is important because this idea was notably lacking in the Greek philosophers, whose work had a huge influence on early Western science. Removing the reference to Alhacen's work from the article altogether is clearly not justifiable. How it is presented and how or whether al-Baghdaadi's work is treated is certainly subject to discussion.

I'm not quite sure why you see the connection between applied force and change of motion as irrelevant to the discussion of inertia. It is precisely this connection that defines inertia: in the absence of an applied force, motion remains constant. --Srleffler (talk) 05:31, 10 April 2008 (UTC)

I think I'm going to have to revert some of the edits you are making tonight as well. The reference does explicitly refer to Alhacen as an experimentalist and an empirical scientist. The fact that it does not explicitly describe what experiments his "discovery" of inertia was based on does not prove that there were no such experiments. You seem to be leaping to a conclusion here, perhaps driven by a personal point of view. Skimming a few pages of the reference, the main point of the section seems to be that these 11th century Islamic scholars were indeed scientists in the modern sense and not mere philosophers. You need to base your edits on citable sources and not on your own opinion. --Srleffler (talk) 05:43, 10 April 2008 (UTC)

i absolutely agree i made mistake by removing the text for sure so i have made the appropriate changes, however it must always be remebered philosophy is not science and attempt to somehow blend speculative and philisophical constructs as science is most definatly flawed which is exactly what was done here, and it is huge difference between just throwing an idea out their rather then putting forward the theory and then verifying it with experiment.Tomasz Prochownik (talk) 05:47, 10 April 2008 (UTC)

I reverted you but softened the original wording a bit. You can't say that Alhacen's work was merely philosophical without a citation to support that claim. The reference provided describes him as an experimentalist. If you want to claim that his work on inertia was merely philosophy, you will have to provide a citation to support that claim. --Srleffler (talk) 05:52, 10 April 2008 (UTC)

By the way, I do understand the distinction you are making between philosophy and science very well, and I agree that it is important not to misrepresent philosophy as science. In this case, however, the cited reference is explicitly making a case that these scholars were scientists, who based their conclusions on experimentation. --Srleffler (talk) 05:56, 10 April 2008 (UTC)

just because the book claims that they were experimentalists doesnt at all prove that they used any science at all to prove inertia, so it seems thats you are the one making conclusions, so unless you have any evidence to back that experiments were performed, well you yourself are making assumptions, also what is the definiton of the word enunciate, well to state pronounce etc, thats a huge leap from that to discovery. Unless u have explicit proof to show that experiments occured and their nature, you 've got nothing, so wouldnt much bother editing that part since am just gonna change it again. If you wanna make such pretty bold claims you're gonna have to back it up with more then a glancing quotation from a book, which doesn't deal much with the history of science.Tomasz Prochownik (talk) 05:58, 10 April 2008 (UTC)

call it philisophical, specualtive, or hypothesizing, unless you have any proof that science was used to back this up, it isn't science soooo you've decided to add the whole section on acceleration, even though thats not related to inertia, as far as foreshadowing, ummm sorry this isnt a fictional novel, thats probably one of the worst ways it can be softened, it pretty much sounds like something one would write on a highschool paper. Hypothesized is the appropriate word here. When making bold claims as you are making you pretty much have to have some evidence to corroborate, which of course doesnt exist. Small piece of advice here for wrtting about historical facts you obtain the evidence to back up the claim, not make the claim, assume it to be true and then look for evidence. If the method u just proposed here was used in history there would be quiet alot of confusion in the field of history. I want proof of these experiments and their nature and relation to inertia, not a sentemce from a book and using second hand circumstantial evidence to prove it to be true. Do also explain what the whole section on acceleration has to do with inertia in the context of this article. Tomasz Prochownik (talk) 06:04, 10 April 2008 (UTC)

I'm OK with "hypothesized". It reduces the claim in this article to a level that is better supported by the reference. The claim that Alhacen did an experiment on motion was not explicitly supported (but neither is the claim that he merely philosophized). "Hypothesized" leaves open the question of whether he did any experiments on the subject.

I restored the section on acceleration because I don't think the deletion of this chunk of cited material has been discussed sufficiently. --Srleffler (talk) 06:20, 10 April 2008 (UTC)

am gonna be removing the section that has to do with acceleration and avicenne becuase it is not related to inertia, unless soneone can demonstrate how ibn sina,Hibat Allah Abu'l-Barakat al-Baghdaadi, and Abū Rayhān al-Bīrūnī concepts related to inertia. More specifically how did they believe their ideas supported or were related to the idea of inertia. I wont be waiting long to make this change so if soneone has any evidence of how these early concepts were believed to contribute to the early ideas of inertia by their respective authors please post this on the discussion page. This whole page is dedicated to the idea of inertia not other conepts such as acceleration and so forth soooo. As far as being sourced thats nice and all but it has nothing to so with inertia and its just clutering the page with not relevant material, plus it gets plenty of space on other pages so eliminating from this page will not totally get rid of it.Tomasz Prochownik (talk) 08:07, 6 May 2008 (UTC)

Higgs Boson or Mass Field

"the equality of inertial and active gravitational mass [...] remains as puzzling as ever".

Open Source (Vacuum Gravity and Inertial Forces) Project: http://www.wiki1.net/groups/pmwiki.php?n=BigCrash.Inertia

If you that gravity around massive objects is a curvature of space accelerating matter toward the massive object, and you accept vacuum energy, then very dense vacuum energy might be expected to generate the same curvature of space accelerating matter toward the dense energy (accelerate elemental particles outward in all directions, felt as resistance to change in motion, see proposed mechanics and math at BigCrash.org > Inertia) --Jtankers (talk) 12:51, 25 April 2008 (UTC)

... Because (astronomical) vacuum gravity force would only act on the smallest particles of matter, possibly quarks, ... seems to indicate that the force may be repulsive with respect to particles/fluctuations that are energy only and only travel at the speed of light. (A fluctuation is strongly pushed by vacuum energy... such a force might require light speed of massless particles. This push force on energy may also push strongly on vibrating strings of energy, possibly folding them into ... every possible bit of empty space. The pull force on matter particles should have the effect of giving the matter particle its size, pulling the quark to the radius that a quark is measured to be, and resisting change in motion, causing inertial forces. --Jtankers (talk) 12:51, 25 April 2008 (UTC)

You mean, something like this [9] 1994 article from Physical Review A? These authors have been developing an extensive theory regarding the vacuum energy and inertia using a formalism called stochastic electrodynamics. They could do with some experimental backup, though. --Wtrmute (talk) 19:50, 19 May 2008 (UTC)

Extensive vandalism repaired. Please check.

I just started reading this article Dec 2008. It looked really torn up. I read backwards through the history and apparently no one had been watching the page for about three months, and the vandalism noise just kept compounding. There was no easy way to use undo to repair it because of "intermediate changes", which were also vandalism. I started copying from old versions and pasting into the new version as I went through the history. This became tedious, so I looked for the end of the last period of activity and did a big diff compare. That helped. I seem to have Humpty Dumpty looking egg-like again. This is a call for anyone that was editing the article before to give the article a once over, to make sure your contributions are still there, just in case I missed something. You can also check my recovery edits starting at http://en.wikipedia.org/w/index.php?title=Inertia&diff=next&oldid=255760972 and ending at http://en.wikipedia.org/w/index.php?title=Inertia&diff=256210358&oldid=256209667 (Later edits fix earlier vandalism.) I asked for semi-protection for the page and Persian Poet Gal protected it until 03:44, 27 December 2008, only 3 weeks. Please put this page on your watchlist so when the protection comes off the same thing doesn't happen again. -- Another Stickler (talk) 10:49, 6 December 2008 (UTC)

"donation toy", I wanna know more

I only saw one of those "black hole" for coins once on this museum this time I was traveling outside my country, I wannna know more (I'll probably not get the answer since the email notification is failing for me, but I felt like asking about this anyway)--TiagoTiago (talk) 23:52, 13 December 2008 (UTC)

That has nothing to do with inertia. It's a spherical-mirror trick. -- Another Stickler (talk) 20:36, 3 January 2009 (UTC)