Talk:Force/Archive 2

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Definition of force

Exactly, Newton said F=ma, but did not postulate it. He was proven wrong later by the entirity of relativity. Lets define force as an external influence. Not all forces cause acceleration - for example, sometime the force of static friction does not cause any acceleration.

I don't agree with User:Crazynas. A force is not defined as an acceleration applied to a mass. If it were, Newton's second law would be a definition of force and not a law of nature linking different quantities. A force is a concept which must be defined without referring to Newton's second law. Look at the causality article. I think it is explained there what is a force. There you find example of causes like :

• The Moon's gravity causes the Earth's tides.
• A good blow to the arm causes a bruise.
• My pushing of the accelerator caused the car to go faster.

Which is exactly what is meant by force. If you want to make this quantitative and provide a numerical value to forces then you need to postulate Newton's second law. Then you use Newton's second law as a quantitative definition of forces but this does not change the original definition which must be independent of Newton's second law. 131.220.44.10 12:11, 12 September 2005 (UTC)

F=ma that, in physics is the fundemental (non calclus) definition of a force. Gravity and magnatism are both good examples of the inverse square law which states that the attratction (force) between two objects is inversely proportional to the product of the two masses times the distance squared. acceleration is distance per time (squared). Acceleration IS fundemental to the definition of a force in physics, why does it have to be independent of the secound law? Crazynas 10:02, 13 September 2005 (UTC)

NO! Read Einstein and Infeld, The evolution of ideas in physics. More seriously look simply at Inertial mass. You will discover that F=ma is a definition of m but not at all of F. You can also have a look at Causality. If F=ma were a definition of force, then it could not have been tested because a definition is always true independent of any observation. The theory of relativity and the MOND model would make then no sense. This is just a question of logic. On one point you are right F=GMm/r2 is a definition of the gravitational force. It is difficult for me to more clear than that. 131.220.68.177 12:13, 13 September 2005 (UTC)

I don't like authoritative arguments but look at [[1]]. Their definition is force: any action or influence that causes an acceleration. They are right. The definition here is a bit more general because WP is an encyclopedia and not a glossary. 131.220.68.177 12:37, 13 September 2005 (UTC)

Once again from [2] force : Any external agent that causes a change in the motion of a free body, or that causes stress in a fixed body. I think this one is better. 131.220.68.177 12:52, 13 September 2005 (UTC)

This one is the best [3]: Force: An agency or influence that if applied to a free body results chiefly in an acceleration of the body and sometimes in elastic deformation and other effects. 131.220.68.177 13:19, 13 September 2005 (UTC)

... all of your definitions use the word 'aceleration' in them... what do you have aginst it being in the tag line for the physics article on force??

The current tagline

"In physics, a force is an external cause responsible for any change of a physical system. For instance, a person holding a dog by a rope is experiencing the force applied by the rope on his hand, and the cause for its pulling forward is the force exercised by the rope."

makes no sense, how can force be a definition of mass??? mass is an independent quality existant in ALL matter, just because a definition is always true, doesn't mean it can't be tested have a look at scientific theory. By your own definitions aceleration IS fundemental to the definition of force, and regardless, F=ma IS the fundemental physics equation for this idea. Crazynas 23:37, 13 September 2005 (UTC)

Now I agree with the current definition in the article: a force is a cause (or an agent) but not an acceleration -- even times a mass. But I still don't agree with you. Though the term acceleration can be used in the definition of force, it must not be because both concepts are independent. Even Aristotle knew what a force was. What he didn't know was the relationship between force and acceleration. Though F=ma may not be seen as the definition of force, it can be seen as the definition of inertial mass. This is the linear coefficient linking force with acceleration (if Newton's second law is valid). A given force induce different acceleration on different systems depending on a linear parameter which is called inertial mass. This inertial mass is not to be confused with gravitational mass which is the coefficient appearing in F=mg (|g|=9.81m/s2). The experimental fact that both parameters are equal has been seen as fortuitous till Einstein recognized this as a basic principle (Mach's principle). For example in modified Newtonian dynamics model one replaces Newton's second law by F=mμ(a)a. When one does this one does not change the definition of F (F is still GMm/r2) but Newton's second law. It becomes a non linear relation between F and a defining both the factor m (the inertial mass) and μ a non linear function. Of course one could see this model also as new definition of the inertial mass with m:=mμ(a) in order to save formally Newton's law. If F=ma were a definition of force one would not need to change it. It would remains the same independent of any observation a bit like $\mathbf {L} =m\times \mathbf {v}$ which is the definition of angular momentum and not a law (axiom) of physics and which validity has not to be checked.131.220.68.177 07:42, 14 September 2005 (UTC)

I am sorry to intrude, but L= mxv is NOT the definition of angular momentum. Angular momentum is defined as a vector product of position r and momentum p : L=rxp Enormousdude 14:48, 2 May 2006 (UTC)

This page is of no help to the aspiring highschool physics student or non technical engineer. Clyde frogg 08:13, 18 October 2005 (UTC)

Am trying to fix this now :) Rpf 14:33, 18 April 2006 (UTC)

Indeed, that's what I was trying to change. Crazynas 23:10, 19 October 2005 (UTC)

Can people please stop mixing cause and effect by thinking you need to define a force as a rate of change of momentum. It's wrong anyway, a compressed spring has a constant momentum and is full of tension. I'm with 131.whatsisface Rpf 06:43, 19 April 2006 (UTC)

"In physics, force is an action that causes a free object with nonzero, finite mass to accelerate or change momentum, relative to an inertial reference frame."

What doesn't work about this is: action is undefined (something that does something is even more vague), gravity is a force and it bends light which has no rest mass, and forces work in non-inertial reference frames as well. Rpf 02:47, 26 April 2006 (UTC)

OK take out the words 'is an action that' . Also light has relativistivic mass, so no problemo! :-)--Light current 02:56, 26 April 2006 (UTC)

that could work, but then we confuse mass and energy and it becomes a mess. Stick with momentum.Rpf 03:06, 26 April 2006 (UTC)

OK stick to rate of change of momentum. But lets not have forces acting alone etc! :-|--Light current 03:08, 26 April 2006 (UTC)

Ok. "Free body" it is. Rpf 03:09, 26 April 2006 (UTC)

Sorry Ive changed it again.--Light current 03:14, 26 April 2006 (UTC)

Looks like we are getting somewhere :) Good stuff. Rpf 05:55, 26 April 2006 (UTC)

Amazing the article has completewly distorted Newton's theory.He never said F=ma actually it was F=Kma .We have described unit Newton in such a way that it becomes 1.Newton in his original version used proportionality,Plz don't replace it with equality sign which is incorrect.I am not doing it myself because I have just joined your discussion and would like to have others view in this matter.Holywarrior 09:32, 3 July 2006 (UTC)

Well, I wouldn't call this a "complete distortion." Maybe Newton did say F = k (dp/dt)= K ma for constant m. But so what? You're left with choice of units, and you are perfectly free to pick units of F, m, or a to make K = 1, so that's what we did.

In fact, many solutions to the scale problem been were used historically. In metric, F was defined as a brand new unit, to make K = 1 when M was in standard kg, and a was in m/sec^2. The new unit of F was called the newton. Easy enough. Occasionally, the metric system uses forces in "kg force units" or "kg weight", and in that case, in order for K to equal 1 in Newton's equation, the a must be given in g's, or gravities. For far as I'm aware, nobody has tried to rescale metric mass in some odd units of 9.81 kg, with some special name.

In the English systems, when they had the same problem, if they used the old unit of pound-mass, they had to invent a new force unit called the poundal, which was 1/32.2 pound-force. On the other hand, when they insisted on using force in the old units of pound-force, they had to rescale their masses in units of 32.2 lbs-mass, a unit they called a slug. Finally, if one insists on using regular pounds-force and regular pounds-mass, then in order to keep K = 1, your a again must come out in gravities. Steve 10:35, 3 July 2006 (UTC)

The article says Newton </wiki/Newton> was first to mathematically define force as the rate of change of momentum: Did he say so????? Plz cite Principia Mathematica version. In physics </wiki/Physics>, a classical force is a name given to a net influence that causes a free body with mass </wiki/Mass> to accelerate </wiki/Accelerate>

What Do you call if force is applied and change is not produced and why this free body concept is important for defining force.

Plz consider Force is an external agency which produces or tries to produce change in the inertial state of body.Holywarrior 11:42, 3 July 2006 (UTC) I would still recommend to stick to theoriginal version by Newton.Equality sign should be replaced by proportionality because it makes more sense.HW 05:39, 15 July 2006 (UTC)

Internal and external forces

This article doesn't talk about internal and external forces and the article's definition makes it seem like all forces are external. Below is a definition from here.

1. Internal force: Forces acting between body parts
2. External force: Forces acting between the body and environment. It can be distant forces (gravity) or contact forces.

Here is a page with diagrams. -- Kjkolb 11:43, 26 November 2005 (UTC)

Yes, but it correctly says the concept of internal and external is relative---depending on how you define the system.Force which produces change in a system are always external ,internal forces are in equilibrium.Holywarrior 11:53, 3 July 2006 (UT

WHAT DO YO MEAN BY PHYSICAL INDEPENDENCE OF FORCE ????????????????????????????????????

Total Net Force

This article's definition of Total Net Force is unclear.

F = m(dp/dt)

More information can be found here.

Actually it's dp/dt...the m is wrong in your source.Rpf 14:32, 18 April 2006 (UTC)

I read the quoted source it says it becomes F=m(dp/dt) after awakening from hypnosis.Holywarrior 11:59, 3 July 2006 (UTC)

Exclusion is not a force

Anyone can correct me if I am wrong..(duh!)

It is meaningless to say that an electron experiences a mysterious new kind of force just because of the exclusion principle. It simply says that fermions are forbidden from coexisting in the same state. It's not like there is a billiard ball you are trying to put on top of another billiard ball, it's two probability densities that simply say you won't find two electrons in the same state once the functions are sufficiently collapsed to tell.

This "force" shouldn't be called as such because it cannot be expressed by the exchange of bosons OR using newtonian ideas.

At the end of the day, the hammer and nail example STILL only uses the 3 forces only, it's just that because of the limits imposed by the fermi statistics, the fermions in matter will always end up existing in awkward positions feeling those 3 forces. The extra term in the L-J potential is a fudge to make sure fermions never sit on top of each other.

No, it is not a "fudge". It follows from identity of fermions.

Enormousdude 16:45, 30 April 2006 (UTC)

For now, it is getting the chop. Rpf 14:50, 18 April 2006 (UTC)

Somebody is trying this again. Can we have a source that confirms statements like this: "All forces, except exchange forces and zero energy pressure force (virtual particles pressure) can be reduced to these fundamental interactions" Rpf 17:20, 24 April 2006 (UTC)

Ineed, how do you reduce exchange force (Pauli repulsion) or Cazimir force to three fundamental interactions ?

Enormousdude 16:45, 30 April 2006 (UTC)

See Standard model on explanation where force(s) mathematically come from. Enormousdude 17:53, 26 April 2006 (UTC)

This is discussed in the "force as particles" section. It is ridiculous to keep insisting that force is defined by a rate of change of momentum because basic mechanics would be totally impractical trying to approach it on a molecular level. Some of us here are *very* familiar with both the standard model and classical mechanics and if you want us to agree with your narrow definition, you need to give us reasons. Put another way, please describe completely how a playground see-saw works using the standard model. Rpf 07:31, 27 April 2006 (UTC)

You said (qote from above) "It is ridiculous to keep insisting that force is defined by a rate of change of momentum..." Please define it otherwise (make sure you define it mathematically because we use it mathematically in mechanics).

Enormousdude 15:47, 28 April 2006 (UTC)

In physics, a force is that which, when acting on a body that is free to move, causes a rate of change of momentum of that body.

In physics, we (=scientists) define quantities MATHEMATICALLY (=via formula or equation). I asked you to provide DEFINITION (=mathematical expression), not words. So far I see none. If you want to use words insted, then you are in wrong place (you have to go to philosophy as it is the place to play with ill-defined terms). So, please provide MATHEMATICAL definition of force.

I disagree. Physics is not math (thank goodness). My textbooks do not mathematically define force the way you define it. Pfalstad 18:24, 30 April 2006 (UTC)

What? Physics is NOT math? That is quite a nonsense (or at least gross misunderstanding of physics). Physics is application of math to objects and systems in real world. Where does, for instance, the shape of p-orbital of H atom come from (in general, explain the origin of the wave function of a particle in radial potential field, say of 1/r potential)? As to the definition of a force -hat textbook do you use? Hope not introductory bok but a good classical mechanics text. How does it define force - can you share it here?

Enormousdude 01:18, 2 May 2006 (UTC)

What you are saying is that force equals rate change in momentum.

Fine. Define it otherwise then.

I doesn't...it CAUSES a rate change in momentum. And it only does this when the body is free.

So, it DOES or it DOES NOT?

You feel gravity now but your momentum is constant.

We don't use FEELINGS in science.

You can carry on about sub-atomic particles, but you still can't do a basic mechanics problem using particle exchanges instead of force can you?

Actually we do it sometimes. It is mathematically intense, and this is the main reason why we stort-cut it using intermetiate quantities (like force F=dp/dt, work, energy, path integrals, etc) - this way insted of operating with tens and hundreds of symbols at a time, we can operate with much fewer. Paperwork and time reduction is obvious (and even some trees conservation - as a side effect bonus).

Did you forget that part of my question? Rpf 04:34, 29 April 2006 (UTC)

I can not answer about the object I don't know (I don't know what "playground see-saw" is). If you explain me what it is, I will explain you how it works.

Force v rate of change of momentum

FWIW, I tend to agree with Rpfs defn above. A force causes a dP/dt if unopposed. In our system (SI) it is also numerically equal to dP/dt. Also, a body with momentum P when striking another body and giving up all its momentum, would create an impulse force equal to the rate of change of momentum of the combined body.

Every force must have an equal and opposite reaction and the total momentum in any closed system is conserved.

Therefore, it can be considered that force and dP/dt are the same (interchangeable) thing in terms of kinetics. When trying to accelerate an object, it is dP/dt that provides the reaction force. Whether they really are the same thing is a matter for philoshophy.

In the case of 'immovable objects', force clearly does not cause any dP/dt because the force is resisted by the static reaction force. I suppose one could say that F = R + dP/dt, where R is any reaction force (friction, opposite push etc) and dP/dt is the reaction force due to accn.

--Light current 14:36, 29 April 2006 (UTC):

If opposed, then obviousely one derivative dp1/dt must be added to another derivative dp2/dt, and the result becomes zero when dp1/dt = -dp2/dt

You got it backward. It is THE rate of change of momentum of that body, which we then TERM as "force"

Physics is NOT a philosophy. In physics, we use mathematical definitions. Philosophy does not (that is the MAJOR problem with philosophy, by the way, - lack of accurate definitions).

It does.

Obviousely, immovabble object is mathematically equivalent to the object which moves with equal acceleration in opposite directions. Thus if the object experiences two equal and opposite rates of change of momentum, it must stay put. If you have "feelings" that this contradict to "common sense" just disregard common sense. Mathematics is much more accurate than common sense or feelings.

Best regards, Enormousdude 16:56, 30 April 2006 (UTC)

As I said before, I think we are entering into the realms of philosophy here. THe mathematics is patently not getting us anywhere! :-|--Light current 17:46, 30 April 2006 (UTC)

Enormousdude, can you produce a source that defines force as rate of change of momentum? I checked several sources, and none of them define it very well; they certainly don't define it that way. One calls it "a quantitative measure of the interaction between two bodies." Pfalstad 18:19, 30 April 2006 (UTC)

This is Newtonian (and the only) definition. It is over 400 years old. Take any classical mechanics textbook. Also - please don't cite popular literature, Star trek script, or boyscout manuals. Also do not cite introductory textbooks. Force in physics is a mathematical quantity (time derivative of momentum) and can be clearly defined only mathematically. Because it requires a calculus to define it, its definition simply can not be found in introductory textbooks. Sincerely, Enormousdude 00:57, 2 May 2006 (UTC)

Rpf made an excellent point above. With a spring under tension, there is definitely force present (tension is a force), but no change in momentum. Force must be something that CAUSES a rate of change of momentum. It's not equivalent to a rate of change of momentum. Pfalstad 18:22, 30 April 2006 (UTC)

With a spring under tension, there is obviously a reaction force holding it there. There is no resultant force in the system. Therefore there is no dP/dt. Also, Paul, cannot dP/dt cause a force? If so, cannot they be considered equivalent? ;-)--Light current 21:30, 30 April 2006 (UTC)

Of course, dp/dt is the CAUSE behind any force. Just kick your car's tire, for example. Your leg gets pushed back by the momenta of nitrogen molecules inside tire. Enormousdude 01:02, 2 May 2006 (UTC)

I'm not going to get sucked into a debate. No original research. I am not aware of any sources that define a force as a momentum change. Pfalstad 22:59, 30 April 2006 (UTC)

Spoilsport! ;-)--Light current 23:01, 30 April 2006 (UTC)

Feynman (Lectures in Physics, 12-1 "What is Force") says that a mathematical definition of Force can be F=dp/dt but goes on to say such a definition is useless. quoting Feynman

"...we could say that must be due to a 'gorce'-a gorce is the rate of change of position. Now we have a wonderful new law, everything stands still except when a gorce is acting. You see, that would be analagous to the above definition of force, and it would contain no information. The real content of Newton's Laws is this: that the force is supposed to have some *independant properties* in addition to the law F=ma; but the *specific* properties of force were not completely described by Newton or anyone else, and therefore the physical law F=ma is an incomplete law." Rpf 03:21, 2 May 2006 (UTC)

COMMENT ON ABOVE Well, Feynman should have amplified his statement. Everybody in Newton's time KNEW that force had properties other than making things move. Anybody holding up a heavy object (keeping it motionless) knows that intuitively. Newton's big point is that THAT kind of force-- the one that causes pushing and tension and the stresses and pressures we're all familiar with in daily life, is ALSO the "thing" or same stuff, which causes motion in free bodies. And when it doesn't, it's because the bodies aren't really "free" but are rather held back by extremely tricky (friction associated) ways, which amount to hidden opposing forces. That's a HUGE insight. I've tried to give some examples on the Force Wiki which will show how that situation kept people historically before Newton, from using Newton's simple definition of "force" for the ordinary "stress and pull and grunt" type of force that they were used to, from ordinary life, and knew long before Newton came along. THAT is Newton's contribution-- to say that this stress type of force can ALSO always be defined by motion. Feynman almost got around to saying that, but didn't. Sbharris 20:17, 15 May 2006 (UTC)

Acting alone

What is the significance of the phrase 'when acting alone'? THis seems to be causing some controversy.--Light current 21:39, 25 April 2006 (UTC)

Well a number of forces may be operating on an object in such a way that they cancel each other out and the object doesn't move. I can stand on a bathroom scale and clearly see the force of gravity as a reading on the scale, yet I do not move. It is only a NET force that results in a change in momentum, so it is not sufficient to use it as a definition. If gravity was "acting alone" i.e.: there was no ground, then that WOULD produce a certain rate of change of momentum as I fell. Any physics text, including Haliday, Resnick & Walker discusses the distinction of force and net force (HRW extended edition pg 83 "Force"). Also note that HRW defines a force in the same cause and affect manner, not by simply using an equal sign. Rpf 01:21, 26 April 2006 (UTC)

In that case all that needs to be said is 'resultant force' :-)--Light current 01:55, 26 April 2006 (UTC)

Why is force not used in higher-level physics?

Enormousdude says that force is not used in higher-level physics because it is redundant to momentum conservation. If that were true, why would it be used in classical mechanics? At least in the case of QM, I think it's not used because it's easier to deal with potential. Force is definitely mentioned in solid-state physics (exchange forces, for example). Pfalstad 18:02, 30 April 2006 (UTC)

Take three fundamental constants (c, G, h) and combine them to get Plank force. What do you get? Surprise - Plank constant h is not there. You see? - force simply does not exist on quantum level. It is just our macroscopic concept to help understand mutual momentum exchange in interactions.

Enormousdude 01:30, 2 May 2006 (UTC)

Enormousdude Violates WP:3RR?

[4] [5] [6] [7] [8] [9] Pfalstad 18:49, 1 May 2006 (UTC)

Oops, the three reverts have to be within a 24-hour period, and they're not (yet). Pfalstad 19:30, 1 May 2006 (UTC)

Oh dear!--Light current 22:55, 1 May 2006 (UTC)

What is 3 revert rule? My accurate definitions are constantly vandalized even more than 3 times.

Enormousdude 01:32, 2 May 2006 (UTC)

Problem is they are YOUR definitions and nobody else's...including Feynman.Rpf 03:24, 2 May 2006 (UTC)

You are plain wrong (again!). I would be glad to take credit for it :-). Unfortunately, it is a little too late for me (and for Feynman, and for anyone else) - Newtion did it about 400 years ago :-(. Just open any classical mechanics text and you'll see this Newtonian (duh - Newtonian, NOT MINE) definition all over it. So far neither you nor anyone else has shown me any OTHER mathematically correct definition of force. Enormousdude 14:38, 2 May 2006 (UTC)

You need to understand the difference between a definition and a mathematical description. 129.78.64.101 04:47, 3 May 2006 (UTC)

1. What is the difference (tell us if you know)? 2. And how this is related to the topic? 3. What is F=dp/dt definition or description? 4. If description, then what is the definition of the symbol F on the left? 5. Given the definition (you give it in questtion 4) prove using that definition that F = dp/dt. Enormousdude 16:20, 3 May 2006 (UTC)

Click on this to find out about 3 revert rule. Your changes are being reverted by multiple people, so as a group we can revert your stuff more than 3 times a day. Pfalstad 01:37, 2 May 2006 (UTC)

If you like edit wars - that is your constitutional right. But I personally do not think that edit war is beneficial for wikipedia. Your "group" then simply remains ignorant (in the dark about the very definition of subject/object of study). Also, you (and your group) restricts spread of knowledge, and even blocks access to the knowledge by other interested people. So, you literally produce and spread ignorance Enormousdude 16:20, 3 May 2006 (UTC)

If more than one person is reverting you Enormouse this is not strictly speaking an edit war, but you are probably wrong and should discuss things on the talk page. Please be aware that it is YOUR actions of continuous reversion (not the community's) that are seen by the community as vandalism !

--Light current 23:40, 3 May 2006 (UTC)

I do not think that introducing accurate definitions (like F = dp/dt, definition of energy, etc) is a vandalism. Removing accurate definitions like you and your "group" does (and replacing them by inaccurate and incoherent mumbling) is not only a vandalism, but a deception of others. YOUR definition of force currently on display for everyone to see is a disgrace to both your uneducated in physics "group", and as well to the Wikipedia. Just READ your own definition: "A force is that which when acting on a body that is free to move, causes a rate of change of momentum of that body" (?!!). You know, even in introductory textbooks you won't find such inaccurate statement. First, force is not "THAT which causes...". Force is a concrete mathematical quantity. Mathematical - because we use it in equations (look up any mechanics text). Secondly, force does not "cause a rate". How can something "cause a rate of change"? And even if you rephrase it to more accurate statement (like: "force is what causes momentum to change...") there still plenty of ambiguity left. For instance, my hand can cause a momentum (of some moving body) to change. Is my hand a force then? I don't think so. Hand and force are DIFFERENT. A magnet can change momentum. Is magnet a force? No, it is not. And lastly - why to invent a triangular wheel (=introduce very unclear proprietory definition) - when a round one was invented long ago (the clear definition of force (F=dp/dt) already exists for over 400 years since Newton)?. If to follow your logic (that a "force is that which..."), then instead of saying that velocity is the derivative of position (v=dx/dt) we must define velocity as "that which causes change of position", or "that which causes rate of change of position". What kind of definition is this? I guarantee that YOUR definition WON'T LAST. When someone with high school (or better) background in physics comes across, he or she WILL CORRECT your definition. I tried several times to write correct (and the only existing in mechanics) definition - but you and your group always deleted or reverted it. What else shall I do? Ignorance is a bliss" - is this your motto? Now you have YOUR OWN laughable and dumb definition on display for everyone. As Ponti Pilates said - I wash my hands.

Sincerely, Enormousdude 20:24, 4 May 2006 (UTC)

It appears you have not yet developed the eloquence (or spelling ability) necessary to convince fellow editors of your viewpoint!--Light current 20:32, 4 May 2006 (UTC)

I do not need to convince anyone. If you and your "team" is 400 years behind in physics, and none of you know elementary definitions of mechanics (not to say, definition of energy and more advanced quantities) - there is nothing I can do here. You simply need to take physics class.

By the way, do you have anything to say about the subject (F=dp/dt)? Or shall we change definition of velosity to (following to your logic of definition of force) "velocity is that which causes a rate of change of position"?

And, by the way - your spelling is not better. Despite that English is your native, by the way. May I ask you to write a couple of sentenses in Russian?

Sincerely, Enormousdude 00:25, 5 May 2006 (UTC)

Dude, my spelling sucks. However, I know how to use a spell checker. It isn't hard. Nonsuch 01:56, 5 May 2006 (UTC)

Im afraid that to get your version of things onto the page you do need to convice us. You seem not to know that WP works by means of consensus. Perhaps you should try explaning your points on the Russian WP?--Light current 08:16, 5 May 2006 (UTC)

Oh, look it up the article "Force". You nonsense definition: "A force is that which when acting on a body that is free to move, causes a rate of change of momentum of that body" has already been removed and is practically replaced with my definition (that force is defined as the rate of chanfe of momentum). I told you many times that your definition is incorrect and mine is correct. Enormousdude 15:18, 14 May 2006 (UTC)

Introduction

I think your missing the point, dude. The object of this exercise is to place a clear and concise definition of force in the introduction. I don't think that we're objecting to your definition as such, but rather the lack of eloquence and clarity. The current consensus is (I think) that although the current introduction isn't ideal, your suggestions aren't obviously better.

Personally, I think that we should avoid any math in the introduction, if at all possible. That leaves room for ambiguity (Mathematics is the language of physics, as they say. (Although I disagree with the view that physics is mathematics, anymore than mathematics is physics.)) but this is the introduction we're talking about right now.

The central confusion, I think, is the difference between force and net force. Only a net force causes a net acceleration, since forces and accelerations can cancel. It's a little counter-intuitive that gravity is accelerating me downwards at 10 m/s^2, but the floor is accelerating me upwards at an equal rate, so that mostly I just sit here not changing velocity at all. That's why many introductory texts talk about net force, or use phrases such as "acting alone", I think.

Anyways, lets hash out an intro. Feel free to edit the following proposal. Nonsuch 21:20, 4 May 2006 (UTC)

Well put!--Light current 23:20, 4 May 2006 (UTC)

In physics, a force is anything that causes a body with mass to accelerate. Intuitively, a push or a pull. For instance, a person holding a dog by a rope is experiences the force applied by the rope on his hand, and the cause for its pulling forward is the force exercised by the rope. Quantitatively, force is a vector quantity defined as the product of the mass and the acceleration induced by the force. The SI unit for force is the newton. The net (or resultant) force is the sum of all the different forces acting on a body. For example, while standing still gravity pulls downwards, but the floor pushes upwards, and the net force is zero. Force should not be confused with stress. Nonsuch 21:20, 4 May 2006 (UTC)

O.K., I'm adding the above paragraph to the introduction. It's not perfect, but it's better than what we have now. Nonsuch 23:44, 4 May 2006 (UTC)

Yeah, Enormousdude has somewhat of a point; I checked Marion and Thornton, and it says that Newton's second law is an "operational definition" of force. But it also says that that is not the only possible interpretation; some see newton's second law not as a definition, but as a physical law. And we saw the quote from Feynman, saying that defining force with Newton's second law works mathematically but is useless. The situation is not as simple as Enormousdude suggests. Another problem is that Enormousdude's definition is totally unsuitable for an introduction of an encyclopedia article. It has to be introductory! One freshman-level physics text calls it ""a quantitative measure of the interaction between two bodies." I disagree with Enormousdude that we should not look at introductory physics texts.. Of course we should! Those texts are aimed at the same level of readers that we are! But we need to talk about the operational definition somewhere lower down. Pfalstad 02:39, 5 May 2006 (UTC)

I do have to object to the idea that Newton's insight is "just" a definition of "force" and not a law of nature, so perhaps doesn't merit the attention given it. You totally miss the point, as has been pointed out. In Newton's time people were acquainted from ordinary experience with all kinds of forces that often didn't change momentum or velocity. Weight was one of them (perhaps the most common). And add to that, all of the tensions associated with material forces in static situations. Newton is not here "defining" force--- that was already defined well enough in everybody's experience, as tensions or weights. What Newton pointed out was that this quantity that people knew, as tension or weight, produced momentum or velocity changes in free bodies, AND Newton gave a quantitation of that by saying that the amount of momentum change is PROPORTIONAL to the force. Double the force and double the momentum rate-change. Both of these are new insights and (yes) they are laws of nature, because they connect long-known separate quantities in ordinary experience to each other, in a way which isn't obvious and wasn't previously known, quantitatively.

If you think not, consider the relation between force and acceleration. Is it "obvious" that doubling the force doubles the acceleration? I hope not, because Einstein showed that at high velocities it actually isn't true. Newton's law speaking of momentum change, however, remains true. These are laws of nature. You can write them down, and they might turn out to be be true, or they might not. You have to do an experiment to see. That's not how mere "definitions" behave. Sbharris 17:41, 15 May 2006 (UTC)

We've been having a lively discussion here about the definition of force, and whether the second law is a definition of it or not, and whether force can be defined mathematically. I don't think the 2nd law fully defines force. I completely agree with the rest of your points, although they don't seem on topic to what I was talking about above. Pfalstad 20:37, 15 May 2006 (UTC)

Examples

I was asked why I used a pitcher of water as the object of my examples. I wanted something that weighed a couple of kilos, so that it would be obvious that it would NOT move if you poked it lightly with a finger while sitting on a table, but WOULD move at a standard slow frictional rate if you shoved it harder. I used water because it's a common object with a uniform density, and a couple of kilos is a bigger container than a glass of it. If you have a better object to use as an example for these resistive and variable frictional forces which screw up the immediate idea of defining force in terms of motion, feel free to suggest them. My examples are aimed at simply trying to elucidate why Newton was a genius, and why his second law, in a world with no zero-g demos or air-hockey games, was genius.Sbharris 23:56, 15 May 2006 (UTC)

We're talking force. Not why Newton was a genius.8-|--Light current 00:21, 16 May 2006 (UTC)

We're talking about whether or not Newton merely proposed a definition for "force" which could have been any word, even a new word (like Feynman's "gorce") which actually explained nothing about the behavior of objects, and taught nothing, and provided no insight, but was just another potential made-up dictionary entry serving no purpose. (This is very much like the Feynman "wakalixes" story for "energy," when he didn't like how a textbook treated THAT concept).

I assert that this isn't the case, and that Feynman somehow misses the point with Newton's idea of defining "force" the way he did. Newton actually took an old idea and an old word, and asserted as a new law of nature that it could be COMPLETELY described, not by scales or balances or rope pulls over pulleys (the way people had previously tried to define it), but INSTEAD, by a very simple quantitative behavior and effect on objects--- BUT one that that people almost never saw it actually produce! And when they did see it, didn't recognize quantitatively because it went too fast (thus, it took Galileo and his inclined planes slowing things down, to get the math right). Newton then required a large part of a classic book to explain how this new idea for defining force by a new description of its behavior in causing motion, wasn't crazy, and really did work. It's all so counterintuitive that most high school students, when first exposed to it, still don't "get" it. Hopefully the Wiki helps. Sbharris 02:31, 16 May 2006 (UTC)

No. On the article page we're talking about force. Other people invesigated force as well as Newton. People still are investigating it, it seems. We should concentrate on 'force' on this page and leave out the Historical controversies if possible. Perhaps some of yout input could go on the Isaac Newton page.8-)--Light current 02:44, 16 May 2006 (UTC)

It could go on the newton's laws of motion page. Pfalstad 12:53, 16 May 2006 (UTC)

Yes--Light current 13:42, 16 May 2006 (UTC)

We are talking about Newton's contribution (his equation or law) to the understanding of force. In the same way that we would talk about Maxwell's equations when talking about electromagnetic radiation, or Newton and Einstein's equations when talking about gravitation or energy. Go to the Wikis on those subjects. Do you see mention of those people and their math? But other people studied EM radiation and gravity as well. Why isn't there more about THEM? Sbharris 16:21, 16 May 2006 (UTC)

Wrong definition

In the present article we define a force as anything which causes a free mass to accelerate. Which would include gravity. But that's a wrong definition, because gravity is not a force. It's a property of space and time, which is why all objects (no matter how massive, or even if they have no mass at all, like photons) all accelerate at the same rate, on the same path in a gravity field. They're not being pulled. On the contrary, one must pull on them to STOP this behavior. Wouldn't it be nice to start out an article with a definition which isn't wrong? We owe at least that to our readers. Sbharris 05:15, 14 June 2006 (UTC)

This isn't the only way of thinking of gravity. You can treat it like any other field if you consider the massless spin 2 graviton as the particle being exchanged in the interactions. Sorry, it is a force. Rpf 07:37, 14 June 2006 (UTC)

If you look at MTW, you will see that a simple massless spin-2 graviton theory does not lead to Einstein's field theory, and is therefore wrong (any good quantum theory must reduce to GR in the field theory limit). No graviton theory has been constructed which does this. If you've got one, you win automaticic Nobel. Be sure to publish first.

I think you are confusing the bahaviour of gravity on a quantum level and a gauge boson derivation of the field equations. I was under the impression the Venutian theory ends up relating $R^{\nu \mu }$ to $\lambda ^{2}T^{\nu \mu }$ in the usual way. You don't need to start with an inertial interpretation of gravity to get the right answer. Yes you get the same old curvature equations Einstein did, but your interpretation need not start with equivalence, you can start with gauge invariance and general behaviour. Point: no need to be militant about interpreting gravity as a ficticious force to get the same answers. Rpf 05:37, 17 June 2006 (UTC)

The Schwarzchilds's Radius is a nice example of how a photon behaves like a free particle under the influence of gravity with an escape velocity. Rpf 07:57, 14 June 2006 (UTC)

That's like saying the Bohr radius is a nice example of how the electron behaves like a planet, because the right size and energy numbers are generated by an incomplete theory which makes many bad (and a few wrong) assumptions. Bad logic.

You're right, the classical approach to deriving the escape energy is out by a factor of two, but the CORRECT approach gives you a value which happens to be precisely the same as the escape velocity of a free particle. You can argue all you like about "truth" in a theory, but what matters to science is the predictive power and the usefulness of an idea. Rpf 05:37, 17 June 2006 (UTC)

I have to agree with Rpf. Force is an old concept anyway, and there is not much mention of it in modern HEP, so using classical ideas about force is not a problem. If I use a standard force meter, eg a spring, then I can measure the force of gravity as easily as I can measure any other force. The Force article is certainly not the place to explain GR, and going half-way - by denying gravity as a force - is even worse - it is just confusing. LeBofSportif 09:14, 14 June 2006 (UTC)

If you use a spring you're not measuring the force of gravity, you're measuring the force of the spring which is required to keep an object from moving in its natural inertial trajectory. Jump off a building with it, and you'll see there's no force at all.

The issue need not be confusing if we start by saying gravity acts in some ways like a force, and in some ways not (as with light, which is bent twice as much as it should be, and acted upon even though it has no mass). Later, the reader can be pointed to the relevant GR pages. Explaining physics simply is no excuse for saying something that is flatly wrong. Be as simple as possible, but no simpler! Sbharris 18:06, 14 June 2006 (UTC)

It is only after considerable cerebral masturbation that one can conclude that the force of gravity is not really there, and we are feeling our efforts to maintain us in an accelerating SR frame. This view is totally theory dependent. Force is not well defined anyway, and you want to introduce a level of precision to the article which is not warranted. LeBofSportif 07:45, 15 June 2006 (UTC)

In the Feynman lectures book I, 12-7, he refers to "gravitational force" as being fundamental. On 12-11 he has a discussion about whether "gravity itself is a pseudo force", but he doesn't stop calling it a force. If centrifugal force can be considered a force, then certainly gravity can. Pfalstad 15:11, 15 June 2006 (UTC)

Well, I'd object to calling centrifugal force a force. You can dispense with the whole notion, and forget there is such a thing. There is a centripetal force which keeps you from traveling in a straight line, and makes you go in a circle instead. But you can see that easily-- it's just a tension in a rope or whatever. That's all that's needed.

When you're in an accelerating rocket, the floor presses against your shoes, and your shoes against the floor. That single force vector exists entirely because the rocket wants to go someplace that you don't naturally want to (i.e., the floor is interfering with your natural inertial motion). So there's only one force vector there, and that's it. There is no "artificial gravity" force which is pushing you down toward the floor, and yet another opposite force provided by the floor, which is keeping you up. The "artifical gravity" in an accelerating rocket (or a rotating space station) is an example of a "pseudoforce", but such forces are called "pseudo" because they are figments of the imagination of people who think they are in an inertial frame, feel the force of the floor against their feet, and CONCLUDE because they don't seem to be moving, that there must be some balancing force somewhere else, to keep them from moving. Well, that's not necessarily true. If you like, you can dispense with the whole idea by dispensing with some of the underlying assumptions.

Gravity is the same way-- it is indeed a "pseudoforce" which people invented by "explain" seemingly inertial motion which actually doesn't NEED explaining, because it's not actually inertial motion. Inertial motion near a mass is free-fall. When you're standing on the ground minding your own business, you're not in inertial motion. You're not a free body which is not accelerating just because you have two sets of forces which balance, acting on you. You are, rather, accelerating to keep you from traveling inertially, which would be your natural state of motion if the ground didn't get in your way. Just as you don't NEED a force of artificial gravity to explain why the rocket floor presses against your shoes (it does, and that's why you're accelerating), you don't NEED a force of Earth's gravity to explain why the Earth presses against your shoes. You should accelerate in curved spacetime, and a force is only required to KEEP you from doing so Sbharris 18:31, 15 June 2006 (UTC)

What is 'cerebral masturbation' Is it the same as mind 'boggling'? Ive always wanted to know what 'boggling' meant.--Light current 15:16, 15 June 2006 (UTC)

I'm not disputing your physics, only your terminology, and the wisdom of introducing GR into the opening sentences of an encyclopedia article on "force". Pfalstad 21:06, 15 June 2006 (UTC)

Well, it's introduced in the opening paragraph of the article on gravity (which redirects to gravitation) and it's introduced for the express purpose of saying that in the view of GR, gravity is not a force. So here you're going to do an article on force for the same encyclopedia, and define it for the reader so that gravity is a force. Do you call that "wisdom"? Why not simply mention the caveat? It takes one sentence.

Of course GR is discussed in an article on gravity. But we're not talking about gravity, we're talking about force. Why introduce something about gravity in the introduction? And if you read the gravity article more carefully, you'll see that whether or not gravity is a force is dependent on which theory you are using (GR vs. quantum gravity). Also, note that the first sentence of this article is talking about a classical force. I object to the caveat because it is not needed and is not appropriate to the level of the article (esp. this point in the article). Pfalstad 00:14, 16 June 2006 (UTC)

There is no such thing as "quantum gravity". It does not exist since there is no theory. Might as well refer to the theory of "Harry Potter gravity."

Why talk about gravity in the intro? Because it's an important force. I think these pseudoforces are very relevent to the article's level. You're going to hit them anyway. Do the the people in a Vomit Comet airplane float around because they have two forces on them which cancel? No. Do the people in an orbiting spaceshuttle float around because they have two forces on them which cancel? Not really-- at least not in their frame. Yet they're still way too close to Earth for "classical" gravity not to act on them (it's about 90% as strong at that altitude). So how do you explain? Easiest to say that in the space shuttle inertial frame (or any free fall frame), all but microgravity is absent. No forces at all to first approximation, so it looks the same as if you were way out in space. Equivalence principle. Not THAT complicated.SBHarris 06:23, 15 July 2006 (UTC)

Effect is a noun, affect is (usually) a verb

The effect of a knock on the head, may be to affect one's judgement.Sbharris 23:50, 14 June 2006 (UTC)

What about to 'effect' a change/ Is that a verb or a noun 8-)--Light current 00:04, 15 June 2006 (UTC)

Okay, effect can also be used as a verb, but it has to be used in the fashion you do. You can effect an effect, though it sounds stilted to say so. You can also affect an affect (eg. by smiling at a depressed person). But it's usually best to use the common verb and noun forms. And some stuff is just wrong. You can say the effect of gravity is to affect weight. But saying the affect of gravity is to effect weight, is just wrong.Sbharris 17:56, 15 June 2006 (UTC)

Then of course you can have the adjective : effective. Yes if the grammar is wrong, just fix it no need for a fuss!--Light current 22:04, 15 June 2006 (UTC)

Those tricky frictional forces

I've semi-reverted this several times, trying to make the examples illustrate the problems our scientific forebears faced. Please remember that Newton's second law arrived so late because friction is so tricky. Static friction PERFECTLY adjusts to balance forces, until it breaks. THEN, as if Nature hadn't been tricky enough there, SLIDING friction shows the same behavior, more or less, allowing velocity to increase quickly until THIS force "adjusts" upward to also perfectly oppose motive force. So now velocity is constant (terminal velocity) and acceleration is zero. Both of these totally incorrect for a free object with no friction. But the usual and normal behavior for objects of ordinary experience: 1) Either it doesn't move at all when you push it, or 2) it takes off and moves at a constant terminal rate. No wonder it took so long to figure out the correct relationship between force and motion. SBHarris 05:27, 12 August 2006 (UTC)

Dear SBHarris, what you are saying is correct for velocity-dependent forces like air resistance, but not for kinetic friction. The kinetic friction force is given by $f_{k}=\mu _{k}N\,$ where $N\,$ is the normal force and $\mu _{k}\,$ is the coefficient of kinetic friction. For a horizontal table, $N=mg\,$, and $\mu _{k}\,$ is a constant that depends on the properties of the surfaces of the object and the table, but not on the velocity. Therefore the force of kinetic friction is fixed. To create a uniform motion, the force of the finger is adjusted to be equal to the kinetic friction. Yevgeny Kats 06:30, 12 August 2006 (UTC)