Wikipedia:Reference desk/Archives/Science/2009 August 5

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

August 5

Knowing Allsorts

I've just been watching the film 'Knowing'. Is it possible that a solar flare would burn up the earth to such an extent as detailed in the film. When I say possible, I am referring to the likelihood of it occuring, rather that the damage caused by such a large-scale event. --russ (talk) —Preceding undated comment added 00:18, 5 August 2009 (UTC).

"Solar flare" is a pop-science term for a coronal mass ejection (it can also mean the less-severe, closed-loop solar prominence and some related phenomena). Gross quantities of solar ejecta are unlikely to ever reach Earth's orbital radius in any significant way. (Read: no giant flameballs will reach us). However, the density and flux of charged particles will increase; and an electromagnetic effect is common; these effects can significantly harm objects in space near Earth. It is very important to understand that solar wind is a charged plasma - as such, it is "deflected" (rather, trapped in cyclotron resonance) by Earth's magnetic field - and the result is the Van Allen belts. During periods of high solar activity, these regions increase in size, energy, and particle content. Also, it's worth noting that 2007-2009 has been "the quietest Solar Minimum ever"[1] - so if there were going to be a catastrophic solar flare (or even any medium-large ones), it would be really unlikely timing. Nimur (talk) 00:36, 5 August 2009 (UTC)

I haven't seen that movie but there is a story called Inconstant Moon on a similar theme. 70.90.174.101 (talk) 07:32, 5 August 2009 (UTC)

I also have not seen the movie, but the book Death from the Skies has a chapter on the actual threat posed by CMEs. I don't have it in front of me, but here's what I recall: the most likely result of a severe CME directed at Earth is rendering satellites inoperative. More severe CMEs could potentially cause widespread damage to the power grid. A particularly severe CME in 2003 was the most violent flare recorded in modern times: it is described here, but there's a good chance you had no idea that anything happened. CMEs are common, and obviously we get smacked by them from time to time. No CME will incinerate the world as in Inconstant Moon; that more properly describes the sun going nova (story claims notwithstanding). We have no reason to expect the sun to go nova. In about one billion years, the sun's energy output will have risen enough (10%) to render Earth uninhabitable; in about five billion years, it will become a red giant. See our article on the sun for details. — Lomn 13:13, 5 August 2009 (UTC)

I've read that Earth will have frozen over in 100 million years due to the cooling of the radioactive core. Imagine Reason (talk) 20:47, 8 August 2009 (UTC)

If earth will survive over white dwarf we don't know for sure. This all depends on how big sun will be. The latter model shows earth might be destroy by tidal force. Original models shows earth could escape to a highier orbit by sun's loss mass and gravity. A good way to state it is earth will be swallow up by sun about a 55% chance. Doesn't matter. Earth will end up been lifeless.--69.229.108.245 (talk) 23:31, 12 August 2009 (UTC)

Steel yourself for this question

When I was a youngster, my family watched too much TV, as evidenced by the fact that we even watched crap shows like That's Incredible! and Real People. Anyway, it was on one of those shows (or similar) that I saw a segment about this dude that could (briefly) touch molten steel with his bare hand. I believe he worked in a steel mill or something of the sort. Anyway, they showed him on camera, quickly flicking his fingers across the liquid metal, flinging globs of steel. I don't think it was a video trick (though I was very young at the time). Does anyone remember the name of that guy or how he did what he did? Firewalking works because ash is actually a poor conductor of heat, but the same is definitely not true of iron/steel! Matt Deres (talk) 00:24, 5 August 2009 (UTC)

I've never seen it done with steel - but I've seen people stick their hands up to the wrists in molten lead - which is probably just as bad. The trick with molten lead is to have your hand be wet - as the water flashes to steam, it insulates your hand from the heat of the metal. It's still insanely hot - and you can't hold your hand there for more than a second or so...but if you know what you're doing it's possible. However, iron melts at 1,370 degC and lead at only 320 degC - that's a very different matter! So I'm frankly a little skeptical about the steel thing - but perhaps if your memory is imperfect, it could have been some lower melting point metal than steel. I certainly don't recommend experimenting! This is an incredibly dangerous thing to try. SteveBaker (talk) 01:01, 5 August 2009 (UTC)

Is there a physical chemist in the room? Does liquid metal have the same thermal conductivity properties as solid metal? If you don't have the same delocalised electrons as you do in a metallic lattice (I'm not sure if you do or not) then the conductivity would be much lower and it would be far easier to touch molten metal than you would expect based on solid metal. --Tango (talk) 01:28, 5 August 2009 (UTC)

76.21.37.87 here (with a new IP address); while I'm a petroleum chemist, not a physical chemist, I'm pretty sure that molten metal has the same delocalized electrons as solid metal, so there's no reason for thermal conductivity to be much lower -- if anything, it'll prob'ly be higher. Besides, with molten metal you also got convection adding to the heat transfer, so you got a lot more of the heat going from the metal to your hand! My advice is, DON'T TRY THIS AT HOME!!! 98.234.126.251 (talk) 01:59, 5 August 2009 (UTC)

I read a book by a 19th century magician, perhaps Robert Houdin, who asserted that molten metal would roll of the hands as described here. At the time I suspected it was a ploy by him to get his rivals to burn their hands to smoking cinders so he would get all the bookings for magic shows. DO NOT ATTEMPT ANYTHING REMOTELY LIKE WHAT IS DESCRIBED. Edison (talk) 02:52, 5 August 2009 (UTC)

As everyone else already says, don't try it. But I've heard the Leidenfrost effect as an explanation of what you're describing. 70.90.174.101 (talk) 07:36, 5 August 2009 (UTC)

It was actually demonstrated on Time Warp (TV series) very recently (I think the episode title is "Hot Stuff and Cold Steel") - the guy gets his hand soaking wet - then calmly dunks it up to the wrist in molten lead - and pulls it back out again within about a second. I've heard of this being done by many people in many situations - it certainly works (although it's obviously dangerous). This is a very different thing than (say) a blob of molten metal landing on you (where the speed of it's arrival could cause it to penetrate the layer of water - or which might allow the steam to escape around the sides. The demonstration is very specific and precise details matter. The guy who did it said that it does hurt quite a bit - and he has what looks like a severe sun-tan on his skin afterwards. SteveBaker (talk) 14:17, 5 August 2009 (UTC)

I think it's a little funny that everyone feels so important to add "don't try this at home!!!" As though, you know, we all have molten iron (at 1300 degrees C) around the house :) 82.234.207.120 (talk) 09:04, 5 August 2009 (UTC)

I guess what I'm getting at is that anyone who has molten steel around would already know :) —Preceding unsigned comment added by 82.234.207.120 (talk) 09:06, 5 August 2009 (UTC)

Of course don't try it. But molten metal touches skin often by accident. (i) When soldering connections in electronics, excess solder may fall on the hand i.e. tin/lead at about 200 °C. (ii) When welding, molten steel may fall on an unprotected part of the body i.e. steel at 1400 °C or more. I can report that (i) hurts and leaves a burn. (ii) hasn't happened to me yet. Cuddlyable3 (talk) 09:26, 5 August 2009 (UTC)

To this day I do not understand how it happened, but once when I was around 15 I was near my uncle while he was welding a steel fence post together. I was supposed to paint over the welds after they cooled. However, after the third one a bit of molten steel flew nearly 15 feet and hit me on the arm. I felt like I had been shot (and yes, I know what that feels like as well, but thats a different story). Considering the molten steel flew a good distance, thus cooling quite a bit, I would say that unless the guys nerves where dead he probably wouldn't be able to touch the molten steel without at least showing some sign of pain.Drew Smith What I've done 10:16, 5 August 2009 (UTC)

We can speculate on how it happened: molten metal dripped from the weld point onto a surface which was either wet, or painted. The heat of the metal vaporized the water or paint, which provided sufficient impetus to launch it in your direction. We'll not, owever interested, speculate on the shooting ;) --Tagishsimon (talk) 13:03, 5 August 2009 (UTC)

There is no question that in general, molten metal will burn your skin. The particular trick that's being described here requires an extremely special set of circumstances for it to work. SteveBaker (talk) 14:17, 5 August 2009 (UTC)

A less special set of circumstances would be for the molten metal in question to be mercury. ;) --Tango (talk) 04:04, 6 August 2009 (UTC)

Of course, sticking your hands in mercury is not a great idea either. --Trovatore (talk) 04:18, 6 August 2009 (UTC)

Question: was colour was this molten steel? red orange?83.100.250.79 (talk) 16:09, 5 August 2009 (UTC)

As Steve noted, it's been a long time since I saw the piece (~quarter century). Any details I might recall are obviously susceptible to imagination, etc. My recollection is that the metal was lighter than that, more yellow-ish than orange. Steve also mentioned getting the hands wet before performing the stunt. Upon seeing that comment, I do recall the guy dipping his hands in a barrel of water. I had assumed or conflated the idea that he was dipping them in water afterwards, but it very well could have been beforehand; my memory is just not that clear on it. Is the TV show scene ringing any bells for anyone? If I could determine the guy's name, it might help me research it. And don't worry folks, I've taken both AAA batteries out of my Acme U-Smelt-It just to prevent curiosity from getting the better of me. Sheesh! :-P Matt Deres (talk) 16:38, 5 August 2009 (UTC)

It's not just the heat- but the density of the molten steel - assuming he could flick it without being burnt, it would (I imagine) be like flicking a concrete bollard - sorry for my unbelieviness.

Unless of course it was this guy [2]83.100.250.79 (talk) 17:36, 5 August 2009 (UTC)

I'm sure I heard somewhere that a guy rolled molten lead around in his mouth. Is this possible? Vimescarrot (talk) 18:28, 5 August 2009 (UTC)

There are a handful of YouTube videos of people doing this with lead...a bit more tentatively than the guy on Time Warp...[3]. SteveBaker (talk) 20:42, 5 August 2009 (UTC)

Hmm. On the one hand, it seems that the stunt is much more likely to be done with lead than with iron or steel (assuming it wasn't a literal "trick"). On the other hand, I have a very low resolution video in my memory banks of a guy playing with yellowish orange molten metal, flicking small globs of it with the tips of his fingers. The more I consider it, the less likely that seems. Unfortunately that makes me all the more curious about the original stunt. Did I assume it was steel because it was shiny and metallic and then over the years alter my memory to make it appear as I now know molten steel to be like? Or was it someone actually playing with molten steel? Or was it an actual trick in the normal sense? Matt Deres (talk) 23:41, 5 August 2009 (UTC)

"My recollection is that the metal was lighter than that, more yellow-ish than orange." -- In that case, it could not have been molten steel: at the temperatures required to melt iron, the metal is hot enough to glow a brilliant white that hurts your eyes just to look at it. It could've been copper, or maybe manganese, but not iron. 98.234.126.251 (talk) 02:41, 6 August 2009 (UTC)

As someone said earlier (though its somehow been ignored) this is probably through the Leidenfrost effect. If you pour a drop of water on a really hot stove the bottom of the water drop will boil immediately and you will have some water vapor between the drop of water and the stove, insulating the water drop. If you put water on your hand and dip it in the metal it will insulate your hand temporarily. It's easy to demonstrate. Take a normal hot plate (the one's at my school worked fine and they're old) and heat it up for a while. Then pour a drop of water on it. If it's hot enough the water will skittle across the plate, like it's bouncing. It's really cool you should try it. 66.133.202.209 (talk) 04:01, 6 August 2009 (UTC)

...But would the steam provide enough insulation to keep out the heat of molten copper? 98.234.126.251 (talk) 04:52, 6 August 2009 (UTC)

Yes, obviously...but the question is: for how long? Evidently (from the dunking your hand into molten lead demonstrations) - it provides enough insulation to allow you to withstand 400 degC temperatures for a second or two with only slight damage (the guy said it felt like sunburn). The effect isn't going to last much longer than a second or two anyway because there isn't a continually replenished supply of water - and in any case, the steam itself is hot enough to do you some pretty severe damage. It's not impossible that this trick would work with steel or copper...but we only have actual evidence of it being done at much lower temperatures using lead. It might be (for example) that because molten steel isn't a dense as molten lead, that the steam layer would be at lower pressure in the case of steel - that would provide a larger volume of protection than in the case of lead - and that might be enough to allow you to survive the much higher temperatures. It's a tough thing to guesstimate - so I wouldn't completely rule out the possibility of doing the trick with molten steel - but my gut feel is that it wouldn't work. However, gut feels are not science. SteveBaker (talk) 13:24, 6 August 2009 (UTC)

Catching flu twice

Is it possible that someone suffering from the current swine flu pandemic strain could fall ill twice? Should one fall ill once, would the immunity gained from combatting it be sufficient to prevent them from falling ill a second time? --russ (talk) —Preceding undated comment added 00:25, 5 August 2009 (UTC).

It might be possible...or at least it might seem to be so. The problem is that identifying the flu strain is pretty tough - the symptoms of H1N1 are pretty similar to 'normal' flu strains - it's possible that the first dose someone got wasn't swine flu at all and they'd just been misdiagnosed - it's also possible that the virus is mutating and mixing with other flu strains - and that too could result in a "new" Swine flu which even people who recovered from the original strain might not have immunity to. So it would be risky to assume that you had immunity. On the other hand, it's believed that people over the age of 52 may have some degree of immunity left over from the 1958 Asian flu - so it's certainly possible that someone who had it once before is now immune. SteveBaker (talk) 00:53, 5 August 2009 (UTC)

There are lots of different strains of 'flu and catching one often gives partial immunity to others. There are strains of H1N1 that are endemic in human populations and have been since long before the recent "swine flu" came on the scene. If at some point in your life you have caught one of those strains (which isn't unlikely, especially for someone in their 50s or older than has had plenty of time to be exposed to various strains of flu), that could well mean you won't get as seriously ill if you now catch swine flu. You shouldn't be able to catch exactly the same strain twice and if you catch a slightly mutated strain you'll still have partial immunity. If you do get flu twice in quick succession, chances are it was a completely different strain. Now, all that aside, there is one important thing to remember - it's just the flu, the cure is bedrest, simple as that (unless you are in a vulnerable group). If in doubt, ask a doctor (or a call centre worker who did a 2 hour course on how to read a script and is now qualified to dispense prescription drugs to people that don't need them). --Tango (talk) 01:19, 5 August 2009 (UTC)

Help identify this aquatic creature

Help identify me!

We received this inquiry via e-mail. The image was donated to us, but part of the stipulation was that we help identify what it is. So please, go at it! It looks like a green sand dollar, only it has long legs (maybe like a sea star), photo was taken at Bali, at Nusa Dua beach. Thanks. Please send me a note on my talk page if you think you know the answer. -Andrew c ^[talk] 04:46, 5 August 2009 (UTC)

Here is what looks like a picture of it. Bus stop (talk) 04:56, 5 August 2009 (UTC)

This too looks like it. Bus stop (talk) 05:03, 5 August 2009 (UTC)

It's called a brittle star, and is in the class Ophiuroidea. —Preceding unsigned comment added by CalamusFortis (talk • contribs) 05:17, 5 August 2009 (UTC)

Hey, thanks everyone!-Andrew c ^[talk] 13:49, 6 August 2009 (UTC)

Forces

Why do forces obey the superposition principal? —Preceding unsigned comment added by 76.69.240.190 (talk) 05:06, 5 August 2009 (UTC)

We observe that this is empirically the case, and set up a mathematical framework based on that assumption. To date, everything which we define as a force tends to obey this mathematical rule, so there's no reason to assume it's invalid. On the deep subatomic scales, more precise definitions of force are necessary (usually a more complex set of physics, like Hamiltonian mechanics is used - where forces are defined as a gradient of a potential field. In the case of certain nuclear interactions, a potential field cannot be defined, so the simple linearly adding forces are probably not applicable to the deep sub-nuclear scale, where really strange quantum physics applies. Nimur (talk) 05:21, 5 August 2009 (UTC)

Also, gravity, which is generally considered to be one of the four basic kinds of force, doesn’t really obey the superposition principle except in the Newtonian approximation. Red Act (talk) 07:21, 5 August 2009 (UTC)

This is not true when one approaches light speed. Imagine Reason (talk) 20:49, 8 August 2009 (UTC)

Thermal Conductivity

According to Thermal Conductivity, the thickness of a material has an influence on thermal conductivity; k. I modelled the equation in Excel using the formula:

k = Q/t * 1/A * x/T as stated in Thermal Conductivity. However, when everything else is constant, increasing the thickness (x) causes an increase in the thermal conductivity and a decrease in thermal resistivity (the reciprocal of k). This seems counter intuitive - I would have thought increasing the thickness of a particular material would cause a decrease in the thermal conductivity as the distance between the source of heat and the colder area would be greater.

Is this an error in my maths or is there something else I am missing? —Preceding unsigned comment added by 157.203.42.175 (talk) 12:29, 5 August 2009 (UTC)

You need (heat flow) Q/t = kA (T2-T1) / x

Increasing the thickness decreases the heat flow all other things being equal.

Q/t (the heat flow) differs depending on the thickness. So it's not independent of x

Thermal conductivity is constant for a given material - the heat flow differs.

You're mixing up thermal conductivity k and heat flow Q/t

83.100.250.79 (talk) 12:40, 5 August 2009 (UTC)

And, of course, whether the thermal resistance (or conductance) of the material increases or decreases depends whether the direction of heat flow is alomg the direction of increased thickness or at right angles to it. Dbfirs 01:47, 7 August 2009 (UTC)

Transverse Processes

Hi everyone I am a little confused about where transverse processes are located on a human vertebrae. The article on transverse processes states: (Hope its ok for me to copy and paste this) 'The transverse or costal processes of a vertebra, two in number, project one at either side from the point where the lamina joins the pedicle, between the superior and inferior articular processes.' Take a look at Grays picture of a cervical vertebrae in the article. I don't know if its just me but I think the Grays picture and the description don't match! The transverse processes don't project at either side from the point where the lamina joins the pedicle. The point where the lamina joins the pedicle is where the articular processes are. I would say that the transverse processes are posterior to the pedicle. This is just a subjective speculation. Can someone else offer an input? Thanks in advance to anyone who helps RichYPE (talk) 12:54, 5 August 2009 (UTC)

I agree that the description does not easily match a cervical vertebra but a cervical vertebra is significantly different from a thoracic vertebra where the description does seem to tally quite well. I guess it is not easy to describe transverse processes on vertebrae without qualifying which type of vertebra you are describing. 86.4.181.14 (talk) 13:34, 5 August 2009 (UTC)

Last's anatomy shows the cervical vertebrae with the "true" transverse processes projecting forwards (anteriorly) from the posterior end of the pedicle. The "anterior" part of the transverse process is described as the anterior/costal bar. The transverse processes of the cervical vertebrae are, if anything, anterior to the pedicles. However you shouldn't be too worked up about this. It's only important if you're going to become a spinal surgeon. Axl ¤ [Talk] 17:55, 5 August 2009 (UTC)

PiCCO cardiac monitoring

I am looking for information for an essay regarding PiCCO cardiac monitoring.

I am also looking for the following information for the same essay:

(i) What does 'stroke volume variation" tell us? Especially a value of 15% , and a value of 9%?

(ii) normal valyues for extra vascular lung water. I have fgound articles about this but no 'normal' reference figures, or information about what a value of 7ml/kg or 5ml/kg would indicate.....

(iii) Intrathoracic blood volume (ITBV). Again, I have found articles about this but no 'normal' reference figures, or information about what a value of 820ml/m2 or900ml/m2 would indicate.....

I would be very grateful for any information that may help me. I would love to be able to complate this essay this week if at all possible.....

Many many thanks

I didn't find a good quality free online journal publication. However this web page has a helpful guide and includes the answers to your questions. Axl ¤ [Talk] 18:12, 5 August 2009 (UTC)

sunspot cycle

The sun seems to have an 11 year sunspot cycle. While we probably have not been able to directly observe sunspots on other stars it seems unlikely that our star would be unique to have sunspots. Would other stars also have an 11 year cycle? Would a red giant have sunspots? Googlemeister (talk) 14:16, 5 August 2009 (UTC)

It is in fact possible to observe starspots on other stars. Other stars have cycles, but not necessarily with the same period as our sun. anonymous6494 14:34, 5 August 2009 (UTC)

Astronomers may have detected a brightness change in a star, but I doubt they have actually "observed" a sunspot by creating an image of a star showing a spot on the disc of the star, like we have long been able to make images of sunspots. "Detect" or "measure the effect of" might be more accurate than "observe." Edison (talk) 17:08, 5 August 2009 (UTC)

The above statement that starspots have been observed is idiomatic in observational astronomy—see the lede of that article for the general sense of what is considered "observation". If they meant to say that they directly formed a picture of the star with spots on it, like we do for the sun, they would have said that they "imaged" or "directly imaged" the starspots. See, for example, this press release concerning the first direct imaging of extrasolar planets. -- Coneslayer (talk) 17:21, 5 August 2009 (UTC)

BTW the sunspot cycle is really 22 years, not 11. It's not really significant but that's what it is. 66.133.202.209 (talk) 04:03, 6 August 2009 (UTC)

Knives

what kind of metal does a knife have to be in order for it to be so sharp, that a gently falling piece paper with easily cut in two in the air? Or, does this metal even exist? --Reticuli88 (talk) 19:26, 5 August 2009 (UTC)

I would think that a lot of different metals could be sharpened to the point where they would do that, but I guess what you're asking is for one that would hold that edge for a useful period of time. For that I think you'd need as hard a steel as possible, which means a high carbon content and perhaps some more exotic things like nickel or chromium thrown in, especially if you don't want it to rust (see stainless steel). TastyCakes (talk) 19:33, 5 August 2009 (UTC)

Surgical scalpels are usually made of obsidian as it is sharper then one made of steel. It would not corrode. That is not metal though. Googlemeister (talk) 19:36, 5 August 2009 (UTC)

It doesn't exist - paper is quite strong for its weight.83.100.250.79 (talk) 19:40, 5 August 2009 (UTC)

Scalpels are usually made of steel. Obsidian scalpels are uncommon. Some surgeons consider obsidian scalpels superior to steel. However the evidence is questionable. Axl ¤ [Talk] 19:51, 5 August 2009 (UTC)

I'm not sure it has too much to do with the sharpness of the knife anyway. When the paper hits the blade, there are perhaps three possibilities:

1. The paper is cut.
2. The paper stops moving and rests on the knife.
3. The paper spins off to one side and continues to fall.

Which of these happens depends on the forces involved. In the case (1), it takes a certain amount of energy for the bonds holding the paper together to be broken. If that energy is greater than the kinetic energy of the falling paper - then the paper will simply stop moving (2) or spin off to the side (3). I don't think it matters (beyond a certain point) how sharp the knife is. Here is a thought experiment for you. A GIGANTIC piece of paper that's 100 feet wide by 100 feet tall comes hurtling out of the sky, landing edge-on onto a 6" wide concrete beam. Obviously - the weight of the paper is huge - and it'll tear in half as it falls past our super-amazingly-blunt concrete "knife". On the other hand, if you dropped a sheet of copier paper onto a 6" concrete beam - there is no way it'll tear. So there is a relationship between the kinetic energy of the falling paper and the sharpness of the knife that is not easy to explain.

Now imagine a large blob of jello falling onto a knife blade - you can imagine it hitting the blade and the blade penetrating a couple of inches - then stopping with the jello balanced on the blade with a couple of inches deep cut on the underside. What happens in that thought-experiment is that the Jello is initially moving quite fast - it has lots of kinetic energy - and that's enough for the knife to start breaking the bonds and cutting into it. However, the act of doing that cutting absorbs some of the kinetic energy - so the Jello slows down a bit. Now it has insufficient kinetic energy for the knife to cut any deeper and everything stops.

With the paper, we could imagine a small cut occurring just at the edge of the paper - then it rolling to one side and falling off...that's when there is just enough kinetic energy to get the cut started - but not enough to go all the way through the sheet.

If you did the experiment in a vacuum - the paper could fall much faster - and I'd expect it to have enough kinetic energy to let the knife cut all the way through it. But the terminal velocity of a sheet of paper is just pathetically low - so there is no way for it to fall fast enough to impart enough energy to make a decent cut.

SteveBaker (talk) 20:49, 5 August 2009 (UTC)

Well, if the knife is sharp enough, you only ever need to break one bond at a time - this effect is why a knife works better at cutting than a brick. --Stephan Schulz (talk) 21:06, 5 August 2009 (UTC)

That's the key thing. The energy involved is the same regardless of the sharpness of the blade, the difference is whether that energy goes into breaking bonds or stopping/deflecting the paper. A standard way to test the sharpness of a blade is to try and cut paper with it. A sharp blade should be able to cut paper by just sliding the blade over the edge of the paper with minimal weight behind it. If you do that with a blunt knife the paper just bends. It has everything to do with the sharpness of the blade and very little to do with energy (there is a minimum amount of energy required to cut the paper, but that is very small indeed). --Tango (talk) 03:10, 6 August 2009 (UTC)

But that's the problem. That "minimal amount of energy" is still larger than a normal-sized sheet of paper falling at it's terminal velocity. Hence, you can't drop a sheet of paper onto a knife and have it be cut in two because there isn't enough kinetic energy present to break enough bonds to make a noticable depth of cut. But no matter how sharp the knife - the energy required to break bonds is the same - and each bond that's broken consumes more of that kinetic energy. It slows the paper down to the point where no more bonds can be broken - even with a one-atom-thick blade. For a large, fast-moving sheet of paper to move past a brick (to pick a canonically blunt object) - only TWO bonds at a time have to be broken - one on either side of the brick...and even those won't have to be broken simultaneously. I really don't think it much matters about the knife. As for your "knife sharpness test" - it's completely bogus. I can poke an unsharpened pencil through a sheet of paper - and using enough force, make a jagged tear through it. It's simply a matter of the amount of force and energy in the system. I don't think a sheet of 8"x11" paper, falling at it's terminal velocity has anything like enough energy to break the bonds of more than maybe a quarter inch length of cut. That being the case, it doesn't matter a damn how sharp the knife is. On the other hand - if the paper has enough kinetic energy (eg if you dropped it from enough height in a vacuum) - then a brick could cut it in half almost as easily as a super-sharp knife. SteveBaker (talk) 13:16, 6 August 2009 (UTC)

This youtube video and this website make me think that Tango's knife sharpness test isn't bogus, although I'm not sure it's exactly what he was describing. TastyCakes (talk) 14:11, 6 August 2009 (UTC)

Those are complicated devices for testing it. My test just requires a piece of paper. --Tango (talk) 22:39, 6 August 2009 (UTC)

The test is not bogus, it's a standard test that I have used plenty of times. As I said, you apply minimal force, you just stroke the blade along the edge of the paper. I think we've interpreting the OP's question differently, though - I'm imagining the blade moving and slicing the paper as it falls like the proverbial samurai slicing a handkerchief in an intimidating manner. --Tango (talk) 22:39, 6 August 2009 (UTC)

You are right, Tango --Reticuli88 (talk) 14:43, 7 August 2009 (UTC)

There is no answer to this question, without knowing particular details. How large is the paper? How stiff is the paper? What is its shear strength? Is there an atmosphere through which it falls (as pointed out by SteveBaker)? At what angle does it encounter the knife? Is it's weight equally distributed on either side of the blade? And only finally, how sharp is the knife? Even after knowing all these details it may take empirical tests, because the calculations are probably beyond those applicable to already known experiences. It is possible to stack the odds in favor of achieving a nice sliced in half rectangle of paper. That I think may be possible. Strength of materials seems to be an article that may have applicability here. Bus stop (talk) 15:04, 7 August 2009 (UTC)

I'm getting a bit sick of these kind of answers, Bus stop. Every question on this desk could have more information added to it, it's not really helpful to list a random list of things you'd like to know, and then even if you are given all this information, you still can't answer it (as you point out). So ask for the details? And most of the details you could reasonably well guess: How big? I'd assume ~A4. How stiff? Assume 80gsm standard printer paper. How stiff? As stiff as paper, ffs! And finally, how sharp? THAT'S pretty much the question! I'm sorry if I'm offending you, but really when you have nothing useful to contribute, you don't need to post. Aaadddaaammm (talk) 13:18, 12 August 2009 (UTC)

Gravity and Mass

I understand that objects of differing mass will accelerate toward the ground at the same velocity, i.e. a ping pong ball and a metal ball bearing (excluding aerodynamics) This can be shown in NASA's vimit comet, when the aeroplane will drop at the same rate as its passengers giving the illusion of zero gravity. What I can't understand is why planets of different mass have different gravitational pulls. I.e. Jupiter would pull us toward it with much greater force than Earth due to it's greater mass! Does this mean that there is a correlation between mass and gravity? Is it simply the case that the difference between the ping pong ball and the heavier metal ball bearing is insignificant in comparison to the mass of the planet earth. Does the origin of the gravitational force come from within the particles or the forces that hold them together? Can it be associated with the possitive and negative charge? From Stuart McPhee86.174.230.38 (talk) 22:41, 5 August 2009 (UTC)

This is an easy mistake to make. The thing to get straight is this: a ping pong ball heads towards earth at an acceleration of roughly 9.8 meters per second per second. This is not relative (at least in Newtonian mechanics); it is indisputable fact that it is the ping pong ball that is accelerating towards the earth, not the earth towards the ping pong ball. The earth is also accelerating towards the ping pong ball, but it is doing so by a much tinier amount, so we only perceive the acceleration of the ping pong ball.

However, suppose we increase the mass of the ping pong ball. Let's replace it with a small planet (but we still want to keep it at the same distance from the center of the earth). The new planet's acceleration towards the earth is still about 9.8 m/s², but the earth's acceleration towards the new planet has increased significantly, to the point that we can actually perceive and measure it.

The earth's acceleration towards an object based on that object's gravity is directly proportional to its mass. For example, if the new "planet" has a mass equal to one tenth of that of earth, then earth will accelerate towards the new planet at a rate of .98 m/s² (one tenth of the new planet's acceleration towards earth).

Similarly, if earth's mass were increased, then the acceleration at which objects fall towards the earth would also increase proportionately. However, the earth's still imperceptible accelerations toward falling objects would remain unchanged (assuming the same distances from the centers of mass are unchanged), since its accelerations toward ping pong balls depends only on the mass of the ping pong ball and the distance between their centers of mass.

You probably don't have the background to understand the equations involved, but here they are for reference. That is, those are the equations Newton formulated over 300 years ago; they have been replaced nowadays by general relativity, which is significantly more complicated. --COVIZAPIBETEFOKY (talk) 23:14, 5 August 2009 (UTC)

To bring that back to your example about Jupiter, the thing to realize is this: Jupiter's acceleration towards earth is exactly the same as that of a hammer if we replace Jupiter with that hammer. It is earth's acceleration towards Jupiter or the hammer that changes in these two scenarios. --COVIZAPIBETEFOKY (talk) 23:28, 5 August 2009 (UTC)

the above answer is pure semantics. The simple answer is: everything attracts everything. (The exception is things that have no mass). And you guess the key already; you say: "Does this mean that there is a correlation between mass and gravity?" YES. Gravity is proportional to mass and inversely proportional to distance. Just remember: everything attracts everything (but the farther away from each other the things, the weaker the attraction). Then you'll be all set. 82.234.207.120 (talk) 01:26, 6 August 2009 (UTC)

I'm not sure how you came to the conclusion that my answer was "pure semantics", but maybe I should clarify the key point in my answer, since I did kind of ramble a bit. Your answer doesn't really seem to address the main question being asked, which was: why should Jupiter have a stronger pull on the earth than earth has on everything else? The error in this 86.174.230.38's train of thought comes from thinking about the acceleration between two bodies, rather than the acceleration of one body due to another. The former is a sum of two instances of the latter (ie. the ping pong ball accelerating due to the earth's gravitational pull and the earth accelerating due to the ping pong ball's gravitational pull), and it is important to distinguish between those two separate accelerations.

Failure to make that distinction can lead to the conclusion that the earth-ping pong ball system should behave identically to the earth-Jupiter system: just increase the mass of the ping pong ball to that of Jupter. The acceleration "between the two bodies" (remember, this is the wrong way to think about it) remains unchanged. Clearly, the earth-ping pong ball system should behave identically to the earth-Jupiter system, at least as far as gravity is concerned. The problem here is that we didn't make the important distinction between the acceleration of one body and that of the other.

Hopefully that's a bit clearer. --COVIZAPIBETEFOKY (talk) 02:10, 6 August 2009 (UTC)

The key detail is that mass appears in two places in the relevant calculations. It appears in the formula for gravitational force, and it appears in F=ma. The reasons hammers and feathers fall at the same rate is because that mass cancels out. However, only the mass of the object we are measuring the acceleration of cancels out, the mass of the other object doesn't cancel out (since it doesn't appear in F=ma, since that is only concerned with the object in question and the force, not the source of that force) so it still has an effect. (That gravitational mass and inertial mass are equal is far from trivial and has been the subject of much discussion.) --Tango (talk) 02:57, 6 August 2009 (UTC)