Wikipedia talk:WikiProject Physics/Archive May 2008

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Centrifugal Force

Scenario (a),

An object sits stationary in an inertial reference frame. As observed from a rotating reference frame, it traces out a circle.

Scenario (b),

Archimedes' principle invokes the heavier particles in a centrifuge to move radially to the outer edge edge.

Are these two cases the same? Which one is centrifugal force? (a), (b), or both? David Tombe (talk) 15:57, 25 April 2008 (UTC)

In your scenario (a), the observer in the rotating reference frame would see the object in a circular orbit because it is constantly being deflected by the Coriolis force which is a fictitious force distinct from the centrifugal force, another fictitious force.
In your scenario (b), an observer rotating with the centrifuge's rotor would see a strong centrifugal force acting on the material. This effectively amplifies the gravity acting on the sample greatly increasing the sedimentation rate.
In either case, in the inertial frame no fictitious forces are seen. The force affecting sedimentation is applied by the bottom of the sample's container. JRSpriggs (talk) 03:37, 26 April 2008 (UTC)

Do the two scenarios involve the same effect?David Tombe (talk) 07:28, 26 April 2008 (UTC)

No. JRSpriggs (talk) 11:50, 26 April 2008 (UTC)

Agreed. Now over on the centrifugal force page, there is an edit war in progress. There are three editors trying to push the view that scenario (a) is centrifugal force.

They are sparing no effort to keep scenario (b) off the page altogether. Any attempts to introduce scenario (b) are being fudged by trying to claim that it is only a reaction to a centripetal force. In actual fact an object being swung in a circle on the end of a string involves a centripetal force that is in fact a reaction to the centrifugal force.

No end of effort is being made to keep this article confused. They have even claimed that centrifugal force is a term applied to two different forces one of which they claim to be (a) and the other which they claim to be a reaction to centripetal force.David Tombe (talk) 14:16, 26 April 2008 (UTC)

See here:

In the case of distant objects and a rotating reference frame, what must be taken into account is the resultant force of centrifugal and Coriolis force. Consider a distant star observed from a rotating spacecraft. In the reference frame co-rotating with the spacecraft, the distant star appears to move along a circular trajectory around the spacecraft. The apparent motion of the star is an apparent centripetal acceleration. Just like in the example above of the car in circular motion, the centrifugal force has the same magnitude as the fictitious centripetal force, but is directed in the opposite, centrifugal direction. In this case the Coriolis force is twice the magnitude of the centrifugal force, and it points in centripetal direction. The vector sum of the centrifugal force and the Coriolis force is the total fictitious force, which in this case points in centripetal direction.

Count Iblis (talk) 15:47, 26 April 2008 (UTC)

Count Ibliss, that is scenario (a). It can't create pressure in the water in a swinging bucket. Furthermore, the maths that you use to justify accounting for scenario (a) in terms of fictitious centrifugal force and fictitious Coriolis force involves the angular velocity vector ω. If you follow the derivation of that maths carefully, you will see that the linear velocity v and the angular velocity ω apply to exactly the same particle. Hence to have any meaning, your equations can only apply to objects that are co-rotating with the rotating frame.

We can play no end of games with mathematics once we detach that maths from the original reality to which it applied. You have just presented a fictitious argument. David Tombe (talk) 06:17, 27 April 2008 (UTC)

Could we stop talking about Coriolis force in all of this, which isn't appropriate for ANY of it? You only see the Coriolus force when the radius of rotation is changing. If it's constant (as in a centrifuge or a rotating anything that isn't deforming) the Coriolus force is ZERO.

The Coriolus force, FYI, is the "catch-up" tangent force that you feel on a rotating system when you move out to larger radis, and thus tend to slide backwards because of your inertia (or, if you need to stay on a radial line on the system, it's a force you have to apply to yourself in a tangent direction to increase your tangent velocity, which you need to do, to move in a radial line).SBHarris 07:04, 27 April 2008 (UTC)

Sbharris, I totally agree. The Coriolis force is a tangential effect which arises from a radial motion. There is an edit war going on at the centrifugal force page because many others disagree with us. David Tombe (talk) 08:16, 27 April 2008 (UTC)

The paragraph quoted by Count Iblis above is correct. My previous answer was incomplete. In scenario (a), the actually stationary object when view in a constantly rotating frame of reference is acted on by two forces: a centrifugal force directed away from the axis of rotation, and a Coriolis force which is twice as large and directed towards the axis. As a result of the net centripetal force, the object is accelerating towards the axis as viewed in the rotating frame. I believe that David Tombe and Sbharris are confused (although they may not be the only ones). Sorry. JRSpriggs (talk) 11:35, 27 April 2008 (UTC)
Count Ibliss's formulae for Coriolis and centrifugal force in rotating frames of reference are mathematically correct, but they have become unanchored from the physical reality upon which they were derived. The angular velocity term always applies to the exact same particle as the velocity term.
This fact has been overlooked. Also, in the final formula, the velocity term only refers to the radial velocity. This fact has also been overlooked.
But we shouldn't need maths at all to sort this problem out. It is quite obvious that the Newton's bucket situation can create an actual hydrodynamical pressure. It can make heavier particles move in relative motion to lighter particles, and this effect can be viewed from all reference frames. see Bucket argument.
Whereas the stars rotating across the sky is merely an illusion.
The two effects are quite different. You should have stuck with your first answer. David Tombe (talk) 07:06, 28 April 2008 (UTC)
Having read through this a few times, it seems that there is some kind of subtle disagreement here which I can't quite pick out. Let me walk through your two scenarios and see what you disagree with.
Scenario (a) involves an object at rest in some inertial frame. As it is at rest in an inertial frame, there are no physical forces on the object. You then define a (for the sake of argument uniformly) rotating frame such that the object is not at the origin. In this frame, the object undergoes uniform circular motion. The explanation for this is the vector sum of the Coriolis (the object has a velocity in the rotating frame) and centrifugal forces, as explained by Count Iblis and JRSpriggs. These forces are fictitious in the sense that they arise solely from choice of coordinate system. They are, if you will, coordinate forces and not physical forces.
Scenario (b) involves a centrifuge (or bucket or similar) filled with some liquid substance(s) which is rotating. In an inertial frame, the centrifuge and liquid are undergoing circular motion, meaning that there must be real forces. In particular the liquid wants to travel in a straight line but is deflected by an inward directed centripetal force exerted by the centrifuge on the liquid. By Newton's 3rd Law, the liquid is then exerting an outward force on the centrifuge, that is the pressure. In a frame co-rotating with the centrifuge, these physical forces still exist, but according to the co-rotating observer nothing is moving, and the system is stationary. Thus, to balance out the real physical forces, there is a centrifugal force acting outward (there is no Coriolis force here as the liquid is not moving in the rotating frame) balancing the physical force of the centrifuge on the water. Thus the net coordinate force in the rotating frame (e.g. the second derivative with respect to time of the coordinate position in the rotating coordinates) is zero.
I think that's an accurate description of what's going on. The fictitious centrifugal force comes in in both scenarios, but of course as it's not a physical force if you put an accelerometer in these scenarios, it would not register the centifugal force. So, is there a disagreement on what I said above (I glanced at the talk page of the actual article and at least for the moment balked at looking at that too closely), or if not that, then what's the disagreement? DAG (talk) 13:37, 28 April 2008 (UTC)

The disagreement begins with whether or not the two scenarios are the same. The team that are controlling the article say that they are the same.

I say that they are different because (a) is an illusion, whereas (b) is a real effect. Heavy particles push past lighter particles. Archimedes' principle is invoked. Hydrostatic pressure is invoked. A diffusion takes place which is visible from all reference frames.

The disagreement then centers on how the article should be introduced. I say it should be introduced with a short introduction stating in simple terms scenario (b) ie. that centrifugal force is an outward force that acts on all objects that are moving in a curved path.

The team that are controlling the article want to ignore scenario (b) and concentrate on what they believe to be the transformation equations for rotating reference frames. They want to emphasize their belief that centrifugal force is fictitious. David Tombe (talk) 14:27, 28 April 2008 (UTC)

Wrong place

This discussion should take place on Talk:Centrifugal force, not here. It's better to just use this page to invite folks to the discussion. Gnixon (talk) 17:47, 28 April 2008 (UTC)

This discussion shouldn't really be happening at all. Much of it seems to involve arguing from first principles what a centrifugal force is, when what needs to be discussed is how the term is used in the literature. --Starwed (talk) 14:54, 29 April 2008 (UTC)
Yes, I know. But that is not what is happening. Please see Wikipedia:Fringe theories/Noticeboard#Centrifugal force -- The Anome (talk) 17:25, 4 May 2008 (UTC)

Boltzmann constant

Lucretius (talk · contribs) is objecting to the following paragraph at the very end of the article Boltzmann constant:

The numerical value of k has no particular fundamental significance in itself: It merely reflects a preference for measuring temperature in units of familiar kelvins, based on the macroscopic physical properties of water. What is physically fundamental is the characteristic energy kT of a particular temperature. The numerical value of k measures the conversion factor for mapping from this characteristic microscopic energy E to the macroscopically-derived temperature scale T = E/k . If, instead of talking of room temperature as 300 K (27 °C or 80 °F), it were conventional to speak of the corresponding energy kT of 4.14×10−21 J, or 0.0259 eV, then Boltzmann's constant would not be needed.

We've gone though the physics a certain amount at the talk page, enough I believe to justify the content of the paragraph. But he's still not happy, and wants still to see it deleted.

Apart from restating what's already been said on the talk page, I'm at a loss as to what more I should do or say to respond to his concerns.

I'd welcome some fresh pairs of eyes on this. Jheald (talk) 09:40, 7 May 2008 (UTC)

Link to discussion (please correct if there is more up there, I have not been following). - Eldereft ~(s)talk~ 17:29, 7 May 2008 (UTC)

List of scientific publications of Albert Einstein

Hi all,

I'm about to nominate List of scientific publications of Albert Einstein as a Featured List candidate, but I'd welcome any of your input(s) before I do. As you might notice, it's been a ton of work, so please be gentle in your criticisms! :) Willow (talk) 19:52, 6 May 2008 (UTC)

That is a very impressive and thorough list indeed. Definitely deserves a featured list status in the near future. I'll head for the talk page. Headbomb (ταλκ · κοντριβς) 22:21, 7 May 2008 (UTC)

Thanks very much, Headbomb, both for your kind words and your excellent suggestions over there! :) I haven't made up my mind about the vertical/horizontal thing, but I daresay we'll reach consensus about it at the FLC. So, I've just nominated it as a Featured List. This is a good opportunity for the WikiProject to get a new Featured Thingy, no? ;) Everyone's input there would be welcome; please follow the link! :) Willow (talk) 20:29, 8 May 2008 (UTC)

Error in image of components of the stress-energy tensor

See Stress-energy tensor. The image Image:StressEnergyTensor.svg contains an error — it says "viscosity" where it should say "shear stress". Can anyone fix it? JRSpriggs (talk) 20:08, 12 May 2008 (UTC)

Is an observer like a camera?

Please notice the new and questionable article Observer (special relativity). It claims an observer is analogous to a particular camera orientation within 3D computer graphics and the terms "observer" and "observation" are being used in an almost completely opposite way to other meanings. JRSpriggs (talk) 20:36, 12 May 2008 (UTC)

I'm of mixed opinion on this article. I like the idea of having such an article; the term "observer" is used in a specific way in physics which is not obvious to the layman. However, the content is completely wrong at points. I'll make some rough changes. Joshua Davis (talk) 01:28, 13 May 2008 (UTC)
I rewrote the article; let me know what you think. -- BenRG (talk) 13:30, 13 May 2008 (UTC)
Thanks to Joshua and especially Ben for improving the article. JRSpriggs (talk) 02:13, 14 May 2008 (UTC)

Featured List for this WikiProject!

Hi all,

The list of scientific publications by Albert Einstein has been nominated as a Featured List. Please go there and, if you think it's worthy, support it! Willow (talk) 23:08, 13 May 2008 (UTC)

Finding reliable sources to back up the "democratic principle" article

I'm finding it hard to find any online reliable sources to back up the Democratic principle article, which refers to a conjecture by John Wheeler regarding Mach's principle.

A Google search for '"democratic principle" wheeler mach' finds some blog posts (eg: [1], [2]), but nothing more. The article itself contains an ArXiv link to a paper by Bahram Mashhoon, (the paper has since been updated to a new edition [3] -- I've fixed the link), but the word "democratic" does not appear to occur anywhere in the paper, and "Wheeler" occurs only once, in a citation used in the context of the statement 'gravitomagnetism ("GEM") has been discussed by a number of authors'. Can anyone find a reference to the "democratic principle" in this paper? Or indeed anywhere in the peer-reviewed physics literature, since an ArXiv search for the exact phrase "democratic principle" only finds two hits, both apparently referring to something else involving branes?

I can find a near-miss reference to "Galileo's democratic principle" in the Google Books preview of Magic universe: the Oxford guide to modern science, that mentions Wheeler in the next sentence, but it doesn't anywhere directly back up what the article says. -- The Anome (talk) 11:21, 15 May 2008 (UTC)


List of baryons nominated for featured list

The list of baryons was nominated as a featured list candidate. In the last month and a half, it went from a poorly referenced and out of date list of 20-something baryons, to a fully referenced and up-to-date with the PDG Review of Particles Physics 2006 list of all baryons in J= 1/2 and J= 3/2 states along with a list of reported pentaquarks, with explanations for naming conventions along with an expansion of the isospin article. There's a lot more, but just head towards the List of baryons for full detail. Headbomb (ταλκ · κοντριβς) 07:08, 16 May 2008 (UTC)

Cosmological decade

After hearing about this concept in a History Channel program, I went to cosmological decade to learn more. I think the article needs some factual work, but I'd like some input before I go any further. See the article's talk page, or jump in to make improvements yourself. --Spiffy sperry (talk) 17:58, 18 May 2008 (UTC)

Johnsen-Rahbek effect - expand

Some months back I created a stub article on the Johnsen-Rahbek effect, which is an obscure effect in which the electrical current conducted between two surfaces changes the friction between them. It was discovered by Elisha Gray, improved by Edison, and used commercially by IBM, but misunderstood until the semiconductor era. The article needs an explanation of the underlying physics. --John Nagle (talk) 04:43, 19 May 2008 (UTC)

Numerous errors in wikipedia's thermodynamics and statistical physics articles  :(

I have been editing and creating a few wiki articles on this subject recently. To my horror I frequently encounter very very stupid errors. One error that is repeated over and over again on many articles (I think I corrected them all by now) is the following erroneous statement:

Which is supposed to follow from the fact that while , the work done by the system is . The latter is not true, of course. In general . This then led to the erroneous statement about the change in the internal energy. Now, it should have been clear to anyone that that inequality is problematic, because U is a a thermodynamic function of state that one should be able to uniquely specify by S and V see also here.

Now one can still argue that it is possible to get an inequality, but that is then not solely due to irreversible changes but rather due to the fact that the system would have to be described by more external variables than just the volume. But if one keeps all the relevant external variables and the corresponding generalized forces you always get the fundamental thermodynamic relation in the form of an equality.

Once a correct treatment derivation is given one can then go on to derive that e.g. the Gibbs energy of a system kept at constant T and P can only decrease. But that certainly does not follow form the nonsensical reasoning like "while , the work done by the system is ".

It actually requires quite a bit of work to explain why the Gibbs energy can decrease. Also in textbooks one typically presents that discussion in terms of finite differences to stress the fact that one considers an initial state of a system in thermodynamical equilibrium and a final state, also in thermodynamical equilibrium. During the change the system is not assumed to be in thermal equilibrium, but the heath bath is assumed to always remain in thermal equilibrium.

One can then e.g. derive that the maximum amount of work in other forms than pressure-volume work that can be extracted from the system is given by minus the Gibbs energy change. And it should be clear that the Gibbs energy change is acounted for by the change in the product of the external variable that describes the change with its generalized force.

Count Iblis (talk) 17:29, 18 May 2008 (UTC)

Not really sure why you are posting this here. This page is usually used to invite contributions to on-going debates elsewhere...in this case I assume you are fixing as you find, since you give no links. This page gets archived relatively quickly; if you want the above thoughts recorded, best put them on a relevant article talk page (not to mention in Gibbs Energy). PaddyLeahy (talk) 18:03, 18 May 2008 (UTC)
...but good work, anyway! :) PaddyLeahy (talk) 18:06, 18 May 2008 (UTC)
Thanks! I just spotted similar problems at Helmholtz free energy which I have now largely fixed. The article on enthalpy also suffers from the same disease, I'll rewrite that page later. Count Iblis (talk) 02:09, 19 May 2008 (UTC)
It makes sense to post systematic issues here. --Starwed (talk) 06:00, 20 May 2008 (UTC)

The first Featured List ever for Physics!

The list of scientific publications by Albert Einstein is being considered as a Featured List. Please check it out and support it if you think it lives up to the criteria. Thanks! :) Willow (talk) 21:27, 20 May 2008 (UTC)

But only because you beat the List of baryons by speed. Damn you, and congrats!Headbomb (ταλκ · κοντριβς) 23:42, 20 May 2008 (UTC)

No, no, no — nothing's been decided yet about the list of scientific publications by Albert Einstein. I'm just pleading with people here to take a moment to review both of our lists; perhaps we'll be promoted together? :) I'll look at yours if you look at mine... ;) Willow (talk) 23:49, 20 May 2008 (UTC)

I already did :P. I'll vote as soon as I get a chance to review the comments.Headbomb (ταλκ · κοντριβς) 23:58, 20 May 2008 (UTC)

I supported the list, and made some revisions to list of baryons , which I hope you like. I was trying to make it more accessible to the average reader. Good luck to us both, Willow (talk) 18:45, 22 May 2008 (UTC)

J. Marvin Herndon

J. Marvin Herndon has recently created an article about himself and his work, and is actively editing it. As a result, the article is now flagged with COI and NPOV templates. I've been in email contact with him, but it would be good if more editors were involved on the article. Is there anyone here with experience with biographical and/or geophysical articles that could lend a hand? Thanks. Mike Peel (talk) 17:23, 24 May 2008 (UTC)

Vortex theory

I just saw an article that said the results of the Michelson–Morley experiment were the final nail in the coffin of the vortex theory, and — lo and behold! — vortex theory was a red link. How is anyone who's never heard of that discredited theory supposed to understand what was being said? That seems like a very surprising omission.

Another place where I came across this was not in Wikipedia. An article said that by 1700, Newton's physics was accepted throughout the whole civilized world. Another article, by a francophone author, said that, as difficult as it would be for his anglophone colleagues to understand, France is actually a part of "the whole civilized world". And moreover, he said, in 1700 many in France still believed in the vortex theory until a crucial test in the year 1733, financed by the French king, consisting of careful geodetic measurements, showed that the earth is oblate, as predicted by Newton's theory, rather than oblong, as predicted by the vortex theory. How can one understand what he's saying without knowing what the vortex theory was? Michael Hardy (talk) 02:43, 26 May 2008 (UTC)

The problem is that there are *two* "vortex theories"... The "real one" (which I honestly never heard of before, but see http://descartes.cyberbrahma.com/vortex.html which seems to indicate a certain reality), and the "crank one". I know wayyyy more than I should about the "crank one", which says that vortexes explain UFO propulsion, and they explain Tesla's results, and they prove that Einstein was wrong, Newton was wrong, Hawking was wrong, everyone was wrong except the patent holder of this new method of levitation and propulsion by means of gravito-magnetic vortex fluxions... linas (talk) 04:17, 31 May 2008 (UTC)
I just made your red link turn blue; however, the side effect may unfortunately be that the vandalism rate on mechanical explanations of gravitation will now increase 100-fold. linas (talk) 04:21, 31 May 2008 (UTC)
I love vortices.
linas (talk) 04:40, 31 May 2008 (UTC)
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