Talk:Friction

Is it μ_s or μs? I'm not sure. Evil saltine 03:08, 15 Nov 2003 (UTC)

It is μ_s

63.205.40.243 05:26, 8 Jan 2004 (UTC)

This article contains some dodgy-looking physics which needs checking and revising by someone who is expert in this field. -- The Anome 12:28, 29 Feb 2004 (UTC)

I'm an engineer, and there's really *not* a lot of physics in here. I can verify most of it (being that I wrote most of it). →Raul654 23:34, Feb 29, 2004 (UTC)

Wow, this article does need some work. I've done what little I can at the moment, but it's still not that great. Especially the information regarding the coefficient of friction should probably be split off, as should acoustic lubrication. 24.118.220.84 09:10, 19 Jul 2004 (UTC)

I originally wrote the co-effecient of friction text for the co-effecient of friction article. Someone came along and merged that content into here. →Raul654 09:25, Jul 19, 2004 (UTC)

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Contents

1 Approximations? 1.1 Dependence on velocity 2 Please translate from German 3 Merging with Static Friction

Approximations?

It is interesting to note that, contrary to common belief, friction is unrelated to the size of the contact area between the two objects.

I've always wondered about that: is it just a first approximation? If not, why does it matter whether a car's tires are pumped up or slightly low? Similarly, isn't F=mu*N just an empirical approximation, and therefore more refined results would perhaps be quadratic or even higher order? If my understanding is true, we should mention it. Otherwise, we should clarify the physical reasons behind it. --zandperl 14:24, 26 Apr 2004 (UTC)

F=mu*N is just a first approximation, just like Hooke's Law and any of a number of other linear appromixations that are often misunderstood by those that use them. And since F=mu*N is an approximation, friction being unrelated to the size of the contact area is also an approximation. It also depends how the normal force is distributed over the area. In any case, I think it is misleading to say friction is unrelated to the size of the contact area between the two objects without qualifying it with for a given normal force because for a given pressure, friction certainly is related to contact area. CyborgTosser 09:10, 22 Aug 2004 (UTC)

It is reasonable to say that under conditions where Coulomb (sliding) friction applies the frictional force is independent of the apparent area of contact (which I think is what is meant here). The real area of contact is just the tips of the bumps (asperities) on the two contacting surfaces and this does vary if the apparent area of contact is changed for the same contact pressure, or visa versa, due to the deformation of the asperities. Slinky puppet 18:26, 20 Sep 2004 (UTC)

Is it really true that the coefficient of friction is always less than 1? There is no physical justification for this statement that I can think of. In fact, one can imagine materials with arbirarily high coefficients of friction. Imagine a surface tiled with microscopic pyramids and another surface into which the pyramids fit exactly. By increasing the slope of the pyramids, arbitrarily large coefficients of friction are possible. Maybe someone can point me to a reliable source. CyborgTosser 09:18, 22 Aug 2004 (UTC)

No, I don't think it's true. IIRC, for rubber on hot asphalt, it's around 5.5. →Raul654 09:19, Aug 22, 2004 (UTC)

On the other hand, I just did some googling and found not one counterexample. →Raul654 09:22, Aug 22, 2004 (UTC)

Yeah, I did some googling too and didn't find a counterexample. But it might make sense to change it is always between 0 and 1 to it is almost always between 0 and 1. But I'll wait to change it in case someone can give a link to something more definite. CyborgTosser 01:18, 23 Aug 2004 (UTC)

It isn't true - in some conditions (e.g. ultra-clean smooth metallic surfaces under vacuum) I believe the friction coefficient can exceed 100. However, in the majority of cases the friction coefficient is in the range 0.1-1. Would is typically in the range work well?. Slinky puppet 18:26, 20 Sep 2004 (UTC)

Dependence on velocity

The article gives no indication that kinetic friction varies with the relative velocity of the objects. There must be variation with velocity, otherwise a block sliding down an incline would accelerate to a very high velocity! Engineers often include a linear velocity term i.e. <math>F_k = \mu_k. F_n + d. v <math> where d is 'damping' coefficient and v is the relative velocity. Others (e.g. hydrodynamicists) include higher order terms. --Richard Stephens 08:42, 8 Jun 2005 (UTC)

Please translate from German

The current German article is more organized and more complete than the English one. Please help to improve it. Andries 20:53, 2 Jun 2004 (UTC)

Merging with Static Friction

It really doesn't make sense to merge this with static friction because static friction is a subset of friction. -Zephyrxero 21:48, 21 Apr 2005 (UTC)

• I agree with Zephyrxero. Merge static friction into this article. - Popefelix 22:11, 26 Apr 2005 (UTC)

• Since "static friction" and "kinetic friction" are subsets of this article, wouldn't it be best to make them pointers to here and get all the information on the one page? --Richard Stephens 08:13, 8 Jun 2005 (UTC)