Talk:Friction/Archive 1

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Text Contradicts Image

The caption of the image says that friction is caused by "imperfections of the surfaces." But the text states that "sliding friction is not caused by surface roughness." (talk) 01:50, 26 April 2008 (UTC)

I went ahead and removed the image. Maybe some of the image caption is worth recovering, but I think it's all in the article. I'm the same person as commented above. (talk) 22:30, 27 April 2008 (UTC)

I don't think there's anything left in the caption that needs to be saved. --Wizard191 (talk) 23:19, 27 April 2008 (UTC)

Indeed, it does sound like the captions contradict the text from those examples.--DavidD4scnrt (talk) 06:57, 1 May 2008 (UTC)


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

It is μs 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)

I believe that ROLLING FRICTION IS A TYPE OF STATIC FRICTION. It is listed as kinetic as of 10/25/2005 at 1:29 pm Eastern Standard Time.

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. 09:10, 19 Jul 2004 (UTC)

The acoustic lubrication part has been moved to a new article with a link here. StuRat 19:42, 1 October 2005 (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)

I'm not the one who merged in the coefficient of friction, but it looks good here, to me. StuRat 19:43, 1 October 2005

What exactly is the product of friction?

Heat and/or motion of bodies or fluids other than the one intended (such as motion of air or water due to passing planes or ships). I added this info to the article as the "Products of friction" section. StuRat 18:59, 21 October 2005 (UTC)


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.
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)
I would say that the vacuum must be considered part of the normal force in such a case, then you should get a value between 0 and 1. As for cases with interlocking teeth, that isn't really the same thing as surface friction. Unfortunately, there is a continuum between actual surface friction and subtle interlocking teeth, as in sandpaper on sandpaper, and big interlocking teeth, as in gears, and we could go right up to two surfaces bolted together, LOL. Of course, the 1.0 theoretical limit doesn't much matter for practical applications, but we should still state it for all the physicists out there and also explain why what looks like friction may well exceed 1.0. I tried to explain these concepts on the main page, with examples. StuRat 19:54, 1 October 2005 (UTC)
I aggree that some warning should be given about the approximate and empirical nature of "friction laws". Clean smooth metal surfaces in contact can have coefficients of friction much greater than 1 (Bowden&Tabor, The friction and lubrication of solids, OUP 1956). Vacuum does not contribute to an increase in normal pressure. It stops the formation of oxide layer which in any other circumstances is constantly re-created at the interface, preventing the formation strong bonds between core metal atoms. It may be usual to add something on the physical nature of the friction force. It is still subject to debate but most people seem to agree than in most every day situations, the friction force is make of an adhesive component of an unclear nature and a "ploughing component" resulting from the plastic deformation of the surface asperities.
Holy moly, this article is highly incorrect when it comes to accounting for surface area. I'll try and fix it later, but I'm taking from memory what I remember in the book: Persson - Sliding friction : physical principles and applications. Sliding friction force is: F = (mu)(N) + (tau)(A). The (mu)(N) we all know, and the (tau)(A) component is the area component. First of all, (tau)(A) is only significant at a certain scale! Say if your object is about <20 microns in size, you should worry about area (the threshold isn't exact). Tau represents the intersurface shear force between the object and surface, and *is* affected by roughness. The rougher your surfaces, the more contact points you have between the surface that generate shear forces in the lateral direction, affecting friction. Intersurface adhesion, on the other hand, does *not* affect friction (though it does affect pull off forces). Furthermore at smaller scales (especially nano scale objects), normal force becomes insignificant, as weights of object are very very small! (weights scale with volume which is L^3, area is L^2). Sliding friction is entirely dominated by surface area and contact roughness at this point. —Preceding unsigned comment added by (talk) 14:41, 7 September 2007 (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. 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)

I suggest you go ahead and add that info, but also keep the simpler formula for cases where the velocity may be ignored. StuRat 20:01, 1 October 2005 (UTC)
The depence of the coffecient of friction with sliding speed is usually weak and in most engineering cases, it also goes the other way round than the one suggested above: friction decreases as the sliding speed increases.

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)
  • Delete the static friction article. All information it contains could fit into this article. This is the more general article and even it doesn't have enough information, so it doesn't make sense for it to have daughter articles. JabberWok 01:09, 12 July 2005 (UTC)
  • I support having static friction included here. StuRat 19:57, 1 October 2005 (UTC)

Static friction and area

The article currently claims

However, for static friction where there is an element of adhesion, the contact area does matter.

Could someone give a reference for this fact? I have seen claims to the contrary, and have never seen this claim anywhere else. The simple fact that slicks are used in racing is not convincing, as there are a host of other possible reasons (such as tire chemistry).

I mean, of course, for a fixed normal force, fixed pair of surfaces in identical conditions, et cetera. This claim would then imply that the coefficient of static friction depends on the area of contact, which I would find surprising. What microscopic model is used to predict this, if any?

Here's one link which doesn't exactly support the claims made here, but at least mentions the possibility:

Tipler (referenced at the bottom of the page) says "to a good approximation, this force is independent of the area of contact" and goes on to discuss why.

I would think this could be proven easily enough by gluing two large blocks together and two small blocks, then testing to see which requires more sheer to break loose. I would think the force needed would be proportional to the contact surface area. StuRat 05:51, 22 November 2005 (UTC)

Using a setting glue is definitely beyond the realm of friction. Tipler's explanation goes as follows: at a microscopic level, surfaces are rough. The shear force required to break them apart is proportional to the microscopic contact area. However, when you put two rough surfaces together, the microscopic contact area per unit macroscopic contact area is small; as you increase the normal force, the fractional microscopic contact area increases linearly with pressure. Increasing the macroscopic contact area for a fixed force decreases the pressure, keeping their product constant, so the total microscopic contact area is about the same.

Adding a glue, of course, fills most of the interstices, producing a vast amount of contact. This is not much like what people usually call friction.

I don't know if Tipler's explanation is correct, but every other reference I have seen states that to a good approximation, static friction does not depend on (macroscopic) contact area. Most go on to state, as an example, that wide tires do not produce more traction than narrow ones on dry roads.

I didn't mean to let the glue set, but was using wet glue as an example of a case where the surfaces have "an element of adhesion", as per the inquiry. The problem with defining friction precisely is that there seems to be a continuum between "pure friction" and cases with adhestion, "interlocking teeth", and/or a vacuum or cold weld. StuRat 15:14, 22 November 2005 (UTC)
The current writing is confusing, because it says that "frictional force is proportional to contact area" and right after says "the frictional force is not entirely independent of the contact area". There is no mention to apparent contact area and that it would be intuitively spected to increase with it. I support that someone add sections from the German. Rend 16:42, 20 May 2007 (UTC)

Direction of friction force

The force of friction is always exerted in a direction that opposes movement.

This is not true. For example, a bike uses this force to accelerate. If there was no friction, the bike wouldn't move (the wheels would be spinning in place).

  • It is true that friction always acts to oppose velocity. If this law is broken we now have systems that are creating energy for free. In the case of a bike, the wheel is rotating and the friction is opposing this rotation. If it were not for this wheels would not roll. Draw a circle, assume it is rolling and draw an arrow around the cirle showing the direction of rotation, now at the bottom of the circle where it is tangent to its rolling surface draw and arrow that is opposing the direction of the rolling. This should point in the direction that you would intuitively think the circle is rolling, and this is the friction force opposing the motion of the wheel.Mechj 19:50, 05 December 2005

What does |v|/v mean? Wouldn't that always be equal to 1?

v is vector, and has both a magnitude and a direction. |v| is just the magnitude of v (also known as the speed). So, v/|v| would yield a normalized direction vector. In English, that means that v/|v| gives you the direction of the velocity without telling you how fast it's going. This notation can be somewhat confusing for those not familiar with more advanced textbooks, since most introductory textbooks use "hat" notation. For more information, try here: Please note that physicists like to write magnitudes like this |u|, while mathematicians like to write it like this ||u||. The reason why we need the direction of velocity at all is because friction opposes motion, so we need our friction to be pointing in the opposite direction of the velocity.

Friction As Probability?

Is there any existing theory about modeling friction as a probability? That is to say, an alternate definition for a coefficient of friction would be as a value between 0 and 1 that represented the probability that the two surfaces in question are part of one rigid body? This might be useful in computer graphics, where using real physics force calculations cause a somewhat unnatural-looking "perfect slide" effect...

That sounds reasonable, especially in the case of sonic lubrication, where the surfaces are only intermittently in contact with each other. StuRat 07:15, 12 February 2006 (UTC)
Unfortunately, there's no reason coefficients of friction need be between 0 and 1 (values higher than 1 occur with certain combinations of tires and road), so probabilities are pretty hopeless.
Tha's true under the standard definition of coefficient of friction. I'm asking if another model has ever been proposed, one that uses a probability instead of the current definition of coefficient of friction. A number that can spin off to infinity can be difficult to work with sometimes...

What does this mean??

Let us have a coefficient of friction which depends on the displacement velocity and is such that its value at 0 (the static friction μs ) is the limit of the kinetic friction μk for the velocity tending to zero. Then a solution of the contact problem with such Coulomb friction solves also the problem with the original μk and any static friction greater than that limit.

So, what is that supposed to mean?

- SundarKanna, not logged in now, typing from 12:20, 23 February 2006 (UTC)

well i dont care.

I can't tell what it means. They need to learn to write for a general audience. StuRat 17:30, 13 July 2006 (UTC)

Also, I think this statement is incorrect. From the static friction section, The static friction is in most cases higher than the kinetic friction. That is why you feel a jerk when starting to move and when stopping. The kinetic friction section also says, Kinetic friction ... is usually less than the coefficient of static friction. Which means that the static friction is not equal to the limit of the kinetic friction for velocity 0, and you cannot treat them as one. --Spoon! 09:05, 21 August 2006 (UTC)

Merge with Friction Force?

Could Frictional force be merged into this article? It seems to be just a shorter version of this article. Maybe a more experienced Wikipedian could organize it? 06:12, 15 March 2006 (UTC)

Agree and have placed merge tags on the two articles. Wait for comments now. Vsmith 02:58, 28 March 2006 (UTC)
I agree, they should be merged. The 2 articles are basically on the same topic. Bodil 16:32, 12 April 2006 (UTC)
I too agree that the two articles should be merged. Joyce c89 15:21, 6 May 2006 (UTC)
I agree with the merge. --Zoz (t) 13:49, 14 May 2006 (UTC)
Ditto to the above. Dragon42 16:41, 17 August 2006 (UTC)

OK - replaced anon copy & paste with copy/paste that includes the formulae. Needs smoothing now. Made friction force a redir. Vsmith 23:44, 23 September 2006 (UTC)

coefficient values

The article states that values can exceed 1.0. I don't understand this at all. Placing an object on an inclined plane and looking at the free body diagram the coefficient of static friction can be expressed as the tangent of the angle at which the body just begins to slide. Values greater than one don't make sense. 15:25, 2 June 2006 (UTC) JXP

When did angles with tangent greater than 1 stop making sense?  :-)

Rracecarr 18:40, 2 June 2006 (UTC)

Nothing wrong with those angles; I apologize for not taking the time to explain my concern with coefficients greater than one. What I was trying to get at was that when the angle is greater than 45 deg, the force down the plane is greater than normal to the plane and that seems counterintuitive to me. However, let pose this differently.

If an object is nailed or glued down, then obviously we are not talking about friction forces. What if the adhesive is not dried or cured but is only tacky, is the force required to slide it due to friction ? How about two very smooth, very clean metal surfaces in contact. These are known to micro weld, or form a solid solution between them. Is the force required to slide them frictional ? It seems to me that if the force required to lift an object is sensibly greater than the force to slide it, the force does not arise from the phenomenon we call friction, excepting minor additonal forces due to surface tension or eddy currents.

I have no expertise in this area, how does one regard these circumstances? JXP it and

It comes down to how you define the word "friction". If only used to describe distinct, solid, non-interfering objects, then the coefficient can't be higher than 1. However, the concept of friction is frequently extended to many other cases where the coefficient is greater than 1. A simple example would be two sheets of sandpaper, where the grains of sand interfere with each other and must be ripped from the paper to allow it to slide. StuRat 17:27, 13 July 2006 (UTC)
The common form in which the general pubic sees friction is that it is the resistive force to motion or the attempt of motion. This then has two contributions, the molecular and atomic interactions, and the interactions due to surface topography. Thus, the Ff=μ*N is the rough approximation where μ can exceed 1. Dragon42 16:51, 17 August 2006 (UTC)

kinetic energy converted to?

Surely the very first paragraph in article should say 'kinetic energy is converted to internal energy' (not heat); increasing atomic vibrations increases internal energy (which then may or may not cause energy to be dissapated as heat). Although it's not important to make distinction between the two in everyday life, I think it should be made in a physics based article. I can't access paragraph, so can someone make changes if they agree, cheers --Taj Bhutta 10:33, 12 July 2006 (UTC)

I disagree. Friction is a common everyday thing, so many people other than physics experts will be reading this article. Thus, it should be written with a general audience in mind, who will have no idea what "internal energy" means. StuRat 17:19, 13 July 2006 (UTC)

Agreed the article should be accessible to non-specialists. But, it should also be suitable for anyone studying science related subjects beyond GCSE. It could be both accesible and technically correct if the term ‘heat’ is dropped. Perhaps ‘In situations where the surfaces in contact are moving relative to each other, friction transfers the kinetic energy of the object to their atoms, which increases their temperature’ Or something similar (if you think this is not clear enough) that doesn’t use the term ‘heat’ as it just adds to the considerable confusion about the term. --Taj Bhutta 12:17, 16 July 2006 (UTC)

That's OK, but just seems like a long way to say "friction generates heat". StuRat 05:17, 29 July 2006 (UTC)

i say merge the co-efficient of friction in with the main friction article. it's inconvient having to switch pages.

types of friction

static friction is the force to start a body moving. kinetic friction is the force to keep a body moving.

There is NO limiting friction!

Could somebody agree/disagree and than edit the article? Some of my undergrads got it wrong in a presentation.

phil (12.12.2006 15:50 London(UK) time)

Exception to kinetic friction

Please add appropriate units - discussion of physics/mechanics topics should include a consistent set of units