# Talk:Friction

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Article merged: See old talk-page here

## Double Image

What's the reasoning for having the same image twice in one article? Annie.barber (talk) 02:46, 12 December 2011 (UTC)

Don't know. Removed one. Materialscientist (talk) 03:26, 12 December 2011 (UTC)

## Effects Of Friction

This article should describe how important friction is in a situation where 2 exactly the same balls that have a different density and thus different masses, fall at a different acceleration towards Earth unlike they would in a vacuum (where they will accelerate equally). And even more interesting, what effect a higher friction would have on this "falling" of objects like in water. Xilliah (talk) 21:42, 17 April 2008 (UTC)

That doesn't belong here because what you are describing is drag. See [Drag (physics)]]. --Wizard191 (talk) 23:41, 17 April 2008 (UTC)
Isn't drag fluid friction? I'm not advocating a merge, just pointing out the connection. -- Another Stickler (talk) 19:00, 16 December 2008 (UTC)
Friction between the surface of a solid object and the fluid through which it passes, sometimes called Skin_friction, is just one component of drag along with form drag and interference drag. -AndrewDressel (talk) 19:06, 16 December 2008 (UTC)
See new talk section Fluid friction. -- Another Stickler (talk) 22:06, 23 December 2008 (UTC)

## Work of friction

<quote>There are exceptions, however, if the surface itself is under acceleration. One can see this by placing a heavy box on a rug, then pulling on the rug quickly. In this case, the box slides backwards relative to the rug, but moves forward relative to the floor. Thus, the kinetic friction between the box and rug accelerates the box in the same direction that the box moves, doing positive work.</quote>

This is wrong explanation because,

1. There is no force applied to box.
2. Please bear in mind that rug moves forward relative to box is not same as box slides backward relative to rug in this case; Again force is not applied to box.
3. Box never moves backward from it's position (inertia) in some case if there is friction between the rug and the box then box might move along the rug against it's inertial force.

-76.184.2.133 (Talk) 10:49, 18 October 2007

1. There definitely is a force applied to the box: the friction from the rug.
2. At the interface between the box and the rug, there is no difference between the rug moving forward relative to box and the box moving backward relative to rug.
3. No the box does not move backward relative to the floor, it moves forward. Since the force of friction of the rug on the box is in same direction the box moves, it does positive work.
-AndrewDressel (talk) 00:17, 24 June 2008 (UTC)
There is definitely a problem with this example. It's a misleading example because you are changing the inertial frame of reference without saying it. The frame of reference here is the earth, so the work is positive. However, in the text the example is trying to prove that friction can do positive work, which it can't. Friction can never to positive work with reference to the two object where the friction exists. As soon as another inertial frame of reference is added the work can be positive or negative, but that doesn't mean anything, because the frame of reference is arbitrary. Wizard191 (talk) 01:52, 15 July 2008 (UTC)
I don't see the change of reference frame. Going with the earth and sticking with it is a good default. Where is it written that friction cannot do positive work? If you had a good reference, then that should replace the existing example, otherwise I don't see a problem here. -AndrewDressel (talk) 20:45, 15 July 2008 (UTC)
Here's why friction can't do positive work: W=(F)(d)cos(theta). Therefore there is never any work with static friction (because there is no displacement). For kinetic friction, if we call the direction of motion of the rug positive, then the direction of the force of friction must be negative by definition. theta is the angle between friction and your displacement. Since friction is in the direction opposite displacement, theta= 180 and work is thus negative. Therefore, friction always does negative work. You'll notice that at no point did i reference the floor (earth) because THAT is where the frame of reference changes. Physics principals are not written where earth is always the inertia frame; it just can't be assumed. Wizard191 (talk) 22:09, 15 July 2008 (UTC)
Since we can't come to some agreement by arguing about it, let's try a citable source: Minds-on Physics by William J Leonard, University of Massachusetts at Amherst. On page 603 he states "Yes, the work done by a friction force can be positive. For instance, when a book is on an accelerating train, static friction does positive work, because the force is in the same direction as the displacement (as seen from the ground). Whenever the firction force is in the direction of motion, the work done by it is positive." -AndrewDressel (talk) 20:03, 17 August 2008 (UTC)
For the record, I changed the text in the article because I thought that you had agreed or at least agreed to disagree by not replying to my last comment. I did not realize that we were still in disagreement. I think the real problem here is that we are not on the same page. Let me flesh out my point a little more:
1. The text currently in the article is *technically* correct, as you have pointed out with your citation. This is not my problem. My problem is that the example is used to explain the wrong phenomenon. The way that it is currently worded it makes it sound like it's completely normal to have a positive work from friction, and thus friction can create energy. Would you not agree that friction cannot create energy?
2. Secondly, do you find fault with my proof above?
3. Thirdly, I think that the text should be rewritten to explain that friction can never to positive work in the most basic inertial frame of reference (i.e. the frame of reference of the two items in which the friction is between). Then, after note that, if the inertial frame of reference is then changed to include a third party then friction can do positive or negative work. Then the above example can be used to show that positive work can be done with a different FOR. --Wizard191 (talk) 16:42, 18 August 2008 (UTC)
Nope, just busy with other articles and even non-Wikipedia life.
1. If you think there is some problem with a point, perhaps you could correct it, not just delete it. As the examples show, it is perfectly normal for friction to do positive work, though perhaps not as often as negative work. Since we know that, short of atomic physics and even then it appears to be just a conversion of form, one cannot create energy, I doubt that there is danger of confusion.
2. This appears to be mostly a semantics issue, and so I don't believe it will be resolved by proof. Besides, Wikipedia is more about references than proof. If you could find a credible reference that asserts that friction cannot do positive work, then we would have to edit the article to say something like "While some authors assert that... others claim the opposite." I've actually had to do that in several fascinating cases, but not yet this one. Most of the Mechanics text books I examined don't state clearly one way or the other.
3. I don't think we use "inertial frame of reference" in the same way. To me, "the most basic inertial frame of reference", for situations that occur on or near the surface of the Earth, is the surface of the Earth. That is the reference point used throughout the entire rug example. To an observer standing still on the surface of the Earth, in an inertial, non-accelerating, Newtonian reference frame, the force of friction is in the direction of the movement of the box and so does positive work. On the other hand, the "frame of reference of the two items in which the friction is between" is accelerating, as stated in both examples, and so cannot be an inertial reference frame.
In any case, I'm enjoying this academic excersise. -AndrewDressel (talk) 20:11, 18 August 2008 (UTC)
You are correct; I shouldn't have just blow the stuff away without double checking with you. You are also correct that a proof isn't as widely accepted and that citations are usually what is requested around here. As such I have been searching all over the place (with my college text being of no avail), but have managed to find this: http://books.google.com/books?id=WRXrtu44W9UC&pg=PA142&dq=friction+work&lr=&as_brr=3&ei=W_upSLqOIIiQjgHk3rj7BA&sig=ACfU3U2YDMXD4Fa2KeBXb7n9Gx-xyGVqwg#PPA142,M2 (Specifically the bold text in the middle of page 142). Therefore work is always negative between two items (which is all I have been trying to say all along). The example is still correct, because, with reference to the ground (earth) the frictional force is doing positive work. I have to admit that I was incorrect in stating that the most basic inertial frame of reference is between the rug and the box, because you are right that the box is in motion and that can't be used as the FOR.
I agree, this discussion has given my mind quite the work out. --Wizard191 (talk) 23:26, 18 August 2008 (UTC)
Nice find. So the internal friction forces in a system of rigid bodies always do negative work. With your reference, that'll make a worthwhile addition. -AndrewDressel (talk) 03:50, 19 August 2008 (UTC)
Andrew, please look over my edit to make sure I didn't screw anything up. I'm glad we got to the bottom of this. It's been a pleasure having this intellectual debate with you. --Wizard191 (talk) 00:01, 20 August 2008 (UTC)
I tweaked it a little. The second reference actually makes an example of static friction, so I generalized you comments about work done in an inertial frame. I also shortened "frictional forces" to just "friction". Please have a look and make adjustments as necessary. Thanks again for sticking with this and finding that first reference. -AndrewDressel (talk) 01:09, 20 August 2008 (UTC)
Looks good to me! --Wizard191 (talk) 13:00, 20 August 2008 (UTC)
Wait a minute. Did someone try to use the surface of the earth as an inertial reference frame? If I understand the sources, inertial FsOR are characterized by straight line unaccelerated motion. The earth is spinning in a gravity well. What we perceive as straight line motion will look curved from a true inertial FOR, and straight line motion in a true FOR will look curved to us. The rug example is good but it shouldn't say the surface of the earth is an inertial FOR. -- Another Stickler (talk) 07:24, 8 December 2008 (UTC)
Seriously? Check out the Coriolis effect article, specifically Length scales and the Rossby number: "Needless to say, one does not worry about which hemisphere one is in when playing catch in the garden." For the purposes of calculating the work done by pulling a rug out from under a box, the surface of the earth is a perfectly valid inertial reference frame. -AndrewDressel (talk) 14:42, 8 December 2008 (UTC)
Seriously. The ground may be a valid reference frame for the purpose of the example, but it's not an inertial one. I've fixed the sentence. -- Another Stickler (talk) 22:47, 9 December 2008 (UTC)
Not that it matters any more, but here finally are the references I was looking for: Interestingly, for all but a small class of engineering problems (e.g., as long as we stay away from relativistic effects or are not interested in orbital mechanics), a frame attached to the surface of the Earth can be considered inertial.[1]; and A reference frame attached to the earth can be considered to be an inertial frame for most practical purposes.[2] -AndrewDressel (talk) 14:18, 5 June 2009 (UTC)
1. ^ Gray, Costanzo, Plesha (2010). Engineering Mechanics: Dynamics. McGraw-Hill. p. 188. ISBN 978-0-07-282871-9.
2. ^ Halliday and Resnick (1977). Physics: Part 1. Wiley. p. 75. ISBN 94801-2 Check |isbn= value: length (help).
Sweet! Thanks for the research. Wizard191 (talk) 21:43, 5 June 2009 (UTC)

(un-indent) Huh? How about this, lets replace the word "earth" with "fixed surface in space-time". Now is it an IFOR? For the purposes of this example the earth is a valid IFOR. Wizard191 (talk) 00:47, 10 December 2008 (UTC)

That sounds like "absolute space". If space had its own permanent coordinate system, then you could use space as an inertial frame of reference, but there's no way to tell where anything is relative to space, only where things are relative to other things in space, so we can't ever know if we have a "fixed surface in space-time". For the purposes of this example, earth doesn't need to be an IFOR, so why even bother trying to fudge the truth by saying it is? -- Another Stickler (talk) 20:00, 16 December 2008 (UTC)

True. I will admit that you are correct that *technically* the earth isn't a IFOR, however, it is close enough to be one, with respect to the example. None of the objects are moving near the speed of light. This is a situation where the difference between FOR and IFOR is negligible. Physicists and engineers often do this to simplify calculations and examples so that they don't need to go through tedious and needless work, calculations, and/or explanations. Wizard191 (talk) 16:38, 19 December 2008 (UTC)
The speed of light doesn't really come in to it. All that matters is whether the FOR is accelerating. If it isn't, then it's an IFOR. -- Another Stickler (talk) 01:43, 20 December 2008 (UTC)

## Engineering vs. Physics

The article sometimes wanders between two incompatible points of view; that of the physicist, and that of the engineer. They are different because an engineer looks for pragmatic solutions to macroscopic, real-world problems; while a physicist tries to construct microscopic explanations for observed phenomena.

For example, lubrication. The engineer says that lubrication reduces the friction between moving parts. They physicist says, it doesn't reduce the friction: It completely changes the problem. From the physicist's point of view, there is no friction between lubricated parts. Instead, he sees friction between each part and the lubricant film, and he sees internal friction within the film.

Same goes for ball bearings and roller bearings. An engineer sees them as a solution to a problem, while a physicist sees them as defining an entirely new problem. 192.55.12.36 (talk) 16:00, 23 June 2008 (UTC)

Good points. What would be the best way to handle this? Two separate articles? Probably not. Sub headings for engineering and physics when appropriate? Might work. -AndrewDressel (talk) 22:17, 23 June 2008 (UTC)
I agree this is a good point. In my opinion, I think that this article should (and mostly does) mainly regard friction from the physics standpoint, because this is what friction truly is. However, it should be noted that non-strictly speaking friction may be thought of in the "engineering" sense (I'm an engineer, and it's true that when we speak of friction we always thing of it as described above). I think another paragraph should be added to the intro to just cover this base. It probably should be cleaned up if it's anywhere else in the article. Wizard191 (talk) 23:07, 23 June 2008 (UTC)

## Sliding Friction

The article states "Contrary to popular credibility, sliding friction is not caused by surface roughness, but by chemical bonding between the surfaces." But the separate article on sliding friction states that "Factors affecting sliding friction include weight (normal force) and the roughness of the two surfaces." And the source of both of those quotes (http://amasci.com/miscon/miscon4.html#fric) doesn't look very trustworthy. Can someone make sure this is right? 99.163.22.236 (talk) 17:14, 14 July 2008 (UTC)

• The other article was incorrect. The above discussion may also be enlightening. Wizard191 (talk) 01:10, 15 July 2008 (UTC)

Should there be more elaboration about the nano-scale causes of friction in the article? Quantalume Wanderer (talk) 02:49, 8 December 2008 (UTC)

Yes. Will you elaborate on them? The way I understand the (separate) roughness issue is that protrusions on both surfaces while seeming to resist the relative motion of surfaces are actually developing normal forces preventing the protrusions from occupying the same space, not friction. -- Another Stickler (talk) 06:33, 8 December 2008 (UTC)

## Incorrect statement about ball bearings

The article seems to claim that ball bearings are used to reduce friction. This is clearly incorrect.

Ball bearings are not used to reduce friction. Instead, as with all other kinds of bearings, they are used to reduce the stress on the materials used by distributing the forces affecting a hub or a joint more evenly. The bearings are lubricated to reduce friction. Actually, a joint or a hub rolling without any kind of bearings resists motion less than a joint or a hub with bearings, at least until the materials wear out enough to cause additional motion resistance (due to uneven shape or incorrect fittings). --Tappel (talk) 10:11, 14 August 2008 (UTC)

This is an interesting point you bring up here. Isn't rolling friction less than kinetic friction? That's what the article says. If that's the case the bearing doesn't need to be lubricated to reduce the friction. The lubrication will just keep the bearing from internally wearing. That's just my though. Wizard191 (talk) 13:05, 14 August 2008 (UTC)
Contrary to popular belief, ball bearings do not replace sliding friction with rolling friction. If you look closely at ball bearings, it becomes obvious that they slide inside their casings. There is at least one device i can think of that actually replaces sliding friction with rolling friction; it is called a planetary gear. --Tappel (talk) 06:17, 15 August 2008 (UTC)
I don't know if I buy it. If it was the case that the balls or rollers slide then why even put it in there? I can put a hardened shaft in a hardened hole and let it rotate and slide just the same. Bottom line...lets see some citations. --Wizard191 (talk) 12:06, 15 August 2008 (UTC)
I just busted out my Shigley book (which is well respected in the academic world) and the intro of the bearing section says:
"Rolling bearings are used to describe that class of bearing in which the main load is transferred through elements in rolling contact rather than in sliding contact. In a rolling bearing the starting fiction is twice the running friction, but still it is negligible in comparison with the starting friction of a sleeve bearing." (italics mine)
I think that says it all. --Wizard191 (talk) 12:57, 15 August 2008 (UTC)
A quick glance at the ball bearing article reveals an animation which clearly shows the balls rolling between the two races, not sliding. -AndrewDressel (talk) 14:44, 15 August 2008 (UTC)
That animation is missing a part. The illustration under the animation shows the part in gold that the ball bearings slide against. This is the part that keeps them equally spaced. If that part were not included, the ball bearings would bunch up and slide against each other instead. -- Another Stickler (talk) 21:32, 28 November 2008 (UTC)
Yes, either the balls slide against each other or they slide against the optional cage, but the normal force at these contacts is negligible compared to the borne load and does not mean that ball bearings do not replace sliding friction with rolling friction. The cage is usually made of stamped steel or even plastic and thus cannot bear more than a trivial load. -AndrewDressel (talk) 22:44, 28 November 2008 (UTC)

## Kinetic > static?

Are there any examples where the (coefficient of) static friction is less than (that of) the kinetic/dynamic friction, as the article implies there are?? —DIV (128.250.80.15 (talk) 07:03, 15 September 2008 (UTC))

This table on this page [1] says aluminum on aluminum has a higher kinetic COF (coefficient of friction) than static COF. It also says steel on steel has the same COF for kinetic friction and sliding friction. Either makes "static COF is always higher than kinetic COF" untrue. -- Another Stickler (talk) 23:18, 28 November 2008 (UTC)
That cannot be right. That would imply that if the external force (to which friction is opposed) is slowly increased until the surfaces slip, the friction force would suddenly jump up (due to the change from static to dynamic friction) and would overpower the (slowly varying) external force preventing slippage. That's a contradiction. What gives? Dauto (talk) 06:37, 14 March 2009 (UTC)
That is actually what happens, its called stick slip. Take a look at stick-slip phenomenon , although it's not that good a page. Many times when something judders to a halt - exhibiting stick-slip motion - it probably has greater kinetic than static friction. This can be the case because "static" and "kinetic" friction are names given to the same thing; friction. The curve of friction force against speed is singular - there's only one point on it for each speed. We call the point at v=0 the static friction force (or, the maximum static friction) and all the other points on it the kinetic friction. There's no particular reason why the curve should be continuous or have the highest point at zero, so it doesn't always (although i suspect that even if it seems to jump at zero it probably just has a very high gradient). Stick-slip happens often on a nano scale also - although i gather its due to the actual periodic shape of the solids in that case. Things get quantum then. - Nyb.Thering (talk) 18:33, 6 April 2009 (UTC)
Stick-slip actually often results from kinetic friction being lower than static friction instead of higher. The important point is that the "coefficient of kinetic friction" is actually an idealization. More generally you might say that the coefficient of friction is dependent on sliding velocity, mu = mu(v), (for v>0). Typically mu decreases with increasing v but not always. This view is itself an idealization too. It describes well situations in which v is constant or slowly changing, but does not describe highly dynamic situations well. These are the subject of ongoing research, look for "rate- and state-dependent friction laws". Edwinv1970 (talk) 13:31, 29 May 2012 (UTC)

## Fluid friction

AndrewDressel, the term fluid friction has a wider meaning than what you are trying to force into the article. It is not only fluid in contact with two solids (as in Newton's rotating-disk experiment), it is also solid-against-fluid, and fluid-against-fluid. I think we have to remember that friction is electromagnetic force historically given different names in different contexts, and that there may be overlap, and even contradiction between those names. We just need to note the usages, not try to correct history. The following are some quotes showing various usages. -- Another Stickler (talk) 23:11, 23 December 2008 (UTC)

"Fluid friction: friction force in which at least one of the object is is a fluid (i.e. either a gas or a liquid)."--http://www.eformulae.com/glossary/physics_glossary.php -- Another Stickler (talk) 23:11, 23 December 2008 (UTC)

"The resistance to an object's motion through a fluid may be termed "fluid friction." It may take the form of viscous resistance in a liquid, or the rather different character of air friction when an object moves through a gas."--http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html -- Another Stickler (talk) 23:11, 23 December 2008 (UTC)

"Fluid friction is observed in the flow of liquids and gases. Its causes are similar to those responsible for friction between solid surfaces, for it also depends on the chemical nature of the fluid and the nature of the surface over which the fluid is flowing. The tendency of the liquid to resist flow, i.e., its degree of viscosity, is another important factor. Fluid friction is affected by increased velocities, and the modern streamline design of airplanes and automobiles is the result of engineers' efforts to minimize fluid friction while retaining speed and protecting structure."--The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2007--http://www.infoplease.com/ce6/sci/A0858296.html -- Another Stickler (talk) 23:11, 23 December 2008 (UTC)

"When we have thoroughly investigated the laws and working of this fluid friction and ascertained its true nature and limits we shall then and not till then be justified in forming an opinion as to whether it be or be not possible to meet and deal with it successfully by methods which are practically applicable to ocean navigation" http://books.google.com/books?id=VY8KAAAAIAAJ&dq=define%3Afluid%20friction&client=firefox-a&pg=RA3-PA676&ci=84,524,899,240&source=bookclip">Annual Report of the Board of Regents of the Smithsonian Institution By Smithsonian Institution Board of Regents, United States National Museum, Smithsonian Institution, page 676 -- Another Stickler (talk) 23:11, 23 December 2008 (UTC)

"...the reduction to a minimum of fluid friction by suitable modifications in the external materials and structure of our ships..." http://books.google.com/books?id=VY8KAAAAIAAJ&dq=define%3Afluid%20friction&client=firefox-a&pg=RA3-PA678&ci=70,1263,924,221&source=bookclip">Annual Report of the Board of Regents of the Smithsonian Institution By Smithsonian Institution Board of Regents, United States National Museum, Smithsonian Institution, page 678 -- Another Stickler (talk) 23:11, 23 December 2008 (UTC)

"When a stream of fluid flows over a solid surface or conversely when a solid moves in still fluid a resistance to the motion is generated commonly termed fluid friction. It is due to the viscosity of the fluid but generally the laws of fluid friction are very different from those of simple viscous resistance.", http://books.google.com/books?id=bVEEAAAAYAAJ&dq=define%3Afluid%20friction&client=firefox-a&pg=PA57&ci=493,407,422,70&source=bookclip">The Encyclopædia Britannica A Dictionary of Arts, Sciences, Literature and General Information By Hugh Chisholm, page 57 -- Another Stickler (talk) 23:11, 23 December 2008 (UTC)

"Fluid friction is developed when adjacent layers in a fluid (liquid or gas) are moving at different velocities." -- "Engineering Mechanics: Statics and Dynamics" by James L. Meriam, page 268 -- Another Stickler (talk) 23:11, 23 December 2008 (UTC)

Well pick a good reference, Meriam will do, and put it in. I can only cite the sources I have, and that isn't one of them. -AndrewDressel (talk) 01:19, 24 December 2008 (UTC)
Never mind. I found a copy of Meriam & Kraige and a copy of Beer & Johnston that confirms it and made the change myself. -AndrewDressel (talk) 00:32, 25 December 2008 (UTC)

## Image

I really believe that an image would enhance the text and should be included, but a quick glance at this talkpage says that the images contradict the text. I would be willing to edit any images from here in a way as so they do not contradict. Which image and what should I change? -- penubag  (talk) 21:19, 24 December 2008 (UTC)

File:Friction.svg is a good picture minus the blow up showing the asperities, implying that they are what induces friction. If you could modify the pic to remove that section view and add an "W" to the arrow pointing down to define the weight and add a "F" to the arrow pointing left to signify the applied load, that would probably make it a worthwhile picture to add. Wizard191 (talk) 01:30, 26 December 2008 (UTC)
Thanks for the reply. How does this look? -- penubag  (talk) 20:26, 31 December 2008 (UTC)
Much better, however could you tweek two things? First, the random mu in the lower left corner doesn't need to be there. Second, the equation "F = (mu)(Nf)" should either be "Ff = (mu)(N)" or just "(mu)(N)". Thanks for the help. Wizard191 (talk) 20:47, 31 December 2008 (UTC)
Done, the equation was actually correct, the XML coding of the image just shifted the position of the f but it should be fixed now. I've added the image to the article, tell me if there is anything else. -- penubag  (talk) 23:41, 31 December 2008 (UTC)
Looks good now. Thanks for your help! Wizard191 (talk) 18:12, 1 January 2009 (UTC)
penubag, I just added the text under the illustration in the article, and I was wondering--is the F arrow longer than the F_f arrow? This would mean that the box is moving, right? If so, I should expand the text. Alternatively, the arrows could be made the same length. -- Another Stickler (talk) 07:04, 13 January 2009 (UTC)
Sorry, I did not see your post but I fixed the image so the F = N and the arrows are the same size except for the force of friction. This means that the box is moving the the left. -- penubag  (talk) 04:25, 14 January 2009 (UTC)

Sorry, but friction is a phenomena between bodies and a friction is a name of a force affecting somebody. Two forces Ff are exist. One force affecting a green body, second affecting brown body (CM).Tadeusz Malinowski (talk) 17:03, 1 February 2009 (UTC)

Excellent point. That's one of the problems with using 'sort of' a free body diagram. The picture should either exclude the lower surface, include friction force arrows in both directions, or show two sketches, as the image used in the free body diagram article does. -AndrewDressel (talk) 19:56, 2 February 2009 (UTC)
But it isn't the free body diagram (look). Tadeusz Malinowski (talk) 00:29, 17 February 2009 (UTC)

Force F isn't necessary. Necessary is a vector velocity (v).Tadeusz Malinowski (talk) 17:03, 1 February 2009 (UTC)

No, Force F isn't necessary, if there is relative motion. At the same time, a velocity vector is not necessary if there is a force vector, as in the case currently depicted. Either, and even both, will do. -AndrewDressel (talk) 19:56, 2 February 2009 (UTC)
It is a part of the text under the picture in the article "Since the magnitude of the applied force is greater than the magnitude of the force of kinetic friction opposing it, the block is moving to the left...". It isn't true.Tadeusz Malinowski (talk) 00:38, 17 February 2009 (UTC)

A good tradition order to located a vector point of application in centre of mass body (CM).Tadeusz Malinowski (talk) 17:03, 1 February 2009 (UTC)

That is depends completely on the model used and the phenomena examined. For example the [Suspension_(motorcycle)#Brake_Dive|brake dive] of a motorcycle cannot be modeled by applying wheel forces at the center of mass of anything. -AndrewDressel (talk) 19:56, 2 February 2009 (UTC)
Are you sure? Here we have the simple model.Tadeusz Malinowski (talk) 00:47, 17 February 2009 (UTC)

The recent change from "Friction is the force resisting relative lateral motion. It is usually subdivided into several varieties:" to " Friction is the force resisting the relative lateral (tangential) motion of surfaces in contact. It is usually subdivided into several varieties:" now specifically excludes four of the five examples cited:

• lubricated friction or fluid friction resists relative lateral motion of two solid surfaces separated by a layer of gas or liquid.
• fluid friction is also used to describe the friction between layers within a fluid that are moving relative to each other.
• skin friction is a component of drag, the force resisting the motion of a solid body through a fluid.
• internal friction is the force resisting motion between the particles making up a solid material while it undergoes deformation.

Can we please put it back the way it was? -AndrewDressel (talk) 04:08, 7 January 2009 (UTC)

Hi. Thanks for not just undoing it. I made that change with the understanding that the term surfaces includes solid surfaces and fluid surfaces, so the fluid examples are not excluded (laminar flow is described as a stack of molecular-thickness layers that rub against each other, and the fluid layer closest to the solid surface also rubs against it). Regarding the internal friction line, an older version used to say something like "the surfaces of the particles making up a solid" rather than just "the particles making up the solid". The newer version excludes itself from the definition and needs to be fixed since friction is between surfaces, not between masses. I specifically didn't say "between solid and/or fluid surfaces" since the list immediately following explicitly shows both the solid and fluid examples and "surfaces" is inclusive. -- Another Stickler (talk) 11:13, 8 January 2009 (UTC)
You may understand that 'surfaces' includes 'solid surfaces' and 'fluid surfaces', but I think it is a stretch without explaining it first. Without that explanation, 'surfaces in contact' seems to exclude 'two solid surfaces separated by a layer of gas or liquid.'
What is wrong with the original text: "Friction is the force resisting relative lateral motion."? -AndrewDressel (talk) 12:04, 8 January 2009 (UTC)
While I completely understand your reasoning, and its completely correct, AS, I think the average reader will not make that connection. I feel that the original text was simpler and still correct, therefore should be restored. There's no need to complicate things in the introduction. Wizard191 (talk) 14:52, 8 January 2009 (UTC)
AndrewDressel, it's not a stretch that surfaces can be fluid. Everybody knows what "the surface of the ocean" means, and that it's between two fluids, water and air, or between a fluid and a solid, such as water and the boat floating on it. But I'll modify the sentence to explicitly state "solid and/or fluid surfaces" just to make sure nobody incorrectly assumes that "surfaces" means only "solid surfaces" since you think it's important to safeguard against that. More-explicit is generally better anyway; ambiguity is our enemy. Regarding the other improvements: "(tangential)" is the companion term to "normal" (as in "normal force") and mathematically describes the friction force vector's direction as residing on a particular plane relative to the surface (and does all that with a single word), a more specific way to say lateral, but put in parentheses because it cannot replace lateral, which is more useful to the general public (I suppose we could even make it more general by saying horizontal too, as in "horizontal (lateral or tangential) motion"). The specifier "of surfaces in contact" is necessary to exclude things which are in relative lateral motion but are not in contact and therefore don't develop friction (even if other forces are resisting their relative lateral motion). Wizard191, it's essential to the definition that friction only develops between surfaces and that those surfaces must be in contact. The earlier version if the sentence left both of those points out. That's why it shouldn't be restored. Forge ahead, not backward. -- Another Stickler (talk) 22:05, 8 January 2009 (UTC)
So now fluids have surfaces between layers, and atomic particles have surfaces? Sorry, but the word 'surfaces' just isn't appropriate nor necessary. Even 'contact' is nebulous for the scale at which friction occurs. Some authors use these terms for some of their definitions, which has already been cited, but not all do and so we shouldn't. -AndrewDressel (talk) 00:33, 9 January 2009 (UTC)
Friction is a tangential force between surfaces in contact. My putting that in the lead sentence is completely defensible. It's going to take me forever to go over the article if you're going to fight me on every sentence. Here (emphasis mine): "The friction force (vector F_f) is a tangential force acting on an object that opposes the sliding of that object on an adjacent surface with which it is in contact." [3] College Physics, By Frederick J. Bueche, Eugene Hecht, George J. Hademenos page 12. -- Another Stickler (talk) 02:53, 10 January 2009 (UTC)
Phew! Finally back home and able to look at my references. Both Meriam and Kraige and Beer and Johnston define fluid friction as between adjacent layers in a fluid moving at different velocities. Mention of surfaces and contact is restricted to dry friction in both books. At one time, this article mentioned fluid friction merely as a synonym for lubricated friction, but now it includes this alternate definition, so the lead paragraph has to be general enough so as not to exclude it.
How much effort is required to get a sentence correct is irrelevent, I think. We're not working on a deadline, are we? -AndrewDressel (talk) 01:36, 14 January 2009 (UTC)

## merge from "Coefficient of friction" article

I sorted the other-language links I think were merged in from the coefficient of friction article. diff They were all together at the bottom of the list and sorted alphabetically there, so I think they were brought in at the same time. I commented each so which-is-which isn't lost. Since I don't actually speak those languages, and wasn't the one that merged them in, I figured I'd better note it here just in case a multi-linguist drops by and can make sure the links point to the right place (such as the link to the Spanish "Coeficiente de rozamiento" which also mentions "coeficiente de fricción" in the article itself, which I guess means its a synonym, so is probably OK). -- Another Stickler (talk) 00:23, 11 January 2009 (UTC)

Ronald S. Davis (talk) 22:08, 24 January 2009 (UTC) The article says "... now understood not to be caused not by surface roughness ...". One of the "not"s should be eliminated.

## Roughness IS important in friction

The article states

Contrary to earlier explanations, kinetic friction is now understood not to be caused by surface roughness but by chemical bonding between the surfaces

and as a source gives link to a web page which reads:

Some books point to surface roughness as the explanation of sliding friction. Surface roughness merely makes the moving surfaces bounce up and down as they move, and any energy lost in pushing the surfaces apart is regained when they fall together again. Friction is mostly caused by chemical bonding between the moving surfaces; it is caused by stickiness. Even scientists once believed this misconception, and they explained friction as being caused by "interlocking asperites", the "asperites" being microscopic bumps on surfaces. But the modern sciences of surfaces, of abrasion, and of lubrication explain sliding friction in terms of chemical bonding and "stick & slip" processes. The subject is still full of unknowns, and new discoveries await those who make surface science their profession

The statement

"Surface roughness merely makes the moving surfaces bounce up and down as they move, and any energy lost in pushing the surfaces apart is regained when they fall together again."

is completely wrong. First because it assumes the sliding bodies are infinitely stiff, when two interlocking groups of atoms forming bumps hit each other they not only make the object rise, they also start oscillating more vigorously and these oscillations propagate through the material as heat, as a result carrying kinetic energy away from the interface. Second the energy used for rising the object is NOT regained when it comes down because now the force is directed down and not along the way the object is being moved. It only works to lower the friction somewhat due to lowering the contact surface.

Roughness definitely does impact the friction, to what extent depends on the materials and other factors. Bonding also plays an important role, in general all electromagnetic effects at the interface can play important role, again it all depends on materials and other factors.

Polishing is a good example of this, in situations where friction is dominated by bonding the more polished and smooth the surfaces the *higher* the friction should be as more atoms have the chance to form bonds, in situations where roughness dominates the friction the more polished the material the lower the friction. For example if you take two very rough metal surfaces roughness will dominate friction, then if you polish it the friction will get lower and lower, but after a certain threshold when the surfaces are almost perfectly smooth further polishing will start increasing the friction again this time it's due to bonding.

So to sum it up it's definitely wrong to state that surface roughness is not important for friction of macroscopic objects, and the source used is of very poor quality and should be removed.195.242.255.83 (talk) 11:49, 26 February 2009 (UTC)

Cool. All you need is a reliable source and you can put that in the article. -AndrewDressel (talk) 13:47, 26 February 2009 (UTC)
I managed to find this explanation from Columbia encyclopedia which is good http://www.bartleby.com/65/fr/friction.html it basically says more or less what I wrote, If needed more sources could perhaps be found on the web. I would however prefer not to rewrite the part of the article myself since my English writing skill is not very good. 195.242.255.83 (talk) 01:09, 28 February 2009 (UTC)

Ok, to try to clear this up and to shed some light on what the source meant - friction is not caused by surface roughness. Friction is caused by attraction between atoms - that is bonds. However, the roughness of the surface is extremely important - it determines where the objects are actually touching. The difference can be made clear if you look at the large numbers of physics papers looking at the frictional properties of single asperities - in order to understand friction they take one very small object and look at what happens when they run it along other objects. Usually they use the tip of an Atomic force microscope as the single asperity. They do this so that the roughness of the surface is taken out of consideration - they are now looking at the friction caused by a small area of contact where the surfaces are actually touching everywhere . So, to summarise - friction, caused by bonds not roughness. Amount of friction, determined by bonds and roughness. Hope this helps a little. I'd give some references here, but i don't know any papers that actually say what's here. Nyb.Thering (talk) 18:51, 6 April 2009 (UTC)

## Traction

This statement: "Friction is distinct from traction. Surface area does not affect friction significantly because as contact area increases, force per unit area decreases. In traction, however, surface area is important." is incorrect and should be removed. Dauto (talk) 17:52, 13 March 2009 (UTC)

## Arc-boutement

Hello,

Arc-boutement
Hinge joint inside the conatct cylinder of a piston

I try to find the translation of the french word arc-boutement, which is a geometrical condition that leads to immobility, as illustrated besides. This phenomenon related to friction is important, e.g. wen designing pistons: the hinge joint must be inside the contact cylinder. This locking phenomenon is also used in freewheels.

Any clue?

cdang|write me 11:08, 26 March 2009 (UTC)

While browsing through patents, I found "over-center locking". Is this correct?
cdang|write me 14:01, 26 March 2009 (UTC)
I understand the phenomenon you are discussing, although I'm unsure of the English translation for it. I do know that you aren't looking for "over-center locking" which is the same as a toggle mechanism. (This link is practically worthless) Wizard191 (talk) 18:50, 26 March 2009 (UTC)
In engineering I've always seen it expressed as the 2:1 rule (or 2:1 ratio) as shown on this website: http://www.lm76.com/newpage6.htm. Wizard191 (talk) 20:10, 26 March 2009 (UTC)
cdang|write me 11:25, 27 March 2009 (UTC)
BTW, I was inspired by a drawing on the webpage and drew File:Arc-boutement porte a faux regle 2 sur 1.svg.
cdang|write me 13:36, 27 March 2009 (UTC
I'm glad to have been of assistance. BTW...That's a really nice drawing you made. We need to show it off in an article now =) Wizard191 (talk) 16:41, 27 March 2009 (UTC)
The Dutch name is 'schranken'. See nl.wikipedia.org/wiki/Schranken. In nl.wikipedia.org/wiki/Zelfremmendheid the background is explained. In German the name is 'Schubladeneffekt'. LaurensvanLieshout (talk) 18:12, 26 August 2012 (UTC)
If anyone is still curious or wondering, the term we use for that phenomenon in an engineering context is "binding of a linear bearing", discussed here: http://www.igus.com/wpck/11047/Stick_slip_TechTalk. 67.40.138.59 (talk) 19:06, 16 July 2015 (UTC)

## cone of friction

Perhaps a better treatment of friction is the cone of friction versus the coef of friction model. In other words the net force exerted by a surface is confined to a cone in nature with a min and max defining the angle of the cone and the force component. Lots of people prefer to refer to it that way. —Preceding unsigned comment added by 122.167.183.22 (talk) 10:46, 30 March 2009 (UTC) http://www.codecogs.com/reference/engineering/mechanics/angle_and_cone_of_friction.php is a good reference It is not always possible to assume that coef of friction is constant etc. —Preceding unsigned comment added by 122.167.183.22 (talk) 10:49, 30 March 2009 (UTC)

## Proposed merge from Traction (engineering)

Traction (engineering) - suggest merging this article into friction - the article is about "coefficient of traction" - basically just a specialised term for "coefficient of friction"FengRail (talk) 15:41, 10 April 2009 (UTC)

Withdrawn proposal - for those still interested in tidying Traction (engineering) please see that talk page.FengRail (talk) 18:15, 11 April 2009 (UTC)

## ratio of kinetic to static friction

It has been proposed that the relavent sentence read

The coefficient of kinetic friction is typically denoted as μk, and can never be greater than the coefficient of static friction for the same materials.

This is a strong claim that requires a reliable source at least. Most sources I find instead state something closer to the previous formulation:

The coefficient of kinetic friction is typically denoted as μk, and is usually less than the coefficient of static friction for the same materials.[1][2]

In fact, Feynman reports that "with dry metals it is very hard to show any difference."[3]

Finally, new models are beginning to show how kinetic friction can be greater than static friction.[4]

Is there a comparable source to confirm the new claim that kinetic friction can never be greater than static friction? -AndrewDressel (talk) 15:47, 16 October 2009 (UTC)

1. ^ Sheppard and Tongue (2005). Statics: Analysis and Design of Sytems in Equilibrium. Wiley and Sons. p. 618. In general, for given contacing surfaces, μk < μs
2. ^ Meriam and Kraige (2002). Engineering Mechanics: Statics. Wiley and Sons. p. 330. Kinetic friction force is usually somewhat less than the maximum static friction force.
3. ^ Feynman, Leighton, and Sands (1964). "The Feynman Lectures on Physics, Vol. I, p. 12-5". Addison-Wesley. Retrieved 2009-10-16.
4. ^ Persson and Volokitin (2002). "Theory of rubber friction: Nonstationary sliding" (PDF). PHYSICAL REVIEW B, VOLUME 65, 134106. Retrieved 2009-10-16.
FYI, there was a discussion about this previously (Kinetic > static?). Wizard191 (talk) 15:59, 16 October 2009 (UTC)

The link (both here, above and in the article) for the Feynman Lectures on physics points to an unrelated physics website, and should be fixed. Furrfu (talk) 17:41, 22 June 2013 (UTC)

## Minor flaw in the top illustration.

The magnitude of the applied force is greater than the friction so the object is accelerating , not just moving. If it was moving with a constant speed the magnitude of the applied force would have to be equal to the force of friction. Magnus.ivarsen (talk) 14:05, 27 October 2009 (UTC)

Quite right. I'll correct that. Wizard191 (talk) 15:09, 27 October 2009 (UTC)

## Citation incorrect

Citation #18 is incorrect; the correct volume is 319, not 316.

Gigabake (talk) 00:18, 28 November 2009 (UTC)

Thanks! I've fixed it. Wizard191 (talk) 19:34, 28 November 2009 (UTC)

## equation could be improved

the current equation lists the friction coefficient as being outside of the integral - this inherently assumes that the mu value is independent of the sliding distance - I submit that this is erroneous; it assumes two things which are not appropriate for a 'general case' formula:

i) that the friction here is between two entirely homogeneous bodies; in practice, the measured coefficient would vary at different locations on the surface, due to varying features of the surface profile, both topological and in terms of chemical composition and/or microstructure;

ii) that the friction coefficient is in itself not a function of the normal force; in practice for some materials (e.g. UHMWPE) the degree of this normal force has been shown to alter the friction coefficient; therefore mu is in fact a function of N and, since N is included as a varying term within the integral, mu should also be included as a varying term within the integral, in order to retain maximal general applicability.

regards,

Mike —Preceding unsigned comment added by 152.78.128.150 (talk) 18:54, 22 December 2009 (UTC)

Thanks, This Helped Me Very Much. Please Carry On Pointing Things Out,

Kind Regards And Sincerity,

An Anonymous User. — Preceding unsigned comment added by 86.169.106.20 (talk) 10:23, 8 November 2015 (UTC)

## Citation for decreased COF with increased load for PTFE

Pleaback (talk) 15:23, 8 March 2010 (UTC)

## Abu Rayhan Biruni

Apparently not in citation given? Rich Farmbrough, 23:53, 8 April 2010 (UTC).

I don't have the source, but if its not there then why don't we just remove it? Wizard191 (talk) 12:20, 9 April 2010 (UTC)
I´ve checked the source and the author never claims Biruni made and important contribution on how we understand friction.He only puts a text were Biruni speaks on the formation of the Earth(nothing to do with the topic) and says that "The earth and the water form one globe, surrounded on all sides by air. Then, since much of the air is in contact with the sphere of the moon, it becomes heated in consequence of the movement and friction of the parts in contact".The fact that friction produces heat was,of course, not discovered by Biruni,because it´s obvious.The credit should go to the Homo erectus that first produced fire!!!
Friction,understood phisically,came much later.So,I´ll remove it --Knight1993 (talk) 19:08, 24 April 2010 (UTC)

## Edit request from Valoyspoerl, 14 May 2010

{{editsemiprotected}} Citation needed: Contrary to earlier explanations, kinetic friction is now understood not to be caused by surface roughness but by chemical bonding between the surfaces.[6]

Reference with a different interpretation: Physics (Text book) A Practical and Conceptual Approach Second Edition Saunders College Publishing Wilson Pages 583 ISBN 0-03-023764-5 Page 12 Examples of Newton's Second Law Friction

Due to local adhesion or "sticking" between the surface irregularities rather than to their fitting together.

Conflicting words: Irregularites rather than chemical bonding. Valoyspoerl (talk) 19:24, 14 May 2010 (UTC)

I would imagine that this book, from 1989, is exactly the sort of earlier explanations it is talking about - whereas the present cited fact comes from a 2007 paper.
If you disagree, please elaborate further below. 21:53, 14 May 2010 (UTC)

Not done

## New even more frictionless material researched

Hello together, I just stumbled upon this page and saw the example of the surface with the "lowest friction" under the "Approximate coefficients of friction" table of number 2.1. On another page I have seen that an even more friction less material has been researched: http://www.azom.com/news.asp?newsID=4186 Quote: "An ultrahard carbon film coating many times slicker than Teflon has been developed by Argonne researchers. The new material's coefficient of friction is less than 0.001 when measured in a dry nitrogen atmosphere--20 times lower than the previous record holder molybdenum disulfide. When tested under the same conditions, Teflon's coefficient of friction is around 0.04." As I do not have a Wiki-Account yet and may anyhow not be allowed to change this article due to its semi protection it would be nice if someone might add this information. Have a nice day and see you around. Best regards Philip R. —Preceding unsigned comment added by 77.9.36.193 (talk) 12:45, 16 May 2010 (UTC)

## Static Friction notation

The section on Static Friction defines "The maximum possible friction force between two surfaces before sliding ...: ${\displaystyle f=\mu _{s}F_{n}\,}$." But in the remaining article it uses the symbol ${\displaystyle F_{max}\,}$. If there are no objections, I will replace the definition of f with ${\displaystyle f_{max}=\mu _{s}F_{n}\,}$ and then replace all references to ${\displaystyle F_{max}\,}$ with ${\displaystyle f_{max}\,}$. cperk (talk) 16:25, 25 September 2010 (UTC)

Seeing how all the other forces use a capital F, it would make sense to just change the one lowercase instance to a capital F (so Fmax). Wizard191 (talk) 23:24, 25 September 2010 (UTC)

## THE OLD AND NEW EXPLANATIONS

Let us begin with the OLD EXPLANATION for the HEAT produced during friction: When the atoms on the two surfaces in contact during motion bump into one another, the kinetic energy of such atoms increase. This means that the INTERNAL ENERGY of both systems increase, specifically the THERMAL ENERGY. This component of the internal energy has two subcomponents: the SENSIBLE ENERGY (internal kinetic energy) and the LATENT ENERGY (one of the many components of the internal potential energy). An increase in the former causes an increment in temperature and an appropiate increase in the latter, eventually, will cause a change in the state (solid to liquid and then to gas). To simplify the analysis, it is supposed that the surfaces are at the same temperature and therefore, there is no thermal contact. This means that this increment in the INTERNAL ENERGY was not caused by a transference of energy in the form of HEAT. Later, if the the objects have different physical properties (heat conductivity), one of them might experience a greater increment in its temperature. That difference in temperature creates the conditions for thermal contact AND THEN HEAT SHOWS UP. The body at higher temperature provides energy to the other (and the environment in non-closed systems) in the form of HEAT.Now the NEW ONE: The atoms of both surfaces bump into one another and CHEMICAL BONDS are BROKEN, this fact causes the atoms to release energy in the form of HEAT. I hope other contributors find these, my insights, helpful to explain better this topic. What I've said is the result of all my learning on this topic. I hope many others can find the sources to support this contribution. I do not have a precise information about what part comes from what book. But I guess it shouldn't be difficult. I have read many books but they are just regular Engineering and Physics books all in all.George Rodney Maruri Game (talk) 13:53, 22 October 2010 (UTC)

## Friction fraud?

Nelkon & Parker (well-known text book in many editions) has an example of sand dropping at 2 kg/s vertically onto a conveyor belt moving horizontally maintaining a steady .1 m/s. It says:

Extra power to keep belt moving is = work/second = force x rate of displacement = force x velocity = 0.2 x 0.1 = .02 watt.
Kinetic energy imparted to sand each second = .5 mv2 = 0.5 x 2 x 0.12 = 0.01 watt.

It then goes on to say that "the extra power is twice the kinetic energy imparted to the sand because the sand does not immediately assume the velocity of the belt so that the belt moves relative to the sand. The extra power is needed to overcome the friction between the sand and belt." Does anyone think this is correct? Ammimajus (talk) 14:26, 24 October 2010 (UTC)

I think they are right. The total force on the belt is constant, as is its velocity. Each grain of sand exerts a force ${\displaystyle F_{f}=\mu F_{N}}$ from the moment it hits the belt until the grain reaches the belt's speed ${\displaystyle v_{b}}$. After the grain gets up to speed, it no longer exerts a force on the belt. A little math will show that the time required to get up to speed is ${\displaystyle t_{b}={v_{b}m_{g} \over F_{f}}={v_{b} \over \mu g}}$, where ${\displaystyle m_{g}}$ is the mass of the grain and ${\displaystyle g}$ is the gravitational acceleration. The total force on the belt at any instant is just the total force exerted by all grains that fell in the previous time interval ${\displaystyle t_{b}}$, which is ${\displaystyle F_{f}\left({dm \over dt}\right)t_{b}{1 \over m_{g}}=\left({dm \over dt}\right)v_{b}}$. The power needed to keep the belt moving is then calculated as they describe. Intuitively, it makes sense that more power is needed to keep the belt moving than is transferred into kinetic energy of the sand. Some of the power has to end up as heat. The fact that the amount that ends up as heat exactly equals the amount that ends up as kinetic energy is surprising, and not at all obvious.--Srleffler (talk) 04:49, 2 August 2011 (UTC)
I think there is a fairly simple way to see why it is the case. The force on each grain is ${\displaystyle F_{f}=\mu F_{N}=\mu mg}$. Hence, since ${\displaystyle F=mA}$, the acceleration of each grain, ${\displaystyle A=\mu g}$, is constant while slipping continues until the grain reaches the speed of the belt, when the slipping stops. With constant acceleration the distance needed to reach the final velocity, ${\displaystyle v_{f}}$, can be found from the standard Newtonian equation, ${\displaystyle {(v_{f})^{2} \over 2}=A*distance}$. (This is derivable from change in kinetic energy = work = force * distance = mass * acceleration * distance with the initial velocity of the grain zero). The work required to carry out the acceleration is force x distance, ${\displaystyle \mu mg*(v_{f})^{2} \over 2A}$ = ${\displaystyle \mu mg*(v_{f})^{2} \over 2\mu g}$. Hence the work required to act against the friction is ${\displaystyle m(v_{f})^{2} \over 2}$, which is equal to the final kinetic energy of the grain. So when the grain is given a certain kinetic energy, an amount of additional energy (equal in size to the kinetic energy) has to be put in (wasted) to overcome friction. Hence the total energy per second, the power, that has to be put in to keep the belt moving is twice the final kinetic energy of the added grains. Of course, it is still slightly surprising (before the calculations are done) that the form of frictional force leads to a situation in which increasing the velocity of slipping sand requires wasting an amount of energy equal to the kinetic energy the sand acquires. Wodorabe (talk) 12:04, 4 June 2017 (UTC)

## Friction coefficient above 1 is ADHESION

Is it not true that for the forces to balance, when you pick up the object being slid, the force to pick it up must equal the weight, otherwise if it is higher there is a missing force in your balance. Talking about rubber having a coefficient of friction above one is nonsense - it is bound to the substrate, and the equations of friction no longer apply. —Preceding unsigned comment added by 74.96.95.76 (talk) 00:47, 16 November 2010 (UTC)

I cannot follow your logic, but you are welcome to find a reliable source that agrees with you conclusion and add it to the article. -AndrewDressel (talk) 14:25, 16 November 2010 (UTC)
When you pick up an object, you create an upward acceleration. The upward force you exert must therefore be greater than the weight of the object.--Srleffler (talk) 23:30, 1 August 2011 (UTC)

## Near zero

In the COF section, it says "Coefficients of friction range from near zero". What does "near zero" mean? It could very well mean 0.1, or 0.000000000000000000000000001. This needs a better definition. — Preceding unsigned comment added by InverseHypercube (talkcontribs) 07:41, 13 January 2011 (UTC)

## Characteristics Of Friction

Point of application: ? Line of Action : ? Direction: ? —Preceding unsigned comment added by 212.28.240.98 (talk) 15:58, 14 March 2011 (UTC)

## internal friction

Internal friction is also used to describe deviations from ideal elasticity, using a picture of a dynamic modulus or viscoelastic model. To me this is even the main meaning. I definitely would not call Plastic deformation of solids main article for internal friction - this article does not even mention internal friction. --Ulrich67 (talk) 20:48, 19 November 2011 (UTC)

## BAM lowest coefficient of friction

The article in question, and the peer edited source article it comes from, only state 0.02 coefficient of friction when lubricated and make no mention of it non-lubricated. This is why the BAM article itself doesn't only says "low coefficient of friction", because noone can find it's non-lubricated coefficient (it appears to not be published). This article should reflect that it's 0.02 lubricated; but I can't change it because it's locked. — Preceding unsigned comment added by 174.253.12.174 (talk) 03:52, 12 December 2011 (UTC)

There are only few refereed articles on this material. doi:10.1016/j.wear.2010.11.044 reports 0.02 in water-glycol-based lubricants and 0.08 in mineral oil. doi:10.1063/1.1615677 mentions 0.04 in dry scratching. I'll wait for comments and then update this and BAM articles. Materialscientist (talk) 04:41, 12 December 2011 (UTC)

## New theory on friction published

ScienceDaily (May 28, 2012) — Friction is a key phenomenon in applied physics, whose origin has been studied for centuries. Until now, it has been understood that mechanical wear-resistance and fluid lubrication affect friction, but the fundamental origin of sliding friction has been unknown. Dr. Lasse Makkonen, Principal Scientist at VTT Technical Research Centre of Finland, has now presented an explanation for the origin of sliding friction between solid objects.

--Cesarakg (talk) 01:51, 29 May 2012 (UTC)

Awesome paper! The key mechanism is that surfaces are disappearing and forming continuously upon entering and leaving the real contact area. Forming new surface area requires (kinetic) energy; disappearing surfaces provide energy that is dissipated. Very nice exposition, with compelling quantitative agreement to atomic force microscope measurements. Edwinv1970 (talk) 17:16, 29 May 2012 (UTC)
I am the author of "A thermodynamic model of sliding friction" published last year in AIP Advances (AIP Advances 2, 012179). The paper is open-access and topped the journal's "Top Most Read list" in 2012. It would seem appropriate to include its explanation of friction and the possibility of quantitative determination of the friction coefficient on the Wikipage. I could update the Friction page accordingly, but editing of that page seems not to be open. — Preceding unsigned comment added by LasseMakkonen (talkcontribs) 12:09, 4 January 2013 (UTC)

## Edit request on 2 June 2012

Excessive erosion or wear of mating surfaces occur when work due frictional forces rise to unacceptable levels.

Not done: please be more specific about what needs to be changed. Mdann52 (talk) 15:06, 2 June 2012 (UTC)

## Requesting pending-edits protection

See my request at Wikipedia:Requests for page protection#Friction (edit.7Ctalk.7Chistory.7Clinks.7Cwatch.7Clogs). --User:Ceyockey (talk to me) 15:47, 25 November 2012 (UTC)

## Calculations from first principles impossible? Or just impractical?

The article introduction says:

The complexity of these interactions makes the calculation of friction from first principles impossible and necessitates the use of empirical methods for analysis and the development of theory.

The first and most obvious problem is a lack of a citation. Besides that, shouldn't it say the calculation is impractical, and not impossible? Surely sufficiently small interactions could be calculable by QED.

Baddox (talk) 21:01, 26 December 2012 (UTC)

The new model of friction <AIP Advances 2, 012179> shows how sliding friction can be calculated from first principles and demonstrates this by practical examples. — Preceding unsigned comment added by LasseMakkonen (talkcontribs) 11:09, 20 May 2013 (UTC)

## Thermal energy

The introduction contains the sentence When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into heat. I suggest this is misleading because the article should say the friction between the two surfaces converts kinetic energy into thermal energy.

The law of conservation of energy states that energy cannot be destroyed but can be converted from one form to another. Our article Friction should demonstrate that kinetic energy is converted to another form of energy, and the appropriate term in this situation is thermal energy.

Heat is one manifestation of energy flowing from one region to another (as a consequence of the difference in temperature of the two regions.) Another manifestation of energy flow is Work (physics). When friction occurs between two surfaces, those two surfaces are likely to be at the same temperature throughout. The temperature of the two surfaces rises due to friction, not due to heat flowing into the region from an adjacent region of higher temperature; it is due to kinetic energy being converted to thermal energy.

When positive work is done on a body we observe that the body’s kinetic energy has increased. We don’t say the body’s work has increased. Considering we have an article about thermal energy, when the temperature of a body increases due to friction we should say the body’s thermal energy has increased rather than saying its heat has increased.

I’m in favor of changing "heat" to "thermal energy" where appropriate in this article. Dolphin (t) 23:13, 21 August 2013 (UTC)

• oppose "heat" is far clearer to a general readership. Andy Dingley (talk) 11:20, 22 August 2013 (UTC)
• Too late for me to oppose now, but I agree that "heat" is clearer for general readership. Dbfirs 20:59, 8 November 2015 (UTC)
@Dbfirs: Never too late, but I'm disappointed I found no-one to support what should be a simple proposition. Throughout Wikipedia, the various forms of energy are identified as state functions whereas heat (and work) are correctly identified as path functions. Heat is identified only while the energy of one region is increasing and that of another is decreasing as the result of a difference in temperature. It appears to be only this article on friction that is inconsistent with the rest of the encyclopedia. In both the above comments, this inconsistency is defended on the grounds of being clearer to a "general readership". In an encyclopedia I think clarity is better achieved by consistency rather than inconsistency. Dolphin (t) 06:31, 9 November 2015 (UTC)
I suppose it depends on what readership you are aiming at. I appreciate your point from the view of an expert in thermodynamics, but those of us who do not come into that category are accustomed to associating heat with friction (in common with most texts below university standard), and don't you agree that your thermal energy has to flow as a result of friction? I hope that you are happy with my compromise. Dbfirs 07:54, 9 November 2015 (UTC)
I have no objection to the notion that friction creates heat, but we would all object to a statement that friction creates thermal energy because energy cannot be created (or destroyed). So, according to Wikipedia, heat and thermal energy are somewhat different things, in the same way that work and kinetic energy are somewhat different things. Similarly, we often say (correctly) that "the exchange of heat is equal to the change in thermal energy", but this isn't a statement of the law of conservation of energy. Conservation of energy would require a statement along the lines that the reduction in kinetic or chemical or electrical energy is equal to the increase in thermal energy. What do you think of the following alternative?
• When surfaces in contact move relative to each other, the friction between the two surfaces creates heat as energy is converted from one form into thermal energy.
Dolphin (t) 11:24, 9 November 2015 (UTC)
I don't like using the word "creates" because it might be misunderstood for the reasons that you set out. I'm happy with the article as it now stands, if you are. Dbfirs 12:30, 9 November 2015 (UTC)
I have just taken a close look at the article as it now stands and, yes, I'm happy with it! Dolphin (t) 12:56, 9 November 2015 (UTC)
Thanks. I agree with you that articles need to be technically correct as well as easy to read. Dbfirs 12:59, 9 November 2015 (UTC)

## Dry friction and instabilities

The present phrase "which are utilized for the reduction of friction in wear in so-called self-lubricating materials" seems awkward. I'm not sure of the original intent. So is:

"which are utilized for the reduction of friction and wear in so-called self-lubricating materials"

OR

"which are utilized for the reduction of friction during "wear in" of so-called self-lubricating materials"

any better? Ward20 (talk) 01:52, 19 October 2013 (UTC)

I agree the section was not well-written. I have made some changes to improve the section, including a change that closely matches the first option offered by Ward20 above. See the diff. Dolphin (t) 02:25, 19 October 2013 (UTC)

## Ambigous Sentence

Under Negative coefficient of friction does improves in "everyday experience that an increase of normal force improves friction" mean increases or decreases? 123.63.207.221 (talk) 05:42, 16 April 2014 (UTC)

I agree that improves was highly ambiguous. Thanks for alerting us to that one. I have changed some of the wording, including changing improves to increases. See the diff. Dolphin (t) 13:06, 16 April 2014 (UTC)

## Correction to Table

I noticed that the values for Steel on Teflon in the table in the section on dry friction don't match the source, http://www.engineeringtoolbox.com/friction-coefficients-d_778.html, which gives 0.05-0.2, not 0.04 which is the value for Teflon on Teflon. Thorbard9 (talk) 10:17, 22 July 2014 (UTC)

Feel free to fix it. Ward20 (talk) 15:51, 22 July 2014 (UTC)

## Semi-protected edit request on 13 January 2015

In the 'static friction' section, there are several mathematical symbols that have italic subscripts but should have upright subscripts. All subscripts are upright in the rest of the article (as they should be). Jkokorian (talk) 14:24, 13 January 2015 (UTC)jkokorian

I've changed some of them (are these the ones you wanted changed?) Should Fmax be changed? Dbfirs 14:56, 13 January 2015 (UTC)

## Semi-protected edit request on 5 August 2015

Please correct typo from "On friction between surfaces moving as low speeds" to "On friction between surfaces moving at low speeds" Please correct typo from "On Friction between Surfaces moving a Low Speeds" to "On Friction between Surfaces moving at Low Speeds" JoeSchlabotnik (talk) 16:35, 5 August 2015 (UTC)

Done Thanks for pointing those out - Arjayay (talk) 16:50, 5 August 2015 (UTC)

## Static friction in fluids

From working with hydraulics and pneumatics, it has always been my understanding that static friction also occurs in fluids. This is the phenomenon that causes the pressure at the far end of a hose to be lower than at the near end, even though no flow is present. (It's analogous to the resistance in wire that causes the open-circuit voltage to be lower at the point of use than it is at the source.) I'll dig up some sources when I get a chance. Zaereth (talk) 23:18, 14 July 2016 (UTC)

Hmmm. If there is no flow in a garden hose, the pressure will be the same at all points along the hose. If the far end of the hose is at a higher elevation than the near end, the pressure at the far end will be lower; but if the far end is at a lower elevation, the pressure there will be higher than at the near end.
One of the differences between solids and Newtonian fluids is that solids experience static friction, whereas Newtonian fluids don't. Weird things such as thixotropic fluids and Bingham plastic might display behaviour approaching static friction. Dolphin (t) 12:32, 15 July 2016 (UTC)
As an example, if you have 100 psi of air in a compressor, connected to a 1/4" hose that is 100 feet long, the pressure at the end of the hose will be around 90 psi and will never equalize to reach 100, regardless of gravity. With a very accurate gauge, this can also be observed in hydraulics, but to a lesser extent. (It becomes far more apparent when liquid is compressed to the 20,000 to 50,000 psi range, where it becomes very compressible.)Zaereth (talk) 16:45, 15 July 2016 (UTC)
Are you quoting something you have read? Or are you describing some measurements you have made yourself? Dolphin (t) 22:22, 15 July 2016 (UTC)
Measurements are found in the Compressed Air and Gas Handbook. These figures are necessary for setting up compressed air systems using the proper diameter per the length of pipe, to reduce pressure loss. As for the cause, I remember reading somewhere that it was due to static friction. I don't recall where, which is why I said I'd do some digging. Zaereth (talk) 22:34, 15 July 2016 (UTC)
Ok, I did a little digging, and static friction definitely does occur in fluids. This describes the resistance to flow requiring a "breakaway force" to begin motion. In example, the book Handbook of Lubrication and Tribology: Volume I says Ford's old. "Type F" transmission fluid was a high static-friction fluid designed to improve low-speed performance. To describe the breakaway force, imagine trying to blow grease through a hose. A certain breakaway force is needed to start the fluid motion. In my example of the air hose it is the same concept in reverse. Zaereth (talk) 00:07, 16 July 2016 (UTC)
This is fascinating stuff if you like fluid dynamics. Several other sources describe static friction in fluids. One example I like is glycerin. Its high static friction is useful for reducing vibration, so is often used to fill gauges where vibration exists. If you wet your fingers with mineral oil and try snapping them, it won't work because of the reduced static friction. If you use glycerin instead, it will work better because glycerin has very high static friction but rather low kinetic friction. When I gather enough information I'll add something to the article about this. Zaereth (talk) 09:06, 16 July 2016 (UTC)

────────────────────────────────────────────────────────────────────────────────────────────────────Thanks for that but I think you are mis-reading the two books you have quoted. The Compressed Air and Gas Handbook gives data for setting up compressed air systems so the hose can carry the required amount of air without the pressure drop being excessive. (If you look closely, you might find some information about the volume of air being passed through a ¼ inch hose, 100 feet long, when the pressure drop is 10 psi.) The Handbook isn’t saying there will be a 10 psi pressure drop when the air is stationary in the hose. (When the air in the hose is stationary there is no shearing motion anywhere and consequently no viscous forces and no pressure variation laterally, only pressure variation vertically.)

Similarly, where the Handbook of Lubrication and Tribology talks about motor vehicle transmission fluid and says it is designed to improve low-speed performance it is talking about the low-speed performance of the vehicle and/or its transmission. It isn’t talking about low-speed performance of the transmission fluid. When the Handbook uses the expression "high static-friction fluid" within a sentence it is most likely talking about a transmission fluid for use in high static friction applications such as motor vehicles and their final drive transmissions. It isn’t talking about a high static-friction fluid. (In the same way that a motoring magazine talking about high-speed tyres is talking about tyres for use on a high-speed motor vehicle; it isn't talking about tyres that travel at high speed all on their own.)

Any mention of breakaway forces needed to accelerate a column of fluid is most likely talking about overcoming the inertia of the fluid in the column rather than overcoming some sort of static friction. This should not be surprising. A force is always required to accelerate anything with mass – vehicle, airplane, ship, bicycle etc. A column of fluid has mass so it also requires a force to accelerate it from one speed to another, including accelerating it from stationary to a non-zero speed.

Pascal's law talks about fluids at rest. It implies that, in a fluid at rest, the pressure is identical at all points the same height above or below any horizontal datum. A practical consequence of Pascal’s law is that, in a hose on a horizontal plane, the pressure at every point in the hose is the same whenever the fluid is stationary. If any person is able to use a Newtonian fluid to satisfactorily demonstrate the phenomenon you describe as static friction, that person will have demolished Pascal’s law! Dolphin (t) 12:47, 16 July 2016 (UTC)

You are correct. I was misreading the source. Once I took the time to sit and read through it, it became clear that the source was talking about surface/fluid interactions and temporary (low speed) adhesive qualities. I also found the source about hoses; something I misread from back in my school days. It actually had to do with system "settling" (a phenomenon in hydraulic hoses affected by the weave and even bends in the hose, where full pressure is reached immediately and the drops a tiny bit as the hose settles under the force.) Sorry for the confusion. My mistake for not checking first. Zaereth (talk) 23:29, 18 July 2016 (UTC)
Acknowledged. Dolphin (t) 10:24, 19 July 2016 (UTC)

## Negative coefficient of friction

As it is written, the description of the "négative" coefficient of friction is confusing: "As of 2012, a single study has demonstrated the potential for an effectively negative coefficient of friction in the low-load regime, meaning that a decrease in normal force leads to an increase in friction. (...)" The definition suggests a pressure-dependent coefficient of friction (COF), but there is no sign change in the COF. A negative COF should add a driving force to a movement when appying pressure (|T| = f |N|, with the direction of T along the velocity vector).

As a matter of fact, the Wiki page corresponds to Nature Materials abstract and title. Note anyway that the Fig. 1 presents only positive COFs, even if they vary negatively to the load. So far, "negative COF" is not a proper expression, and one might say "negatively pressure-dependent COF". Sorry but I don't have access to the full paper: it might be confirmed by someone else.

I suggest to put this work as a remark in the limitations of Coulomb model. — Preceding unsigned comment added by Jerome molimard (talkcontribs) 05:46, 4 August 2016 (UTC) +1 on moving to limitations of Coulomb model. I would even avoid using negative COF, as this is just a kind of eye catcher. The main point is haven a material pair, where under certain conditions more normal force does not cause higher sliding force.--Ulrich67 (talk) 10:28, 5 August 2016 (UTC)

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