Talk:Friction

Article merged: See old talk-page here

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

In reply to:

<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.; and A reference frame attached to the earth can be considered to be an inertial frame for most practical purposes. -AndrewDressel (talk) 14:18, 5 June 2009 (UTC)

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)

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)(N_f)" should either be "F_f = (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)

Hi guys. I don't have a lot of time to edit anymore. I have so many notes and citations I've worked on off-line to eventually add, but they will have to wait a few months. Anyway, about the arrows in the illustration, I agree with Tadeusz Malinowski that some vectors are missing. Unfortunately, this is very common in the teaching literature and as wikipedia editors we're supposed to report what's out there, not create our own methods or try to correct history. User penubag's illustration, as it currently stands, is a pretty accurate reflection of the most-common illustrations out there in text books now. Occasionally, rarely, you can find more-complex illustrations that include all the force vectors. Even though this is more accurate, it is less common. (Is this evidence of the dumbing-down of education? I don't know.) I have created an illustration off-line with more vectors, but would prefer to find an un-copyrighted one from an existing source that we can use, for veracity. The importance of showing all the vectors is that the equal-and-opposite pairs become apparent and people aren't misled into matching up the wrong pairs. As Tadeusz Malinowski said, both bodies are subject to friction forces, in opposite directions. That is the true matched pair, not friction versus applied force. Similarly, both bodies are subject to gravitational attraction. The earth is pulled up just as much as the block is pulled down. That is another true matched pair, not weight versus normal force. Also, both bodies are feeling the normal force, in opposite directions, which is what keeps them from occupying the same space--another matched pair of equal opposites. This seems to leave the applied force with no equal opposite, however, depending who you ask, an engineer or a physicist or a scientific historian, you may get one of at least two answers: first, the applied force has no opposite and is considered a "net force" or "unbalanced force" causing acceleration, or second, the applied force is matched by D'Alembert's "force of inertia" or "inertial force" (defined as "the negative of the product of mass times acceleration" [2]), which prevents any body (having mass) from instantaneously accelerating to the speed of light. In the latter case, there would have to be two additional vectors added to the illustration, inertial force on the block, opposite in direction to the applied force, and rotational inertia or moment of inertia on the earth, which opposes the torque on the earth caused by the applied force on the block, also opposite in direction to the applied force on the block. The magnitudes of those two inertial force vectors would have to add up to the magnitude of the applied force they oppose, meaning the force vectors on any body (in this case, gravitational, normal, frictional, applied, and inertial) always add up to zero, even during acceleration. The text description of the illustration would also have to explain not only that the block is moving to the left, but that it is accelerating to the left, and that the earth is also accelerating (rotationally and maybe directionally) to the left (even if it is moving to the right), although at a much smaller rate of acceleration than the block because of the difference in masses. A complete illustration should also show what form the applied force is and explain if there are any force vectors involved in the mechanism, for instance, if a rocket is strapped to the block to provide propulsion, then it must be noted that the rocket exhaust is being expelled under force in the opposite direction and that it has its own mass and inertial force, gas pressure forces, etc. The illustration could get pretty complex. Unfortunately, the simple illustration is easier to look at, and is common in the literature, and will probably stand for a long time, and people will accidentally be mislead into incorrectly learning that "weight opposes normal force." Good luck guys. See you later. -- Another Stickler (talk) 19:50, 6 February 2009 (UTC)

Lead section wording

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)

Thanks for the link. After reading, the term I'm looking seems to be "higher-than-acceptable edge loading". In fact, arc-boutement seems to mean both "over-center locking" and "higher-than-acceptable edge loading".

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)

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 μ_k, 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 μ_k, and is usually less than the coefficient of static friction for the same materials.

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

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

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)

FYI, there was a discussion about this previously (Kinetic > static?). Wizard191 (talk) 15:59, 16 October 2009 (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)

Citation for decreased COF with increased load for PTFE

http://www.fluorocarbon.co.uk/downloads/Fluorocarbon%20Slide%20Bearings%20and%20skidways%20FCL-SL-001.pdf

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.  Chzz  ►  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 ...: $f=\mu _{s}F_{n}\,$." But in the remaining article it uses the symbol $F_{max}\,$. If there are no objections, I will replace the definition of f with $f_{max}=\mu _{s}F_{n}\,$ and then replace all references to $F_{max}\,$ with $f_{max}\,$. cperk (talk) 16:25, 25 September 2010 (UTC)