Talk:Friction

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work

Quoting from the section 'Work of friction':

===Work of friction===

The work done by friction can translate into deformation, wear, and heat that can affect the contact surface properties (even the coefficient of friction between the surfaces). This can be beneficial as in polishing. The work of friction is used to mix and join materials such as in the process of friction welding. Excessive erosion or wear of mating sliding surfaces occurs when work due to frictional forces rise to unacceptable levels. Harder corrosion particles caught between mating surfaces in relative motion (fretting) exacerbates wear of frictional forces. As surfaces are worn by work due to friction, fit and surface finish of an object may degrade until it no longer functions properly. For example, bearing seizure or failure may result from excessive wear due to work of friction.

In the reference frame of the interface between two surfaces, static friction does no work, because there is never displacement between the surfaces. In the same reference frame, kinetic friction is always in the direction opposite the motion, and does negative work. However, friction can do positive work in certain frames of reference. 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 frame of reference in which the floor is stationary. Thus, the kinetic friction between the box and rug accelerates the box in the same direction that the box moves, doing positive work.

At present, I don't intend to try to edit this section. I prefer to leave that to local editors. But I do want to comment.

It doesn't make good sense to speak of 'work done by friction'. It makes good sense, instead, to speak of 'work lost through friction'. It may be a colourful turn of phrase to speak of 'negative work', just as it is a colourful turn of phrase to speak of 'radiant coolth'. But such colourful turns of phrase should be diligently avoided in a Wikipedia article such as this one. A source that positively supports such colourful turn of phrase is not reliable in that respect, no matter how reliable otherwise.

For example, for the box on the rug, force is transmitted from the rug to the box, but it is just force that is transmitted, not kinetic friction. The transmission is by cohesion or adhesion, not friction. The cohesion limits the loss of work through kinetic friction.

Friction is always dissipative. Friction never does work; neither 'positive work' nor 'negative work'; it always dissipates work. In this line of thinking, so called 'static friction' is properly spoken of as cohesion or adhesion, or some such. Chjoaygame (talk) 14:40, 7 October 2024 (UTC)

Chjoaygame: You say "Friction never does work ... it dissipates work." This is contrary to what we read in the best reliable sources. Consider objects or materials placed on a conveyor belt that is moving at speed V_c. Using the Earth's surface as our reference frame, these objects or material initially have a velocity of around zero. After they come into contact with the conveyor belt they soon reach the speed V_c. The force responsible for the acceleration from zero to V_c is the friction between the objects or materials and the surface of the conveyor belt. The increase in kinetic energy of each object or parcel of material (plus the increase in internal energy due to frictional heating) is equal to the positive work done by this force of friction. Or do you have an alternative explanation? Dolphin (t) 10:38, 6 November 2024 (UTC)

Dolphin51: Thank you for your response. I see it as useful to try to clarify this matter. I would say that the force responsible for the acceleration is not friction: it is grip, adhesion or cohesion or somesuch. Friction expresses the slippage between the belt and the body, the remaining deficiency between the belt speed and the body speed, the relative motion between the belt and the body, that is to say, lack of grip. The acceleration of the body with the belt is due to non-friction, that is to say, it is due to non-slippage, that is to say, it is due to grip, adhesion or cohesion. Perhaps the literature on friction likes to think of non-slippage as a sort of "static friction", but I would say that it is a misnomer to speak of "static friction" as if it were really a kind of friction. Static friction is grip or non-slippage. It is a mistake of logic to think of grip or non-slippage as if it were 'negative slippage', because slippage is always positively dissipative. There is no such thing as negative dissipation. In a way, that is the burden of the second law. My point is that friction is dissipative, always a loss of potentially useful kinetic energy, or a waste of force. Another way of saying this is that friction is entirely expressed by generation of heat, and is always a form of waste of otherwise potentially useful mechanical energy. Another way of saying this is that friction always involves slippage and generates heat, not coolth. I am not familiar with the sources, but if they deny this, then I think they are probably partisan or highly specialized and, for logic for non-experts, should be called out for it. I am not remotely expert on friction, but if the general way of 'frictionist' thinking is illogical, then Wikipedia should make that clear for the non-expert, even if it takes some searching amongst sources to establish it. If you are expert on 'frictionist' thinking, perhaps you will be more easily able to check out sources on this than I will be.Chjoaygame (talk) 16:55, 6 November 2024 (UTC)

Hmmmm. We need to be careful both among ourselves and with readers about how we use "Force" in the context of friction. There are a range of clear forces, for instance electromagnetic, strong, weak, gravity etc. These exist independent of whether there is movement or not. In contrast to this "frictional force" is the derivative of energy as a function of movement. These energy dissipation processes can be elastic as in adhesion and elastic deformation, or it can be completely unrecoverable, for instance dislocation motion, grain boundary motion, phonons etc. Please note that adhesion is part of friction, see for instance JKR theory. The current text is general enough that it is adequately rigorous.

I will also comment that the tribology community will be very upset with a page that says that static friction is not a component of friction. Indeed, stiction is a sequence of static friction and slip events.

I strongly suggest KISS as the derivative of energy with position, otherwise we will be misleading readers and perhaps even guilty of lie to children oversimplification. I will add that while science discussions always have value, to me a higher priority should be finding appropriate sources for the many unsourced statements/paragraphs in the existing article. Ldm1954 (talk) 17:22, 6 November 2024 (UTC)

Thank you, Ldm1954, for your sober comment. The article is highly learned and supplied with many references. Because I am far from expert in friction, it is obvious that I am at risk of mistakes, both formal and substantial. I am not familiar with the textbooks of friction, nor with the literature. Perhaps you will correct some of my possible mistakes on this talk page. I have noted above that, broadly speaking, I do not intend to edit the article. I guess that you will be able to find sources more effectively than I.

Looking at the article as a whole, I may start with the lead:

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of the processes involved is called tribology, and has a history of more than 2000 years.

And, in particular, with the first sentence. The leading definition cites a dictionary and another encyclopaedia. The dictionary citation is of the item 1b in Merriam–Webster, where I found

1a the rubbing of one body against another (the friction of sandpaper on wood); 1b the force that resists relative motion between two bodies in contact (oil in a car engine reduces friction); 2 the clashing between two persons or parties of opposed views : disagreement (friction between neighbors; friction between state and federal authorities); 3 sound produced by the movement of air through a narrow constriction in the mouth or glottis.

Another reliable dictionary is the Oxford English Dictionary, which lists

1. a. The action of chafing or rubbing (the body or limbs). (Formerly much used in medical treatment.) b. Hairdressing. A massage of the scalp. 2. The rubbing of one body against another; attrition. 3. Physics and Mech. The resistance which any body meets with in moving over another body.

The cited external encyclopaedia definition reads:

friction, force that resists the sliding or rolling of one solid object over another.

In my view, Wikipedia is an encyclopaedia in its own right, not a dictionary, so I don't think that dictionary and other encyclopaedia definitions are necessarily the right sources.

Looking for possible reliable sources from the field of tribology, I went to Persson (2000), where I found

"Thus one may think that friction is a simple and well understood subject."

That isn't a definition, but it shows reasonable usage of the word 'friction'. There, the word seems to refer to a topic of investigation, rather than to the highly specific idea of frictional force. That book repeatedly uses the phrase 'friction force', apparently using the word 'friction' in a wider sense than simply the force, requiring the extra specifier 'force'.

Also that book writes:

Neglecting wear processes, friction arises from the transfer of collective translational kinetic energy into nearly random heat motion. Friction can formally be considered as resulting from the process of eliminating, or "integrating out", microscopic degrees of freedom in the following manner: ...

and

This approach to sliding friction is similar to the renormalization group approach to second-order (or continuous) phase transitions.

Many of the references in the article are to more or less primary sources. At present, I don't have convenient access to other textbook-status reliable sources. I agree with your above comment that we "should be finding appropriate sources". I don't know how many textbook-status sources are suitable.

For myself, I prefer to think of friction as a word that encompasses a topic as well as a kind of process. 'Friction force' can then be defined more precisely. If it is practicable, I would prefer a leading definition from a reliable source textbook, though I accept that such may be easier to ask for than to produce. Leading definitions can be, and perhaps preferably are, unreferenced and synthetic because they are a summary of the article.

Chjoaygame (talk) 08:22, 8 November 2024 (UTC)

Leonardo da Vinci {Friction}

Leonardo did not publish any of his findings; however, some of his notebook pages discovered more recently contain amazing illustrations and observations related to friction. His ideas included the thought that friction was the result of the roughness of the material and smoother materials resulted in less friction. 165.228.48.230 (talk) 04:02, 6 November 2024 (UTC)

Please draft a few sentences to a paragraph that you are suggesting gets added to the History section. I think adding it would be appropriate, but we need to see a specific change proposal. Ldm1954 (talk) 13:18, 6 November 2024 (UTC)

Edit Request: µ of graphite

The referenced paper [44] Superlubricity of Graphite states µ below 0.001 instead of 0.01 139.30.239.150 (talk) 07:59, 25 April 2025 (UTC)

Declining the change. Martin Dienwiebel's paper does not define superlubricity, for which different people have slightly different values, so "below 0.01" is not incorrect. I added a ref to Martin Muser's paper which is in the Wikipedia link and the source of that number. In this article I don't think more is needed. Ldm1954 (talk) 08:29, 25 April 2025 (UTC)

Confusing edit

@Chjoaygame, unfortunately your recent change could be read to indicate that the force due to friction is a fundamental force. It is not, it is only the consequence of elastic/plastic deformation and others such as triboelectricity etc. Whatever goes into those is manifested as friction. Rather than my reverting your edit, can you please change it. Ldm1954 (talk) 11:11, 26 April 2025 (UTC)

@Ldm1954, thank you for your helpful comment, which I regard as valid. I will shortly try to do better, hopefully within a day.Chjoaygame (talk) 11:29, 26 April 2025 (UTC)

Looking again, I note that my initial edit https://en.wikipedia.org/w/index.php?title=Friction&diff=1287427495&oldid=1287295431 was only to the first paragraph; I left the other paragraphs untouched. I wonder if what you found unsatisfactory was actually in the second paragraph? I agree that the second paragraph has problems, but I planned to touch up the first paragraph before tackling the second paragraph. I have retouched the first paragraph. Enough for the moment. Your thoughts?Chjoaygame (talk) 11:55, 26 April 2025 (UTC)

Much of that first paragraph was a digression -- polishing etc has nothing to with the work done. I trimmed it.

I need to do some work on this article, parts are a bit of a mess. Toooo bust at the moment. Ldm1954 (talk) 12:18, 26 April 2025 (UTC)

Delete the "Energy due to friction section"

That section is a combination of somewhat vague statements plus some unsourced opinions about thermodynamic relevance that has no bearing on friction. I propose to take the first paragraph of the "Work" subsection, ensure that is goes somewhere earlier, and delete the rest. I will wait a few days for comments. Ldm1954 (talk) 12:31, 26 April 2025 (UTC)

There is no section titled “Energy due to friction” so I assume you mean the section titled “Energy of friction”. There is no subsection titled “Work” so I assume you mean the subsection titled “Work dissipated through friction”.

Are you proposing to retain the paragraph Friction is a response to a primary active force. Part or all of the energy transferred as work done by the primary force is dissipated as heat, deformation, wear, triboelectic charge transfer and changes of the contact surfaces. The dissipated energy does not enter the target body as work, and the dissipated energy per unit distance is what manifests as the frictional force., and delete the remainder of the subsection?

I agree that the remainder of the subsection is designed to confuse rather than enlighten. Much of it is unsourced. What value there is in the remainder can be salvaged and restored to the article, with sources, if someone is motivated to do so. I don’t object to your proposal. Dolphin (t) 13:38, 26 April 2025 (UTC)