Talk:Friction

Article merged: See old talk-page here

Edit request

Plinius the elder and Vitruvius are Romans not Greeks and not even remotely contemporary. 151.35.217.119 (talk) 14:29, 6 July 2023 (UTC)

Done by removing the "Greek" as is was superfluous. Ldm1954 (talk) 20:20, 31 August 2023 (UTC)

Wiki Education assignment: 4A Wikipedia Assignment

This article was the subject of a Wiki Education Foundation-supported course assignment, between 21 August 2023 and 16 December 2023. Further details are available on the course page. Student editor(s): FranciscoCa27, Esteban282 (article contribs). Peer reviewers: ThomasDLV, GoldenGecko.

— Assignment last updated by Kmijares (talk) 22:40, 15 November 2023 (UTC)

"Kinetic energy is converted to thermal energy whenever motion with friction occurs"

The lead states

Kinetic energy is converted to thermal energy whenever motion with friction occurs

Is that always true? If you drop (or perhaps gently lower) a brick onto the roof of a train moving at a constant speed, so that friction accelerates the brick till it moves at the same speed as the train, the kinetic energy of the train is constant, and that of the brick is increasing. - My mechanics classes were long ago; I may have gotten this wrong. The train engines may have to work a little harder, so it is not a closed system. Also, seeing things from the inertial frame of the moving train, the statement is true. Still, is the statement too absolute? Nø (talk) 11:00, 24 August 2024 (UTC)

The statement is true. In your case the train engine is doing some work on nanoscale protrusion at the brick/train interface which will elastically bend then be restored. (Think of the contact as a comb, not quite right but a decent analogue.) Since it is not 100% elastic then some energy is lost to heat.

Note that not 100% of the energy has to be heat, some could be light or potential energy but that does not make the statement wrong. Ldm1954 (talk) 11:19, 24 August 2024 (UTC)

Editor Nø raises a good question. I am thinking from the point of view of clear language, not seeking to debate the particular case that he raises.

The language of the quoted sentence might be criticized in various ways.

The phrase thermal energy is one to be avoided wherever possible. It is often used as a cover for loose thinking. The present Wikipedia article on thermal energy starts:

The term "thermal energy" is used loosely in various contexts in physics and engineering, generally related to the kinetic energy of vibrating and colliding atoms in a substance. It can refer to several different physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, defined as a type of energy transfer (as is thermodynamic work); and the characteristic energy of a degree of freedom, $k_{\mathrm {B} }T$, in a system that is described in terms of its microscopic particulate constituents (where $T$ denotes temperature and $k_{\mathrm {B} }$ denotes the Boltzmann constant).

The clause whenever motion with friction occurs is vague, perhaps deliberately so, but still vague. It seems to me that it would refer, amongst other things, to a case in which an imposed force, not described simply by the term kinetic energy, causes a dragging or rubbing of surfaces against each other, through a distance. The operative factor will be the mechanical work done other than that ascribable to the kinetic energies of the rubbing bodies.

The meaning of motion with friction is not clear as to whether moving friction or static friction is intended. In the present case, the brick may simply stick to the surface of the top of the train. In that case, there would be some possibility of elastic or inelastic interaction between the top of the train and the bottom of the brick. Or perhaps, in addition, the brick may slip along the top of the train.

Editor Nø's talk of "a train moving at constant speed" is a little loose. The forces between the brick and the train top will not be transmitted instantaneously to the engine, so that the train's speed will hardly be perfectly "constant". Editor Nø's use of the phrase "not a closed system" is confusing. If the engine is a diesel engine that is working all the time, the system is not closed.Chjoaygame (talk) 22:16, 24 August 2024 (UTC)

Sorry, but no, here the term thermal energy is exactly correct. Furthermore the lede is certainly not where fine details should be discussed. Ldm1954 (talk) 22:51, 24 August 2024 (UTC)

Let me expand slightly; tribology is one of my research areas. When two bodies contact it is not flat-on-flat, it is asperity contacts as mentioned with relevant sources at the end of Friction#History, as I mentioned before. Friction is due mainly to elastic/plastic deformation of these asperities, perhaps with some Triboelectric contribution. The anelastic component is mainly converted into thermal energy (phonons), although some can go into other inelastic channels.

N.B., of course you don't have motion with stiction, and kinetic energies are not relevant unless the bodies are moving at a sizeable fraction of the local speed of sound.

Sorry @Chjoaygame and @Nø, as written the article lede is rigorous. Ldm1954 (talk) 01:27, 25 August 2024 (UTC)

I agree with Ldm1954. The original question is complicated by the fact that we don’t know how the locomotive’s engine will respond as the brick tries to slow the train. The engine may produce constant power so the train decelerates; or it may deliver increasing power so the train’s speed remains constant; or somewhere in between.

The concept can be examined so easily, and deliver some useful information, if we consider the train, momentarily, to be travelling without friction or air resistance, and with no forward thrust between the steel lines and the train’s wheels. It is then possible, and very easy, to consider conservation of momentum. Assume we know the mass and the initial speed of the train so we can calculate its momentum; and we assume the horizontal speed of the brick is zero, so its momentum is zero. Assume we know the mass of the brick, so we can easily calculate the speed of the train plus the brick because we know the combined mass of the train plus the brick; and the final momentum of the train plus brick is the same as the original momentum of the train alone.

After we have calculated the speed of the train plus brick, we can check total kinetic energy before, and after, the brick joins the train. By comparing the two we can calculate precisely the amount of kinetic energy converted to heat and/or thermal energy by friction acting between the brick and the roof of the train. Dolphin (t) 11:45, 25 August 2024 (UTC)

Thank you, Editor Ldm1954, for your response. I always enjoy being condescended to. You write "the term thermal energy is exactly correct." Then you clarify by adding "(phonons)". For my taste, it is right that the lead of the article Thermal energy writes "The term "thermal energy" is used loosely in various contexts in physics and engineering." That's why I prefer to avoid it. The frictionally transferred energy is received as heat by both the giver and the receiver. The internal energy that comes from the heating is indeed, as you say, manifest microscopically mostly as phonons.Chjoaygame (talk) 22:18, 25 August 2024 (UTC)

Sorry, there was no intent to condescend, I was just stating the established science. In fact I think the article Thermal energy is quite wrong, and I will post to WP:PHYSICS Ldm1954 (talk) 22:32, 25 August 2024 (UTC)

Thank you for your response. I don't like the term 'thermal energy', because it is not a very well defined term in classical thermodynamics. In my opinion, it is used as a convenient short cut to say various things. It is a kind of hybrid term between microscopic and macroscopic thinking. Evidently, it seems that you like the term 'thermal energy'? I have no intention of trying to alter the lead of the article on Friction; I was just making a passing remark. I generally avoid editing the Wikipedia article on Thermal energy because I think such editing gets too close to feeding the trolls.Chjoaygame (talk) 00:24, 26 August 2024 (UTC)

Thermal energy in the sense of (measurable/measured) temperature increases as atomic vibrations is well established for friction. Stored energy in the form of dislocation creation, new grain boundaries, fracture, (triboelectric) charge etc is there, but has been measured to be relatively small in almost all cases. Light (triboluminescence) is also there but again small.

How it ends as thermal energy is not so clear, and there are multiple pathways and dissagreements. However, the end result is heat for 80-99% of the dissipative energy.

Of course there is a major difference in temperature as a state variable and as a direct observable. However, that would be way off topic. Ldm1954 (talk) 00:46, 26 August 2024 (UTC)

It may be acceptable to Nø and all other participants if the text in question is adjusted to say “Some kinetic energy is converted to other forms of energy whenever motion with friction occurs.” This will accommodate the possibility that extra light, sound or potential energy may be observed after an event with friction. Dolphin (t) 06:42, 27 August 2024 (UTC)

I thought it would suffice and be correct to say that dynamic friction produces heat. But I am not sure! Nø (talk) 12:59, 27 August 2024 (UTC)

Unfortunately it would not be quite correct. As an example, push your hand onto a surface and vibrate it from side to side without enough force for sliding. The hand does not move due to static friction. However, the inelastic part of the deformation of your hand and the surface goes into heat. Ldm1954 (talk) 13:40, 27 August 2024 (UTC)

I don't see how an example about static friction + inelastic deformation can invalidate the statement "dynamic friction produces heat". Nø (talk) 06:30, 28 August 2024 (UTC)

N.B., sites such as this may be useful as it has some more details on asperity contacts. (I have no connection to that source.) Ldm1954 (talk) 14:04, 27 August 2024 (UTC)

"the end result is heat for 80-99% of the dissipative energy." The example of vibrating one's hand "from side to side without enough force for sliding' is a case in which motion with friction goes into heat. In this case, it is hardly kinetic energy alone that goes into heat, I guess; more that mechanical work (as distinct from thermodynamic work) goes into heat. Suggestions: 'Motion with friction produces heat.' OR 'Friction converts mechanical work into heat.' ??Chjoaygame (talk)

I restructured the lede so it better follows the MOS, and is has a more logical flow. The different dissipation channels were mentioned later, and are now briefly alluded to in the lede. A lot more can (and probably should) be added as a lot of the recent literature is not mentioned. Ldm1954 (talk) 01:51, 28 August 2024 (UTC)

Do you have a comment on my above remark, that, rather than kinetic energy, it is often mechanical work that friction converts into heat?Chjoaygame (talk) 06:11, 28 August 2024 (UTC)

My comment is that this type of issue does not go into the lede, it might go into the section Friction#Energy of friction. That section could be rewritten to include what you are referring to (if you feel it is not covered), plus more details such as phonons/electron/Tomlinson/third body/dislocation slip/travelling crack and others that are currently discussed in the literature. I think it would be far better to draft a separate page. Ldm1954 (talk) 08:14, 28 August 2024 (UTC)

Thank you for your response.Chjoaygame (talk) 09:07, 28 August 2024 (UTC)

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)