Talk:Friction

Article merged: See old talk-page here

Proposed merge of Friction Acoustics into Friction

I don't see any particular reason for "Friction Acoustics" to be an independent article at this time. I recommend merging into the section under "Energy of friction" Polyamorph (talk) 13:17, 3 February 2022 (UTC)

Weak oppose: while I agree that the current article is weak, the topic has sufficient scope to be developed. Hence, I think that ímprove rather than merge, would be the better action. Klbrain (talk) 19:59, 23 August 2022 (UTC)

Closing, given the uncontested objection and no support. Klbrain (talk) 10:08, 27 December 2022 (UTC)

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)