# Source of frictional force?

1. Apr 23, 2013

### coconut62

Since friction is a "force" and is able to stop a moving object, why doesn't it possess energy?

For example, light, which is an electromagnetic wave, is emitted from a source which have the energy to "do light". Then what is the source of friction? An object cannot just intrinsically possess this 'stopping' ability once it is produced, right?

2. Apr 23, 2013

### A.T.

Forces don't poses energy. Forces are related to energy via the definition of work:
http://en.wikipedia.org/wiki/Work_(physics)

3. Apr 23, 2013

### davenn

Hi there :)

That statement doesnt really make much sense sorry.
Have you read up on how an EM wave is generated, that is, how the photons ( regardless of if they are radio waves, visible light or say Xrays ) are created in an atom ?
There is no friction involved

here's a link to a reasonable not too technical explanation of how photons are emitted

cheers
Dave

4. Apr 24, 2013

### CWatters

What do you mean by "possess energy"?

The concept of friction is just another way of modeling electromagnetic forces...

http://en.wikipedia.org/wiki/Friction

5. Apr 24, 2013

### jambaugh

Keep in mind, there is energy and there are processes which transfer energy which thereby involve forces. Friction is the conversion of motive energy (kinetic energy) to heat (randomized kinetic energy at the microscopic scale). Friction is just this randomization. The brakes on a car at the microscopic scale introduce random collisions between the atoms on the surfaces. These collisions arbitrarily transfer kinetic energy between the atoms. Since the atoms on one surface are mostly moving uniformly relative to the other in the case of an active brake, this uniform motion and its energy becomes randomized motion i.e. heat. It is analogous to a thrown piece of glass hitting a wall and shattering. The energy of the uniform motion gets redirected in all directions in a random way.

6. Apr 24, 2013

### physwizard

Frankly, the cause of friction is quite poorly understood today and I would even doubt if it remains purely in the domain of physics or more in the domain of chemistry involving chemical processes and bonding. Empirically though, we know the laws governing it well enough. In particular, we know that it is a non-conservative force, meaning that there isn't a concept of potential energy associated with it. In this sense, we may say that it doesn't have a concept of 'energy' associated with it. It is capable of taking energy away from an object, however it is not capable of giving energy to an object. The energy taken away by the frictional force is almost always converted to heat.
Electromagnetic fields, by contrast, have a concept of energy associated with them and are capable of both giving energy to an object, and taking energy away from it.

7. Apr 24, 2013

### jambaugh

I don't rightly agree, there is no mystery to the process in terms of some as yet unidentified force, only to the details and how that relates to the engineering problems of e.g. designing better lubricants or materials which provide friction without wear.

8. Apr 25, 2013

### physwizard

oh, really. then what is the cause? can you explain it?

9. Apr 25, 2013

### A.T.

And friction can do positive or negative work on an object. It depends on the reference frame, like with all forces.

10. Apr 25, 2013

### jambaugh

I just did. If you need more details we'll need to get down to cases, e.g. drag in compressible flow, viscous friction between solid and a fluid, solid surface to surface.

In nearly all cases there is relative motion between two parallel surfaces. The solid surfaces at the microlevel are jagged. With that there is random interaction exchanging momentum and energy. The net momentum exchange results in a net force opposing motion. The net energy exchange is imperfect due to the imperfect rigidity so some of the energy is converted to random vibration a.k.a. heat.

Fluids get a bit more fun with the interplay of viscosity and turbulance.

11. Apr 25, 2013

### physwizard

can you cite an authoritative source for this, preferably publication in a reputed journal?

12. Apr 25, 2013

### physwizard

okay, i should clarify. i was referring to kinetic friction, not static friction. is this what you are referring to, also?

13. Apr 25, 2013

### jambaugh

Sure, here's one:
Benjamin Thompson, "An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction", (1798), Philosophical Transaction of the Royal Society p.102.

but in return, YOU state which of my four statements you are specifically doubting. Put your ante into the pot. You say there's a mystery. Can you cite authoritative sources, preferably reputed journal publications that state there still exists a mystery as to the basic laws of physics and thermodynamics regarding friction and the nature of heat?

14. Apr 25, 2013

### A.T.

Kinetic friction can also do positive or negative work, depending on the reference frame. Any force can. Kinetic friction is just less efficient than static friction in transmitting energy.

15. Apr 25, 2013

### Staff: Mentor

Please provide a reference supporting this claim. My understanding is in line with jambaughs. The details may be computationally intractable, but I don't think that is the same as being poorly understood.

16. Apr 25, 2013

### physwizard

lol is this a joke? that paper only establishes that friction produces heat. that too, it does this experimentally, not theoretically. it does not talk anywhere about the cause of frictional force. quite an outdated paper, no doubt it was a novel concept at that time but today its contents are known even to school kids.
a far cry from saying that you really understand friction.

17. Apr 26, 2013

### physwizard

Rather, I would ask you to cite a reference concluding that all aspects of the cause of friction are known and understood. Since you are "Mentor", you would be expected to be able to support your claims and hopefully in a better way than poor jambaugh. It would be difficult for me to cite a paper stating that it is not understood. As such papers wouldn't get published in the first place. You don't get to publish a paper for 'not having understood' something, you only get to publish if you 'have understood' something.
However, I can quote from textbooks written by distinguished physicists. For eg. you can refer to Feynman Lectures on Physics, Volume 1, pg. 12-5:
" Although the law F = (mu)N is fairly accurate once the surfaces are standardized, the reason for this form of the law is not really understood."
" At any rate, this friction law is another of those semiempirical laws that are not thoroughly understood, and in view of all the work that has been done it is surprising that more understanding of this phenomenon has not come about. At the present time, in fact, it is impossible even to estimate the coefficient of friction between two substances."

18. Apr 26, 2013

### jambaugh

Again you need to "put up or shut up". For all I can tell from you, that is something you dispute. Now we are making (slow) progress. Shall I now progress through the history of thermodynamics or will you please guide me to the specific "mystery" you are asserting?

My next citations will be Joule and then Thompson, establishing mechanical equivalence and kinetic theory of heat. This may take a while unless you help out.

edit: This btw speaks to the nature of the cause of the frictional force. That it produces heat and the nature of heat itself establishes stat mec and physical parameters. I.e there must be randomized interaction between surfaces.

Last edited: Apr 26, 2013
19. Apr 26, 2013

### jambaugh

OK, Thank You! Now we're getting somewhere. There is a distinction between "cause of the frictional force" and the "reason for the validity of a semi-empirical approximate rule for the value of the frictional force".

Is there a "mystery" in the latter? Of course until someone has analyzed to death every possible form and case where it is applied. It isn't a fundamental law but rather a first order approximation for some very complex non-linear interactions in condensed matter physics. For ice at a certain temp you even see a reduction in friction with increasing normal force due to the pressure induced melting of a lubricating surface layer. But all due respect to Feynman, you can sit down with some basic calculus and work out that....

The frictional force may depend on the normal force applied,
that dependence for many cases will be a smooth function $F_f(F_n)$,
that we can take a first order approximation to that function hence $F_f(F_n) \approx F_f(0) + F'_f(0) F_n$.

So the form of the equation is no real mystery. The calculation of the normal force dependency is the messy "mystery" you get in all practical condensed matter problems. You have on the one hand an intractable many particle interaction picture if you decompose down to atoms or subatomic particles and you have a lack of omniscience as to the make up of each and every possible surface at the larger scales. But we have some very good qualitative models and can classify cases. It is of course a rich area of research with great industrial application.

20. Apr 26, 2013

### jambaugh

An additional point. Why should the linear approximation work so well? Consider that its validity in calculus depends on the independent variable changing on a small scale. This is imposed in the physics by the fact that the details of the non-linearities will come down to the intermolecular forces in the bulk matter of the "wooden block sliding down the granite surface". In the domain of the problems you will not of course apply normal forces producing pressures beyond the bulk strength of the materials. You will stay some orders of magnitude away from that scale to prevent even shape deformation on the large scale from invalidating your assumptions (that you have a rigid block with fixed shape). This limits the range of normal force based on the same intermolecular forces we imagine producing the friction. But of course not entirely. The surface roughness will mean the two planar surfaces will in fact be touching at only a subarea of their continuum idealization. The local pressures will be sufficient to deform the little peaks where they touch an intermesh. Increased force we can imagine will increase the surface of interaction in typical cases