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My lecturer at the engineering school that I study said that friction is not yet completely understood and that smooth surfaces could have a higher friction coefficient than rough ones. He attributed this counter-intuitive phenomenon to the presence of resonance between the two sliding surfaces.

I have seen an identical question on this site but resonance is not mentioned. Also a google research did not give anything. Do you think that there is any relation between resonance and inreased friction?

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    $\begingroup$ It would be helpful for you to link to the other question you looked at. $\endgroup$ – Jacob Maibach Jan 25 '17 at 16:54
  • $\begingroup$ I think so. Friction is a microscopic kinetic energy interaction between atoms at the surfaces. The motion of those atoms is constrained in solids as they are in a lattice, creating the potential for their oscillation. If they can oscillate at a frequency that increases the overall momentum of their next interaction with an atom in the other surface it would increase energy exchange and therefore friction. $\endgroup$ – JMLCarter Jan 25 '17 at 17:09
  • $\begingroup$ Did you ask your lecturer for an explanation? $\endgroup$ – sammy gerbil Jan 25 '17 at 20:30
  • $\begingroup$ The increase in friction for very smooth, clean surfaces might be attributed to "cold welding" - eg why making a surface "super" smooth increases the coefficient of friction? and Why friction between two smooth surfaces is high?. Unless there is some evidence that resonance is involved, providing an explanation for an unverfied phenomenon is not a sensible thing to do. $\endgroup$ – sammy gerbil Jan 25 '17 at 20:43
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Yes, certainly.

"Friction" means mechanical energy is wasted when two surfaces are in contact and move relative to each other. The only way to waste this energy is if both surfaces are not completely smooth, and one or both surfaces can be distorted (i.e. is not perfectly rigid). That way sound, and thus eventually heat, can be induced in the material. Obviously there's no such thing as a perfectly rigid surface--otherwise it would shatter as soon as the bumps on its surface collided with another like surface, because the collision would be instantaneous, and therefore have infinite force.

I think there are many effects that make friction non-linear, like chemical adhesion, effects of air pressure depleted in the gap, non-gaussian patterns in the roughness, and resonances.

Resonance is just the 'squeeking' one might get--like when you rub your clean fingers on clean, wet glass surface. (although your lecturer was probably thinking of much higher frequency and likewise shorter wavelengths). If there is squeeking and the friction coefficient is non-linear (pretty much all are), then the periods of strong force between the surface can have extra-strong friction compared to the friction during periods of lower force, the induced sound is stronger, and so more energy is lost.

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