Why is it that, as soon as the 'required' static friction for no relative motion between two objects exceeds the maximum static friction, kinetic friction 'takes over'? Shouldn't static friction continue acting at it's maximum value, in addition to the kinetic friction?

Essentially, if there is no relative motion between two objects, we have static friction trying to to maintain that state, but if there is relative motion, then static friction just completely 'gives up'. Why is this?

  • $\begingroup$ Actually it cannot: the bodies are either in relative motion or at rest. Friction is quite complicated and poorly understood. A starting point is here: en.wikipedia.org/wiki/Friction. From this wiki article please note that "[n]ew models are beginning to show how kinetic friction can be greater than static friction" so really pretty much anything goes. $\endgroup$ – ZeroTheHero Oct 15 '17 at 21:53
  • $\begingroup$ Sorry, but why does bodies being in either relative motion or rest imply static friction 'gives up' if they are in relative motion? $\endgroup$ – John Oct 15 '17 at 22:24
  • $\begingroup$ It’s not so much that static friction gives up, just that it changes in character. The kinetic friction can generate heat which can function as a lubricant, there are chemical bonds broken that are no longer favoured, etc. You can get a sense of the complexity from this: web.pa.msu.edu/cmp/csc/eprint/DT060.pdf $\endgroup$ – ZeroTheHero Oct 16 '17 at 1:05

The static and kinetic friction are phenomenological forces that have the same microscopic origin, viz., the EM interactions between the molecules.

The "static" friction is still there, just that it's magnitude is reduced, and so given another name.

  • $\begingroup$ But the behavior of the 'new' static friction is completely different from that of the original.. there is no maximum kinetic friction, and for a given normal force, it is a constant, unlike static friction. Also, why is the magnitude reduced? Why does friction suddenly decide to stop 'trying its best' to keep the bodies in (relative) rest once they are in relative motion? $\endgroup$ – John Oct 15 '17 at 22:26
  • $\begingroup$ When there is relative motion, there is no enough time for the molecules to form strong interactions. $\endgroup$ – velut luna Oct 15 '17 at 22:29

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