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My physics textbook says that friction mainly arises due to intermolecular attraction between atoms of the objects in contact and clashes between peaks are only significant for rough surfaces. I was wondering, if friction arises due to intermolecular attraction, why doesn't it oppose lifting a body as well(considering that the atoms are attracted to each other). Why doesn't it oppose rolling? (even when there is no slippage, if atoms of the contact patch are attracted to the atoms in the ground, there should be a force opposing movement)

Another related question: Is there an intuitive reason why(for most pairs of surfaces) kinetic friction is lower than static friction?

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Let me start at the end. Indeed, the attractive forces between atoms may make it harder to lift an object. The actual reason is adhesion or cohesion – if the surfaces are "sticky", it's hard to separate them.

Adhesion is only counted in friction if we study the effect of the adhesion on the sliding motion when the surfaces remain in contact. If the surfaces are separated, the adhesion obviously stops acting as soon as the surfaces are separated. So this part of the "friction" immediately drops to zero.

To answer the remaining two questions, the "clashes between the peaks" rather than "adhesion" is the key to understand the right explanation. Imagine the two surfaces as cogwheels.

The friction arises because the teeth have to jump out from the holes on the other surface which requires one to move the object perpendicularly to the surface and perhaps break some of the teeth. Work has to be done for these events to occur which means that some extra force has to be exerted for the surfaces to move relatively to one another (to slide).

Why is the kinetic friction smaller? Because when the surfaces are already moving, the "teeth" of the cogwheel are no longer sitting in the "holes" of the other surface – they have already been lifted from the holes. The surfaces have been separated – by a distance comparable to the "depth of the typical holes". While the peaks may still "drop" to some of the holes on the other surface, they don't drop as deeply as when the surfaces were at rest. So the "lifting" one has to make is smaller than it was at rest, and so is the corresponding friction force.

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  • $\begingroup$ "To answer the remaining two questions, the "clashes between the peaks" rather than "adhesion" is the key to understand the right explanation." Do you mean adhesion is not the primary cause of friction? $\endgroup$ – user42991 Jul 31 '14 at 11:47
  • $\begingroup$ "Adhesion is only counted in friction if we study the effect of the adhesion on the sliding motion when the surfaces remain in contact. If the surfaces are separated, the adhesion obviously stops acting as soon as the surfaces are separated." My book says attraction between atoms is the main reason for friction on ordinary, non rough surfaces. Why don't we feel the 'stickiness' then? $\endgroup$ – user42991 Jul 31 '14 at 11:58
  • $\begingroup$ Yes, I do, the clashes are the key to the explanation, not adhesion. Concerning the second question: stickiness is always nonzero but sometimes it is so weak that our sense of touch isn't refined enough to feel it. $\endgroup$ – Luboš Motl Jul 31 '14 at 12:13
  • $\begingroup$ Resnick and Halliday's Principles of Physics 9th edition Chapter 6 Page 118 doesn't even mention crests and troughs and attributes friction to cold welding between the surfaces. I'm getting different explanations everywhere. Is there no consensus? $\endgroup$ – user42991 Jul 31 '14 at 14:10
  • $\begingroup$ There is really no contradiction here. Cold welding also involves matching of crests and troughs. See the picture powerguru.org/wordpress/wp-content/uploads/2012/07/… at powerguru.org/reliability-of-pressfit-connections $\endgroup$ – Luboš Motl Jul 31 '14 at 17:36

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