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Viscous force depends on relative motion of the two surfaces and also the surface area. Does friction also depend on these two factors? (I'm not sure.)

What's the main difference between both forces according to a perspective from molecular level and what's the reason behind that?

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  • $\begingroup$ At not too high loads (normal forces) (or in other words, not too big micro deformations, which means most everyday cases), friction does not depend on surface area / contact area $\endgroup$ – Steeven Jul 4 '17 at 15:29
  • $\begingroup$ So, why is it easier to make a sphere run on a surface than a cube of same mass? The sphere gets less friction than the cube although both have same mass, getting same normal force from the surface. Please, help me here. $\endgroup$ – Hisab Jul 4 '17 at 15:39
  • $\begingroup$ @Hisab In that scenario there are several things that can happen. First and foremost; when determining friction there is some "coefficient of friction" (for the type of friction that "doesn't depend on contact area"). The thing is, this coefficient is experimentally determined; and will vary if you go between extremes like a cube with high surface area to a sphere with low surface area. Another big difference is that the sphere might not slide. A sphere is able to roll, and therefore the point of contact has no relative movement. This means the resistance comes from inertia. $\endgroup$ – JMac Jul 4 '17 at 15:43
  • $\begingroup$ @Hisab Uh, that is indeed a good question. When i say friction here, I mean kinetic friction. The thing is that a sliding box feels kinetic friction, while a rolling ball feels static friction. And they behave differently. In your case, when comparing to drag forces, only kinetic friction makes sense. $\endgroup$ – Steeven Jul 4 '17 at 17:00
  • $\begingroup$ So, does kinetic friction depend on surface area? $\endgroup$ – Hisab Jul 4 '17 at 17:09
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In the extreme case of a gas, viscosity comes mostly from the diffusion of molecules between layers of the flow (this diffusion transports momentum between the layers) — this is why a hard sphere gas is viscous for example. In the opposite extreme of the friction between two solids, this clearly does not happen: in that case, friction comes mostly from short distance repulsion between molecules or atoms.

The problem to go further is that there is a continuum of material between solids and gases, not to mention that a thin layer of the surface of a solid can melt under friction.

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