Question

Does friction oppose motion or relative motion of surfaces in contact?

Answer 1

I have always treated friction as 'opposing' force rather than motion, either at its limit or to maintain equilibrium. I have just realised, however, that if an object is at rest on an inclined plane, but is not at limiting equilibrium and you give it a nudge it will accelerate while you apply the force on to it but then (usually) decellerate to rest. If friction opposes force you would expect the object to maintain a constant velocity once it is moving, so there must be friction greater than the weight parallel to the plane. Does this mean that friction opposes motion rather than force and would you calculate the change in friction as the object deccelerates?

Reference https://www.physicsforums.com/threads/does-friction-oppose-force-or-motion.695597/