My prof really confused me. Given a rolling sphere, which way does the friction point to? http://img187.imageshack.us/img187/7968/rolling1.png [Broken] http://img338.imageshack.us/img338/8753/rolling2.png [Broken] I think it probably matters if the ball is purely rolling or partially skidding, but I'm not sure which direction friction points to in either case. So can someone please tell me under which cases the friction will point which way? Thanks!