Hey, I have this task that may seem simple, but really isn't: When a box moves across a surface the force of friction does a negative work. Can a friction force ever do a positive work? Possibility 1: Yes, because else what is it then that moves your car forward when you accelerates? Possibility 2: No, because the force of friction is always opposit the direction of motion, and therefore W = F*s*cos(180) = -F*s.... Anybody who can offer help?