1. The problem statement, all variables and given/known data A disk roll without slipping down a incline plane. Identify the forces acting on the disk, explain qualitatively which of these forces do work. 2. Relevant equations The rotational work is given by [itex]W=\int \tau_z d\theta[/itex] (1) , where [itex]\tau_z[/itex] is the component of the torque parallel to the axis of rotation. 3. The attempt at a solution I'm ok with the first part of the exercise, my main doubt is about works. In particular I know that only gravity do work, while static friction force does not (the contact point does not move in every time istant considered). Nevertheless, looking at the definition of (rotational) work (1) I came up with a big doubt. Consider the motion of the disk as a combination of a traslational motion of the CM and a rotation about the CM, take as pivot point for the calculation of torques the CM. Then static friction force exerts a torque, which causes the rotation, while gravity does not. Hence, from (1), static friction force should do (rotational) work. But that's not possible, because static friction does no work. How can that be?