By employing the angular momentum and mechanical energy theorems, we analyse the increase of the
angular velocity for an ice skater who changes his arms, initially in an horizontal position, along
the axis of the body, first with respect to the lab frame, in which a variation of kinetic energy is
observed, and then with respect to the rotating frame moving with the skater. In the last one, while
there is no variation of kinetic energy between initial and final positions, the work of the
centrifugal force is the opposite to the variation of kinetic energy in the lab frame. We discuss
the explanation given by Richard Feynman in his Mechanics course.