Feynman and the kinetic energy of an ice skater

International audienceBy 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