A small disc of mass m slides down a smooth hill of height h without initial velocity and gets onto a plank of mass M lying on the horizontal plane at the base of the hill. (figure). Due to friction between the disc and the plank the disc slows down and, beginning with a certain moment, moves in one piece with the plank.
(1) Find the total work performed by the friction forces in this process.
(2) Can it be stated that the result of obtained does not depend on the choice of the reference frame?