a string is wound around a uniform disk of radius r and mass m. the disk is released from rest when the string is vertical and its top end is tied to a fixed bar (fig. p11.47). show that (a) the tension in the string is one-third the weight of the disk, (b) the magnitude of the acceleration of the center of mass is 2g/3 , and (c) the speed of the center of mass is (4gh/3)^1/2 as the disk descends. Verify your answer to part (c) using the energy approach.