A horizontal circular disk with mass m and radius 2r can rotate easily about its vertical axis of symmetry O. A small boy with mass km starts in A, when the system is at rest, and runs along the outer edge of the disk. At B he radially inwards to C and continues along a quarter circle arc to D. How big an angle has the disc turned when the boy has returned to A? You have to draw the free body diagram AO goes D. A 1+2mr2 The answaer of the question is; w1 Wo (mb2 +m 12