The puck in Figure P8.51 has a mass of 0.120 kg. Its original distance from the center of rotation is 40.0 cm, and it moves with a speed of 85.0 cm/s. The string is pulled downward 15.0 cm through the hole in the frictionless table. Determine the work done on the puck. (Hint: Consider the change of kinetic energy of the puck.)
I tried doing it by finding the Kinetic Rotational Energy before and after but its not coming out right. For the KE after do I use the radius as 15, or is it 40-15?