You have a horizontal grindstone (a disk) that is 85 kg, has a 0.39 m radius, is turning at 85 rpm (in the positive direction), and you press a steel axe against the edge with a force of 19 n in the radial direction. 50% part (a) assuming that the kinetic coefficient of friction between steel and stone is 0.20, calculate the angular acceleration of the grindstone in rad/s2.