You have a grindstone (a disk) that is 90.0 kg, has a 0.340-m radius and is turning

You have a grindstone (a disk) that is 90.0 kg, has a 0.340-m radius and is turning at 90.0 rpm, and you press a steel axe against it with a radial force of 20.0 N.

(a) Assuming the kinetic coefficient of friction between steel and stone is 0.20, calculate the angular acceleration of the grindstone.
(b) How many turns will the stone make before coming to rest?