A grinding wheel, with a mass of 20.0 kg and a radius of 22.4 cm, is a uniform cylindrical disk. 

(a) Find the rotational inertia of the wheel about its central axis. 

(b) When the grinding wheel's motor is turned off, friction causes the wheel to slow from 1200 rev/min to rest in 60.0s. What torque must the motor provide to accelerate the wheel from rest to 1200 rev/min in 4.00s? Assume that the frictional torque is the same regardless of whether the motor is on or off.