Each of the two grinding wheels has a diameter of 5.6 in., a thickness of 0.75 in., and a specific weight of 477 lb/ft3. When switched on, the machine accelerates from rest to its operating speed of 3840 rev/min in 4.9 sec. When switched off, it comes to rest in 31 sec. Determine the magnitudes of the motor torque and the frictional moment, assuming that each is constant. Neglect the effects of the inertia of the rotating motor armature and assume that the grinding wheels are uniform disks.