I don't really understand the answer, could someone help me, please :,) A motor drives a shaft of radius R/8 that is attached to the center of a wheel of radius R. The motor is turned off, and the force that the motor exerts on the shaft, Fmotor, varies with time as shown. There is also a constant frictional force of 0.4 N applied to the rim of the wheel in the opposite direction of the motion. During which time interval does the rotational kinetic energy increase and then decrease? (ans: from 3 sec to 5 sec)