Amotor keep a ferris wheel (with moment of inertia 6.97 × 107 kg · m2 ) rotating at 8.5 rev/hr. when the motor is turned off, the wheel slows down (because of friction) to 7.5 rev/hr in 15 s. what was the power of the motor that kept the wheel rotating at 8.5 rev/hr despite friction?

Explanation:

Initial angular speed of the ferris wheel is given as

final angular speed after friction is given as

now angular acceleration is given as

now torque due to friction on the wheel is given as

Now the power required to rotate it with initial given speed is

real, smaller and upside-down.   so the answer should be b.