Consider the following situation. An electric motor is coupled to a flywheel [ m

Question

Consider the following situation. An electric motor is coupled to a flywheel [ moment of inertia = 150 kgm^2 ] and accelerates it from rest to an angular speed of 30 rad/sec. During the aceleration, axle bearing friction does 25,000 J of work against the flywheel. If the acceleration takes place in a 5 second time interval. what average power does the motor supply? (in watts) a. 18,500 b. 22,000 c. 10,000 d. 13,500 e. 8,500 4. At left, a certain type of disk [ mass = M; radius = R; I_cm = (3/4)MR^2 ] rolls (without slipping) down an inclined plane. What is the magnitude of the acceleration of its center of mass ? a. (4/7) gcos theta b. (4/3) gcos theta c. gsin theta d. (4/3) gsin theta e. (4/7) gsin theta

Explanation / Answer

3.

W = final angular speed = 30 rad/s

I = moment of inertia = 150 kgm2

net work done W = (0.5) IW2 = (0.5) (150) (30)2 = 67500 J

work done by friction = Wf = - 25000 J

work done by motor , Wm = W + Wf = 67500 + 25000 = 92500 J

Power = Wm / t = 92500 / 5 = 18500 Watt

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