Question

A car is designed to get its energy from a rotating flywheel with a radius of 1.50 m and a mass of 420 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 4,650 rev/min. (a) Find the kinetic energy stored in the flywheel. 1128x Your response differs from the correct answer by more than 10% Double check your calculations. ] (b) If the flywheel is to supply energy to the car as a 20.0 hp motor would find the length of time the car could run before the flywheel would have to be brought back up to speed.

Answer 1

A)

Angular velocity, w = 4650 x 2 pi/60 = 486.95 rad/sec

Moment of inertia, I = 0.5 x 420 x 1.5^2 = 472.5 kgm^2

Kinetic energy, E = 0.5 I w^2 = 5.60 x 10^7 J

B)

Power, P = 20 x 746 = 14920 W

Time, t = E/P

t = (5.6 x 10^7)/14920 = 3753.35 sec = 1.04 hrs

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