A car is designed to get its energy from a rotating flywheel with a radius of 1.

Question

A car is designed to get its energy from a rotating flywheel with a radius of 1.00 m and a mass of 460 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 4,350 rev/min.

(a) Find the kinetic energy stored in the flywheel.
J

(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.
h

Easiest way for me to memorize if its possible! please!

Explanation / Answer

a)

KE of flywheel = 0.5*I*W^2

where I = moment of inertia = M*R^2 = 460*1^2 = 460 kg.m2

W = angular speed = 4350 rev/min = 4350*2*pi/(60) = 455.5 rad/s

So, KE = 0.5*460*455.5^2 = 4.77*10^7 J

b)

Power of the motor, P = 20 hp = 14914 W <----- as 1 hp = 745.7 W

We know, P = W/t

where W = energy of the flywheel = 4.77*10^7 J

So, t = 4.77*10^7/14914 = 3198.3 s = 3189.3/3600 = 0.89 h

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