Question

A car is designed to get its energy from a rotating flywheel (solid disk) with a radius of 1.00 m and a mass of 575 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 5,000 rev/min.
(a) Find the kinetic energy stored in the flywheel.
Answer in J

(b) If the flywheel is to supply energy to the car as a 10.0-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 in h

Answer 1

Given: r=1m, m=575kg, V = 5000rev/min.
KE = ?

a. C = pi*D=3.14 * 2=6.28m= Circumference.

V=5000rev/min * 6.28m/rev *(1/60)min/s
= 523.3m/s.

KE=0.5mV^2=0.5 * 575 * (523.3)^2 =
78,729,831J.

b. Po = 10hp * 746W/hp = 7460 Watts =
7460 Joules/s.

Po = 7460J/s * 3600s/h = 26,856,000J/h.

T = KE / Po = 78,729,831 / 26,856,000 =
2.93h.