Physics Homework Help

Question

A car is designed to get its energy from a rotating flywheel with a radius of 1.82 m and a mass of 687.7 kg. Before a trip, the disk-shaped flywheel is attached to an electric motor, which brings the flywheel’s rotational speed up to 1943.0 rev/min.

a) Find the kinetic energy stored in the flywheel. Answer in units of J.

b) If the flywheel is to supply as much energy to the car as a 7374 W motor would find the length of time the car can run before the flywheel has to be brought back up to speed again. Answer in units of s.

I already have the answer for part A, need help with part B

Answer 1

Let

r = 1.82 m be the radius of the flywheel,

m = 687.7 kg be the mass of the flywheel,

I = mr²/2 be the moment of inertia of the flywheel,

ω = 1943 s^-1 be angular velocity of the flywheel,

E = Iω²/2 be the energy stored in the flywheel.

a) Substituting:

E = mr²ω²/4 = 687.7 kg × 1.82² m² × 1943² s^-2 / 4 = 2149945298.35813 J

b) Let

P = 7374 W be the power drawn from the flywheel,

t (to be computed) be the time the flywheel is to run.

Given that the car draws power P from the flywheel for t seconds till all the energy

of the flywheel is exhausted,

E = Pt

t = E/P = 2149945298.35813 J / 7374 W = 291557.53978276 s

(That is about 80 hours)