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 425 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 4000 rev/min.
(a) Find the kinetic energy stored in the flywheel.
(b) If the flywheel is to supply energy to the car as would a 15.0-hp motor, find the length of time the car could run before the flywheel would have to be brought back up to speed.
The Attempt at a Solution
First I used the equation to find the moment of inertia: I=(425kg)(1)^2=425
I know that the angular velocity is 4000 rev/min, so I converted that to 418.879 rad/s
Then I plugged that into the the equation for KE: 1/2(425)(418.879 rad/s)^2 =3.73e7
This was wrong...
For part b, I used the equation relating power to kinetic energy, so I converted 15 hp to 11185.5 Watt
So, 3.73e7/11185.5 = t = 3334.67s
Is there something I'm missing here? I can't figure out for the life of me what I'm doing wrong.