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persephone

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## Homework Statement

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.

## Homework Equations

KE=(1/2)Iw^2

where I=Mr^2

P=KE/t

## 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

Also wrong..

Is there something I'm missing here? I can't figure out for the life of me what I'm doing wrong.