Find the kinetic energy stored in the flywheel

1. Oct 26, 2008

persephone

1. The problem statement, all variables and given/known data

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.

2. Relevant equations
KE=(1/2)Iw^2
where I=Mr^2

P=KE/t

3. 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.

2. Oct 26, 2008

G01

At first glance, I noticed that you are using the wrong moment of inertia.

You are using the moment of inertia for a circular hoop, not a solid disc. The moment of inertia for a disc is:

$$I=\frac{1}{2}MR^2$$

Try working the problem with this moment of inertia and see if it helps.

3. Oct 26, 2008

persephone

ah ha! that equation worked! I'm a total newb at physics so I wasn't aware there was a separate equation for the moment of inertia for a disc

Thank you so much!

4. Oct 26, 2008

G01

No problem. I'm glad to be of help!