# Kinetic energy stored in the flyweel

1. Oct 25, 2009

### jeneekim

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

A car is designed to get its energy from a ro-
tating flywheel with a radius of 2.19 m and
a mass of 679 kg. Before a trip, the fly-
wheel is attached to an electric motor, which
brings the flywheel’s rotational speed up to
7310 rev/min.
Find the kinetic energy stored in the fly-
wheel.

2. Relevant equations

KE = 1/2I$$\omega$$2
I = mr2

3. The attempt at a solution

given:
r = 2.19 m
m = 679 kg
$$\omega$$ = 7310 rev/min = 765.50141 rad/s

I = mr2
= (679 kg)(2.19 m)2
= 3256.5519 kgm2

KE = 1/2I$$\omega$$2
= 954157346 J

When I enter this answer, it says that it is incorrect? Could someone please let me know what I'm doing wrong?

Thank you!

2. Oct 25, 2009

### jeneekim

I figured it out! I was using the wrong equation for Inertia...it should be:

I = 1/2mr^2