# Kinetic Energy of Rotation

## Homework Statement

An automobile is traveling at v = 32 m/s and has wheels with radius R = 22 cm. Assuming that the wheels are disks which roll without slipping and have a mass m = 11 kg, what is the kinetic energy of rotation of the four tires in Joules?

## Homework Equations

w = v/R

The kinetic energy for each tire is then

KE = 1/2 Iw^2

There are four wheels.

## The Attempt at a Solution

I tried converting 32 m/s to rad/s for angular velocity. So 32m/s divided by .22m (radius) to get 145.45.

Then I squared that answer. I multiplied it by .5.

Then for I, I used mr^2 and got .5324.

I got 5631.648. Tried multiplying that by four and it didn't work.

I've tried entering

3 Incorrect. (Try 1) 5630
5 Incorrect. (Try 2) 5.6x10^2
7 Incorrect. (Try 3) 2.3x10^4
9 Incorrect. (Try 4) 22500

None of these were correct. I'm wondering where I messed up....

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ehild
Homework Helper
"Then for I, I used mr^2 and got .5324."

The wheels were assumed disks, and the moment of inertia of a disk is 0.5 mr^2.

ehild