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Homework Help: Kinetic Energy of Rotation

  1. Mar 28, 2010 #1
    1. The problem statement, all variables and given/known data

    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?

    2. Relevant equations

    w = v/R

    The kinetic energy for each tire is then

    KE = 1/2 Iw^2

    There are four wheels.

    3. 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....
  2. jcsd
  3. Mar 29, 2010 #2


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    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.

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