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Homework Help: Find the kinetic energy stored in the flywheel

  1. Oct 26, 2008 #1
    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
    where I=Mr^2


    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. jcsd
  3. Oct 26, 2008 #2


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    Homework Helper
    Gold Member

    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:


    Try working the problem with this moment of inertia and see if it helps.
  4. Oct 26, 2008 #3
    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!
  5. Oct 26, 2008 #4


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    Homework Helper
    Gold Member

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