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Homework Help: Rotational kinetic energy of a flywheel

  1. Mar 21, 2010 #1
    1. The problem statement, all variables and given/known data
    A car is designed to get its energy from a rotating flywheel with a radius of 1.12 m and
    a mass of 736 kg. Before a trip, the flywheel is attached to an electric motor, which
    brings the flywheel’s rotational speed up to 7270 rev/min.
    Find the kinetic energy stored in the fly-wheel.
    Answer in units of J.

    2. Relevant equations
    KE = 0.5*Inertia*w^2
    Inertia = m*r^2

    3. The attempt at a solution

    convert 7270 rev/min to rad/s = 761.314 rad/s

    KE equation: 0.5*(761.314)^2*(736 kg)*(1.12m)^2 = 267554029.7

    answer is apparently wrong... help please?

    PART 2, I don't think I can do this without a correct answer from part 1

    If the flywheel is to supply energy to the car as would a 11.8 hp motor, how long could the
    car run before the flywheel would have to be brought back up to speed?
    Answer in units of h.
  2. jcsd
  3. Mar 21, 2010 #2
    Is it possible the flywheel is modeled as a disk rather than a hoop, in which case

    I = .5MR^2, not I = MR^2 ?
  4. Mar 21, 2010 #3


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    Yes, try!

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