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Homework Help: Kinetic energy stored in the flyweel

  1. Oct 25, 2009 #1
    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-
    Answer in units of J.

    2. Relevant equations

    KE = 1/2I[tex]\omega[/tex]2
    I = mr2

    3. The attempt at a solution

    r = 2.19 m
    m = 679 kg
    [tex]\omega[/tex] = 7310 rev/min = 765.50141 rad/s

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

    KE = 1/2I[tex]\omega[/tex]2
    = 1/2(3256.5519 kgm2)(765.50141 rad/s)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. jcsd
  3. Oct 25, 2009 #2
    I figured it out! I was using the wrong equation for Inertia...it should be:

    I = 1/2mr^2
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