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Homework Help: Flywheel with weight - Rotational Inertia

  1. Nov 8, 2009 #1
    Hey everyone! I have a small problem with part b. Im wondering if i miscalculated something. Any ideas? Thanks!

    1. The problem statement, all variables and given/known data

    A 15 kg flywheel has all its mass around its outer rim. A string is wrapped around it and a m = 4.7 kg weight is hanging on the string. The flywheel has radius R = 0.25 m.

    a) When the 4.7 kg weight is dropping with a speed of 1.1 m/s, what is the angular velocity of the flywheel?
    w = rad/s *
    4.4 OK

    b) When the 4.7 kg weight is moving with a speed of 1.1 m/s, what is the kinetic energy of the entire system? This one seems really easy, but for some reason I keep getting the wrong answer.

    2. Relevant equations

    K(rotational) = 0.5Iw^2
    I = 0.5m(wheel)r^2
    KE = 0.5m(block)v^2

    3. The attempt at a solution

    KE(system) = 0.5[0.5m(wheel)r^2]w^2 + 0.5m(block)v^2

    0.5*[0.5*15*0.25^2]*(4.4^2) + 0.5*4.7*(1.1^2)
    KE(system) = 4.5375 + 2.8435 = 7.381 J

    For some reason 7.381 is not correct.
    Last edited: Nov 8, 2009
  2. jcsd
  3. Nov 8, 2009 #2
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