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Rotational dynamics question

  1. Dec 10, 2013 #1
    1. The problem statement, all variables and given/known data
    A large wheel has a radius of 7.1 m. A rope is wrapped around the edge of the wheel and a 7.6 kg box hangs from the rope. A smaller disk of radius 1.38 m is attached to the wheel. A rope is wrapped around the edge of the disk as shown. An axis of rotation passes through the center of the wheel-disk system. What is the value of the mass M that will prevent the wheel from rotating?


    2. Relevant equations



    3. The attempt at a solution
    all moments of inertia should equal zero so
    m1r1^2 + m2r2^2 = 0
    m1 = -m2r2^2/r1^2
    m1 = -(7.6)(7.1)^2/1.38^3
    m1 = 201

    answer is actually 39kg
     
  2. jcsd
  3. Dec 11, 2013 #2

    ehild

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    Homework Helper
    Gold Member

    [/QUOTE]

    The torques have to sum up to zero.

    ehild
     
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