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Homework Help: Rotational motion of a Spool

  1. Dec 3, 2009 #1
    1. The problem statement, all variables and given/known data
    A narrow but solid spool of thread has radius R and mass M. If you pull up on the thread so that the CM of the spool remains suspended in the air at the same place as it unwinds,
    (a) what force must you exert on the thread?
    (B) how much work have you done by the time the spool turns with angular velocity w?

    2. Relevant equations

    k=1/2Iw^2 + 1/2MR^2w^2

    3. The attempt at a solution

    Ive tried to make an understanding that when your pulling upward for problem (a) you are canceling out the force being put on the spool. by pulling up. I'm just having trouble trying to find the correct formula to use to find Force. any help in the right direction would be great.
  2. jcsd
  3. Dec 3, 2009 #2


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    Draw a free body diagram of the spool. How many forces act on the spool? How must these forces be related if the spool remains suspended in air, i.e. its center of mass does not accelerate?
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