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Confused about the speed of a point on a rotating body

  1. Nov 11, 2013 #1
    1. The problem statement, all variables and given/known data


    The system consists of a 20-lb disk A, 4lb slender rod BC, and a 1-lb smooth collar C. If the disk rolls without slipping, determine the velocity of the collar at the instant theta=30 degrees. The system is released from rest when theta= 45 degrees.



    2. Relevant equations

    v=wr



    3. The attempt at a solution

     
  2. jcsd
  3. Nov 11, 2013 #2

    Simon Bridge

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    The radius is measured from the center of rotation.

    But I suspect you have misunderstood the question - point C (the collar) is not rotating.
     
  4. Nov 12, 2013 #3
    Would the center of rotation be C or G?
     
  5. Nov 12, 2013 #4

    Simon Bridge

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    The rod is certainly turning - but it's also translating (well, it's center of mass is).
    It's rather like those problems with a ladder leaning against a wall and sliding isn't it?

    How could you go about figuring the center of rotation - could it be B?
    Why not draw a free-body diagram and place the forces on it and let them tell you?
    (Your attachment, btw, is not a free body diagram.)
     
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