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Homework Help: Angular Velocity of a pivoting rod

  1. Mar 2, 2009 #1


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    1. The problem statement, all variables and given/known data
    A uniform rod 5.8m long weighing 10kg is pivoted at its center and a small weight of mass 5.15 kg is rigidly attached to one end. You may neglect the size of the weight and assume it is located right at the end of the rod. It is held at a 37 degree angle at rest, then released.
    What is the angular velocity when the rod is vertical?

    2. Relevant equations
    After some work, I earlier solved that angular acceleration at it's release point is 1.6295 Don't know if that will be needed or not...
    Angular Velocity = Tangential Velocity /R
    VF^2 = VI^2 + 2A (X1-Xf)

    3. The attempt at a solution
    I dared to hope I could do this on my first try, but was disappointed. (Again.)
    5.8/2 = 2.9, so radius = 2.9
    sin37 2.9 = 1.745
    2.9-1.745 =1.1547 m of fall

    VF^2 = 0 + 2 (9.81) (1.1547)
    VF^2 = 22.65
    VF = 4.76

    4.76 / 2.9 = 1.64
    It looks so right.... but is so wrong. *sigh*
  2. jcsd
  3. Mar 3, 2009 #2


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    Hi TG3! :smile:
    Sorry, not following your proof at all :confused:

    using conservation of energy is right :smile:

    but what's happened to the mass, and the moment of inertia ?
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