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Homework Help: A rim unwinding from a string due to gravity.

  1. Nov 15, 2011 #1
    1. The problem statement, all variables and given/known data

    A string is wrapped around a small hoop several times. the radius is 8cm and mass 0.18 kg. the string is released from rest after the hoop has descended 75 cm what is the angular speed of the rotating hoop and the speed at its centre?
    2. Relevant equations

    mgh 1/2 Iw^2

    3. The attempt at a solution

    MGH energy is conserved so mgh=1/2Iw^2
    i sub in and solve except this does'nt give the right answer.
  2. jcsd
  3. Nov 16, 2011 #2
    If I is the moment of inertia of hoop about its centre then,
    intial energy = final energy
    GPE = KE of translation + KE of rotation
  4. Nov 16, 2011 #3
    mgh = (1/2)mv[itex]^{2}[/itex] + (1/2)I[itex]\omega[/itex][itex]^{2}[/itex]
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