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Rotational Physics

  1. Aug 6, 2011 #1
    So the question is this:

    A disc is released from rest. A block is causing it to rotate. After a time t the block has fallen a height h and the disc has rotated through an angle theta. (in rad)
    Express h in terms of r (the radius of the part of the hub around which the string is wound) and theta.

    I came up with this answer:

    PE lost = KE gained
    mgh = (1/2)mv^2 + (1/2)I(omega)^2
    mgh = (1/2)m(r*omega)^2 + (1/2)*(mr^2)omega^2
    gh = (1/2)r^2*omega^2 + (1/2)omega^2*r^2
    gh = omega^2*r^2
    omega = d(theta)/dt
    subs for omega and solve for h: h = (r^2*theta^2)/(t^2*g)

    The question asks me to express h in terms of r and theta but my answer has g and t in it. Any help?

    Thanks in advance!
  2. jcsd
  3. Aug 7, 2011 #2


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    welcome to pf!

    hi scifan! welcome to pf! :smile:

    (have a theta: θ and try using the X2 icon just above the Reply box :wink:)
    read the question!! :rolleyes:

    energy has nothing to do with it …

    this is just geometry!
  4. Aug 7, 2011 #3
    Re: welcome to pf!

    Would that be correct if I say h = r*theta then?
  5. Aug 7, 2011 #4


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    Last edited by a moderator: Apr 26, 2017
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