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Homework Help: Finding angular velocity using work energy theorem

  1. Nov 3, 2012 #1
    1. The problem statement, all variables and given/known data
    cmPVt.png

    2. Relevant equations
    Inertia about pin = ml^2/3
    Work done by rotation = M*theta = 0.5(Inertia about pin) *ω2

    3. The attempt at a solution
    I tried two different methods and both came out to be the same answer. However they did not correspond to any of the multiple choice answers... What am I doing wrong?

    zsts9.jpg

    Thanks!
     
  2. jcsd
  3. Nov 3, 2012 #2
    Method 1: The angular acceleration varies with [itex]\cos(\theta)[/itex]. So using constant acceleration formulas doesn't work. You simply can't use that method.

    Method 2: The potential energy is still [itex]U=mgh[/itex], not [itex]U=M\theta[/itex].
     
  4. Nov 3, 2012 #3
    Thanks, but why does Mtheta not work?
     
  5. Nov 3, 2012 #4
    I'm not even sure what M is.
     
  6. Nov 3, 2012 #5
    Moment about the pin.
     
  7. Nov 3, 2012 #6
    It's still wrong. It should be [itex]\frac{1}{2}M\omega[/itex]. But that by itself contains no information about the fact it's in gravity.
     
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