Finding angular velocity using work energy theorem

  • Thread starter theBEAST
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  • #1
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Homework Statement


cmPVt.png


Homework Equations


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

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!
 

Answers and Replies

  • #2
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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].
 
  • #3
364
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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].

Thanks, but why does Mtheta not work?
 
  • #4
265
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I'm not even sure what M is.
 
  • #6
265
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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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