# Homework Help: Finding angular velocity using work energy theorem

1. Nov 3, 2012

### theBEAST

1. The problem statement, all variables and given/known data

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?

Thanks!

2. Nov 3, 2012

### frogjg2003

Method 1: The angular acceleration varies with $\cos(\theta)$. So using constant acceleration formulas doesn't work. You simply can't use that method.

Method 2: The potential energy is still $U=mgh$, not $U=M\theta$.

3. Nov 3, 2012

### theBEAST

Thanks, but why does Mtheta not work?

4. Nov 3, 2012

### frogjg2003

I'm not even sure what M is.

5. Nov 3, 2012

### theBEAST

Moment about the pin.

6. Nov 3, 2012

### frogjg2003

It's still wrong. It should be $\frac{1}{2}M\omega$. But that by itself contains no information about the fact it's in gravity.

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