# Homework Help: Gre 08 q97 Angular Momentum

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1. Mar 15, 2013

### pluspolaritons

Hi I attempted to solve this question but got a different answer, I don't know what's wrong with my solution.

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

3. The attempt at a solution
At point P before collision, the angular momentum from the top disc has two components, rotational and translational.

For the translational L=rxp=-MRv_0=-0.5*M*R^2*w_0. This value is negative as I choose out of the page as my positive axis.

For the rotational part, I applied parallel-axis theorem, thus I=(0.5MR^2+MR^2)=1.5MR^2. And the angular momentum is L=1.5*M*R^2*w_0

Adding these two components up I obtain: 0.5*M*R^2*w_0. The answer does not have this choice. The given solution is (A).

If I do not apply parallel-axis theorem, I will get the correct answer. In fact in most solutions I looked up, they do not apply parallel-axis theorem, which I think is incorrect since the axis of rotation is not the center of mass of the top disc but rather at point P, so we have to apply parallel-axis theorem to shift the rotational angular momentum axis of rotation to point P. Can someone please help to see where I did wrong and why my argument is invalid?