# Moment of Inertia problem [closed]

I'm revising for my exam and there's this one problem where I can't get the correct answer, so I'd like to know how to solve it. The problem is:

Two boys, each with a mass of 30kg, are sitting on the ends of a horizontal girder which is 2.5m long and has a mass of 15 kg. The girder rotates at 7 turns/minute around a vertical axis through its center.

a) What is the angular speed if each boy moves forward 29 cm?
b) What is the change in kinetic energy?

(I added the homework tag, but it's not like I have to solve this, I just don't know how to.)

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Hi Sirzh, and welcome to Physics Stack Exchange! First, thanks for adding the homework tag (it is entirely appropriate here), but as our FAQ says, we don't just offer full solutions to homework or self-study problems. To give you a useful answer, we need to know what exactly you're having trouble with: what did you try, and where did you get stuck? As soon as you narrow down your question to the specific physical concept that's giving you trouble, I'll be happy to reopen it. See this meta post for more info. – David Zaslavsky Aug 22 '12 at 20:54
Part (a) is done using conservation of angular momentum. Angular momentum is equal to $I\omega$, so just calculate the moment of inertia before and after the boys move. Google for how to calculate $I$ - the Wikipedia article is pretty helpful. Once you've calculated the new value of $\omega$ the kinetic energy is just $0.5I\omega^2$. – John Rennie Aug 23 '12 at 8:52

## closed as too localized by David Zaslavsky♦Aug 22 '12 at 20:51

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