# Homework Help: Moment of inertia

1. Nov 10, 2008

### asi123

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

Hey guys.
So I got this poll attached to 2 balls as you can see in the picture.
The question is, what is the moment of inertia at point A (at the middle).
I didn't get the same answer as them so I'm guessing I'm doing something wrong, any idea?

10x.

2. Relevant equations

3. The attempt at a solution

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2. Nov 10, 2008

### G01

If I'm reading the problem correctly, then you have two spheres of Radii R1 and R2 a distance a and b away from the center of rotation. You are finding the moment of inertia.

You do not need to go back to the definition of moment of inertia to solve this problem.

HINT: Use the parallel axis theorem.

3. Nov 10, 2008

### asi123

First of all, is the center of rotation is the center of mass?
you did noticed that there is a poll between them, right?
Is the moment of inertia for C that I wrote is correct?
10x.

4. Nov 10, 2008

### Staff: Mentor

Since you want the moment of inertia about the middle (if I understand the problem), why mess around with the center of mass?

5. Nov 10, 2008

### asi123

Ok then, should that be the moment of inertia about the middle (in the picture)?

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6. Nov 10, 2008

### Staff: Mentor

I don't understand how you got the 2nd and 3rd terms.

7. Nov 10, 2008

### asi123

It's the moment of inertia for the balls.
How can I calculate the moment of inertia for the all thing?

10x.

8. Nov 10, 2008

### Staff: Mentor

About what axis?

For each ball: What's Icm? What's the distance from cm to axis of rotation? Apply the parallel axis theorem.
By adding up the moments of inertia of the parts. (Which is what I thought you were doing.)

9. Nov 10, 2008

### asi123

About the middle of the poll.

So, is it correct what I did?

10x.

10. Nov 10, 2008

### asi123

Oh, now I got it.
Good thing I got it before the test

Thanks a lot.

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