Calculate Moment of Inertia at Point A

[asi123]

Homework Statement

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.

Homework Equations

The Attempt at a Solution

 

[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.

 

[asi123]

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.

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.

 

[Doc Al]

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

 

[asi123]

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

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

 

[Doc Al]

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

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

 

[asi123]

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

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

10x.

 

[Doc Al]

It's the moment of inertia for the balls.

About what axis?

For each ball: What's Icm? What's the distance from cm to axis of rotation? Apply the parallel axis theorem.

How can I calculate the moment of inertia for the all thing?

By adding up the moments of inertia of the parts. (Which is what I thought you were doing.)

 

[asi123]

About what axis?

About the middle of the poll.

By adding up the moments of inertia of the parts. (Which is what I thought you were doing.)

So, is it correct what I did?

10x.

 

[asi123]

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

Thanks a lot.