Calculate Moment of Inertia at Point A

AI Thread Summary
The discussion centers on calculating the moment of inertia at point A, located at the middle of a setup involving two spheres attached to a pole. Participants emphasize using the parallel axis theorem to find the moment of inertia, suggesting that the center of rotation does not need to be the center of mass. Clarifications are made regarding the calculation process, including determining the moment of inertia for each sphere and the distances from their centers of mass to the axis of rotation. The conversation concludes with one participant expressing relief at understanding the solution before an upcoming test. Overall, the thread provides guidance on applying the parallel axis theorem to solve the moment of inertia problem effectively.
asi123
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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

 

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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.
 
G01 said:
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.
 
Since you want the moment of inertia about the middle (if I understand the problem), why mess around with the center of mass?
 
Doc Al said:
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)?
 

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asi123 said:
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.
 
Doc Al said:
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.
 
asi123 said:
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.)
 
Doc Al said:
About what axis?

About the middle of the poll.

Doc Al said:
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.
 
  • #10
Oh, now I got it.
Good thing I got it before the test :smile:

Thanks a lot.
 
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