Calculate Moment of Inertia at Point A

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Homework Help Overview

The discussion revolves around calculating the moment of inertia at a specific point (Point A) for a system involving two spheres attached to a pole. Participants are exploring the implications of the setup and the application of relevant physics concepts.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants are attempting to clarify the problem setup, including the distances from the center of rotation and the relevance of the center of mass. There is a suggestion to use the parallel axis theorem, and questions are raised about the correctness of specific calculations and terms related to the moment of inertia.

Discussion Status

The discussion is active with participants questioning assumptions and clarifying their understanding of the moment of inertia in relation to the system. Some guidance has been offered regarding the use of the parallel axis theorem and the need to consider the contributions of individual components to the total moment of inertia.

Contextual Notes

Participants are working under the constraints of a homework assignment, which may limit the information they can use or the methods they can apply. There is an emphasis on understanding the definitions and relationships involved in calculating moment of inertia.

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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