How do you calculate moment of inertia for circle?

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Discussion Overview

The discussion centers on calculating the moment of inertia for a circle that is not centered at its center of gravity, particularly in the context of a complex shape composed of multiple circles. The scope includes theoretical considerations and practical applications in two-dimensional space.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant seeks guidance on calculating the moment of inertia for a circle not at its center of gravity, expressing frustration over a lack of clear answers found online.
  • Another participant suggests that the calculation generally involves integration and mentions the parallel axis theorem as a potential simplification.
  • A participant clarifies that the discussion pertains to two-dimensional space, indicating the z-axis as the relevant axis for the moment of inertia.
  • There is a question regarding the clarity of the Wikipedia page on the topic, indicating some participants may find it insufficient for practical application.
  • One participant distinguishes between the center of gravity and the centroid of a circle, suggesting that the term "cg" may be misapplied in this context.

Areas of Agreement / Disagreement

Participants express differing levels of understanding regarding the calculation methods and the terminology used, indicating that multiple competing views remain on how to approach the problem.

Contextual Notes

There are unresolved aspects regarding the application of the parallel axis theorem and the specific definitions of terms like center of gravity and centroid, which may affect the calculations discussed.

Josh The Cool
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Hey, not sure if this is the right place to post this, but here it goes. how do you calculate the moment of inertia for a circle that is not at its center of gravity. I am trying to find the moment of inertia for a complex shape made of many circles, and this seems like a good place to start. Forgive me if the answer is simple, because I have spent hours googling this question and have come up empty handed. I would greatly appreciate any help!
 
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Around which axis? In general, with an integral, but sometimes there are some tricks to avoid this, like the parallel axis theorem.
 
This is in two dimensional space. So I think the z axis if I understand correctly.
Edit:
I also looked at that wiki page, but couldn't apply it in any useful way.
 
What is unclear about the Wikipedia page?
 
Are you thinking of a reference point in the plane of the circle but not at the center of the circle?

A circle is a purely geometric figure, and as such, has no cg. It does have centroid; is that what you have in mind?
 

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