Moment of inertia of a polygon

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Discussion Overview

The discussion focuses on calculating the moment of inertia of a polygon, specifically in a 2D system where the polygon's area, centroid, and vertices are known. Participants explore methods to compute this without resorting to complex integrals over the area or mass.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about calculating the moment of inertia of a polygon, seeking methods that do not involve difficult integrals.
  • Another participant suggests that if the polygon is regular, it could be divided into triangles with a vertex at the center, allowing for the summation of moments of inertia.
  • A clarification is made that the polygon in question is arbitrarily shaped, referred to as an "n"-agon, prompting further inquiry into alternative mathematical methods.
  • A participant presents a specific equation for calculating the moment of inertia about the origin based on the polygon's vertices, providing a link to a detailed derivation.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the best method for calculating the moment of inertia of an arbitrarily shaped polygon, and multiple approaches are being discussed.

Contextual Notes

The discussion does not resolve the limitations or assumptions involved in the proposed methods, such as the applicability of the triangular decomposition to non-regular polygons or the completeness of the presented equation.

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How can one calculate the moment of inertia of a polygon?
Assuming that one knows the polygon’s total area, centroid
and vertices, which are connected by straight lines in a 2D system.
Is it possible to avoid a difficult integral over the area/mass?

Any helpful information is highly appreciated.
 
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Is it a regular polygon? You could break it into triangles each with a vertex at the center of the polygon and add up the moments of inertia appropriately.
 
Tide said:
Is it a regular polygon? You could break it into triangles each with a vertex at the center of the polygon and add up the moments of inertia appropriately.


hi tide, in response to ur question, it is an arbitrarily shaped polygon, which basically means any shape with any number of vertices..maybe can call it "n"-agon.

i get the triangular part, but just wondering whether there is any other mathematical methods.

regards
 
I have calculated the moment of inertia about the origin of a polygon based on its vertices.

The equation is

1/12 * sum { (y_{i+1} - y_{i} )(x_{i+1} + x_{i})(x_{i+1}^2 + x_{i}^2)
- (x_{i+1} - x_{i} )(y_{i+1} + y_{i})(y_{i+1}^2 + y_{i}^2)

I put the details of its derivation on my web page.

http://www.enel.ucalgary.ca/~shannon/v2/green/
 
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