# How to define inertia of a body block in SimMechanics?

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Chih-Heng 2011 年 11 月 9 日

Hello,
The inertia has to expressed in 3-by-3 matrix. I just don't really know how to put in this section and the help menu doesn't explain it that well. Can someone give me an example like a circle or a cube and show me how to define its inertia?
thanks,
Saxon

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### 回答 (1 件)

Steve Miller 2022 年 12 月 21 日
For symmetrical objects, such as a sphere or a cylinder, the inertia matrix will have the moments of inertia along the diagonal and zeroes on the off-diagonal elements.
For a sphere, you can enter it as [1 0 0;0 1 0;0 0 1], where 1 is the moment of inertia about an axis. You can easily find moment of inertia formulas online and in many textbooks, and you can write MATLAB code to calculate those values based on the dimensions of your solid.
--Steve

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