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POCalculating inertia for a multi-shape rigid body
 

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    <p>I figured someone probably asked this question before but I wasn't able to find an answer.</p> <p>I'm writing a physics library for my game engine (2d, currently in actionscript3, but easily translatable to C based languages).</p> <p>I'm having problems finding a good formula to calculate the inertia of my game objects.</p> <p>The thing is, there are plenty of proven formulas to calculate inertia around a centroid of a convex polygon, but my structure is slightly different: I have game-objects with their own local space. You can add convex shapes such as circles and convex polygons to this local space to form complex objects. The shapes themselves again have their own local space. So there are three layers: World, object &amp; shape space.</p> <p>I would have no problems calculating the inertia of each individual polygon in the shape with the formulas provided on the <a href="http://en.wikipedia.org/wiki/List_of_moments_of_inertia" rel="nofollow">moments of inertia Wikipedia article</a>.</p> <p>or the ones provided in an awesome <a href="http://elancev.name/oliver/2D%20polygon.htm#tut7" rel="nofollow">collision detection &amp; response article</a>. </p> <p>But I'm wondering how to relate this to my object structure, do I simply add all the inertia's of the shapes of the object? That's what another writer uses to <a href="http://lab.polygonal.de/?p=57" rel="nofollow">calculate the inertia of triangulated polygons</a>, he adds all the moments of inertia of the triangles. Or is there more to it?</p> <p>I find this whole inertia concept quite difficult to understand as I don't have a strong physics background. So if anyone could provide me with an answer, preferably with the logic behind inertia around a given centroid, I would be very thankful. I actually study I.T. - Game development at my university, but to my great frustration none of the teachers in their ranks are experienced in the area of physics. </p>
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    1. CO@cha - I think you might want to read the [W3C guidance on link text](http://www.w3.org/TR/WCAG10-HTML-TECHS/#link-text).
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    2. CO@Flexo I didn't get you .... please clarify
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