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Moment of Inertia for a Circle?

Topics: Developer Forum
Dec 4, 2008 at 8:53 PM
In BodyFactory.CreateCircleBody, the moment of inertia is calculated as follows:

body.MomentOfInertia = .5f * mass * (float)Math.Pow(radius, 2f);

Looking at the moment of inertia for a solid disk on Wikipedia (, I would think the equation should be:

body.MomentOfInertia = .25f * mass * (float)Math.Pow(radius, 2f);

... for a 2-dimensional object.
Dec 5, 2008 at 12:35 AM
The MOI is calculated around the "z" axis. Which for 2d is perpendicular to the screen.  I think the .5 is correct.