Stretch rule

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In physics, the stretch rule states that the moment of inertia of a rigid object is unchanged when the object is stretched parallel to the axis of rotation, (without changing the distribution of mass except in the direction parallel to the axis}. That is, cylinders oriented parallel to the axis must not change in radius.

This rule can be applied with the parallel axes theorem and the perpendicular axes rule to find moments of inertia for a variety of shapes.