Perpendicular Axis Theorem

The Perpendicular Axis Theorem states that for a flat, flat object like a lamina, the total rotational inertia around an axis perpendicular to its plane equals the sum of its moments of inertia around two perpendicular axes lying in the plane and crossing at the same point. Essentially, if you know how the object resists rotation around two perpendicular in-plane axes, you can find out how it resists rotation around an axis perpendicular to the plane by adding those two values together. This theorem helps in analyzing how objects spin and rotate in three-dimensional space.