Theorem of Perpendicular Axis

Theorem of perpendicular axis states that if I_XX and I_YY be the moment of inertia of a plane section about two mutually perpendicular axes X−X and Y−Y in the plane of the section (as shown in Figure 11.15), then the moment of inertia of the section I_ZZ about the axis Z−Z, perpendicular to the plane and passing through the intersection of axes X−X and Y−Y is given by,

Figure 11.15 Perpendicular Axis Theorem

Hence, I_ZZ is also known as polar moment of inertia.