This engineering calculator will determine the section modulus for the given cross-section. For a rigid body moving about a fixed axis the laws of motion have the same form as those of rectilinear motion with the moment of inertia replacing mass angular replacing linear velocity angular momentum replacing linear momentum etc. Derivation Of Moment Of Inertia Of Rectangle Maximum Bending Stress Equations. . This engineering data is often used in the design. The polar section modulus also called section modulus of torsion Z p for circular sections may be found by dividing the polar moment of inertia J by the. I Moment of inertia about new axis I I A d 2 centroid d new axis Area A I Moment of inertia about the centroidal axis A Area of the region d perpendicular distance between the two axes. Hence the kinetic energy of a body rotating about a fixed axis with angular velocity ω is ½ω² which corresponds to ½mv² for the kinetic.