Moment of inertia: rectangle iy = hb3/12; Where 3 represents the relationship of cubes; round iz = 3. 14d 4/64； The four after d stands for the fourth power. Square: IX = B * H 3/ 12, Iy = H * B 3/ 12, where h refers to the high circular cross section: IX = Iy = D 4 * π/64-D 4 * π/64, and the moment of inertia operation satisfies the addition, that is, the ring. Polar Moment of Inertia: Because ρ 2 = x 2+y 2, we can get the mathematical relationship between polar moment of inertia and sub-wheelbase of section as shown in the left figure above, that is, the polar moment of inertia of section to any point is equal to the sum of sub-wheelbase of section to any set of orthogonal coordinate systems with this point as the origin.