截面极惯性矩（Ip=面积X垂直轴二次）。
扭转惯性矩Ip: the torsional moment of inertia
极惯性矩：the polar moment of inertia
截面各微元面积与各微元至某一指定截面距离二次方乘积的积分Ip＝ P↑2dF。
a quantity to predict an object’s ability to resist torsion, to calculate the angular displacement of an object subjected to a torque.
