当作用在质点系上所有外力对某一固定轴的力矩之和为零,则质点系对该轴的动量矩保持不变。例如花样滑冰表演者,可以绕通过重心的铅垂轴高速旋转,也可以展开双臂缓慢旋转,在这一过程中,摩擦力矩可以忽略,因此表演者对旋转轴的动量矩守恒。When the sum of all the moments of force acting on a fixed axis on the particle system is zero, the moment of force of the particle system on the fixed axis remains constant. A figure skater, for example, can spin at high speed around a plumb axis passing through the center of gravity or slowly rotate his arms. In this process, the friction moment of force can be ignored, so the moment of motion of the skater toward the axis of rotation is conserved.