In the motion , the direction of angular momentum vector changes every moment , because there is a torque caused by gravity act on the wheel , then the wheel rotate itself and rotate around the fixed point 车轮进动实验:在进动中,角动量的矢量方向不断改变,是因为受到一个由重力产生的力矩,使车轮在自转的同时又绕支点转动。