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 車輪進動實驗:在進動中,角動量的矢量方向不斷改變,是因為受到一個由重力產生的力矩,使車輪在自轉的同時又繞支點轉動。