计算梯度向量Grad U和圆饼的移动规则:将所有的力都正因分解，分解成x轴方向和y轴方向的力，整个问题就简单化了，通过分别求出x轴方向和y轴方向的力之和，就可以知道该圆饼将要移动的轨迹，而x轴方向和y轴方向的合力方向应该就是该圆饼的梯度向量的方向。而所有圆饼的梯度向量就组成了Grad U-Because of all the forces are decomposed into the x-axis direction and the y-axis direction of the force, the whole problem simplistic, were obtained by x-axis direction and the y-axis direction of the force of and to know that the pie track to be moved, and x-axis direction and the y-axis direction of the force should be the vector of the pie the direction of the gradient. All round cake on the formation of the gradient vector Grad U