例:一根直杆可绕轴O转动,在直杆的中点挂一重物,在杆的另一端施加一个方向始终保持水平的力F,如图所示,在力F使直杆从竖直位置慢慢抬起到水平位置的过程中, Example: A straight rod can rotate around axis O, hang a heavy object at the middle point of the straight rod, and exert a force F at a horizontal direction at the other end of the rod. As shown in the figure, Straight position slowly raised to the level of the process,