High school physics lesson before you leave. High school physics teacher used to hold up a ball. She would say this ball has potential energy, but nothing will happen to it, it can't do any work until I drop it and it changes states.
在我们离开之前，说一下高中物理课 物理老师经常举起一个球， 说这个球 有势能， 但它不能做功， 除非我让它落下从而改变它的状态。