有时物体受到几个外力作用，但这些力均不对物体做功，此时，物体是否能对外做功呢？现在我们分析一例：人上楼梯时，人受到重力、支持力的作用如果人的体重为G，登的高度为h，我们都知道人克服重力做的功为W＝Gh．但此功是什么力做的呢？重力的方向向下，而物体向上运动，可见 Sometimes the object is subjected to several external forces, but these forces do not work on the object. At this time, can the object do work externally? Now we analyze an example: When people go upstairs, people are affected by gravity and support. If a person weighs G and a height is h, we all know that the work done by a person overcoming gravity is W=Gh. But what is the power to do this? The direction of gravity is downwards, and the object moves upwards, visible