题干
如图，两个完全相同的空心塑料球A和B，用两根完全相同的绝缘丝线将两球悬挂在天花板上的一点，当M、N两处都带上适量负电后，A，B两球“恰好接触而不挤压对方”．请画出B球受力示意图．
答案(点此获取答案解析）
【解答】解：B球受丝线的拉力F、重力G和A球对它的排斥力N，作用点可以都在重心，如图所示：
如图，两个完全相同的空心塑料球A和B，用两根完全相同的绝缘丝线将两球悬挂在天花板上的一点，当M、N两处都带上适量负电后，A，B两球“恰好接触而不挤压对方”．请画出B球受力示意图．
【解答】解：B球受丝线的拉力F、重力G和A球对它的排斥力N，作用点可以都在重心，如图所示：
It is noon now. The sun is high in the sky. It is very hot. Look at the tree. The shadow is short. 
It is evening now. The sun goes down in the west(西方). Look at the tree, the shadow is long again. 
It is night now. The moon is high in the sky. There are some stars in the sky too. 
