The water flows from the tank by gravity to the houses below.水箱里的水由于引力的作用流到下面的房子里。The stone rolled down the hill by gravity.石头在重力作用下向山下滚去。Since light has no mass, Newton's equation predicts that it will not be attracted by gravity towards anything.既然光是没有质量的，牛顿的方程式便预言光不会受到引力作用。