1、 The stone rolled down the hill by gravity、石头在重力作用下向山下滚去。

2、 Anything that is dropped falls towards the centre of the earth because of the pull of gravity、物体脱手即向地心方向跌落是由于重力吸引的缘故。

3、 He could hardly keep his gravity、他忍不住要笑出来。

4、 gravity

5、 She does not know the gravity of her illness、她不知道自己病情严重。