A star is born when gravity forces a huge volume of mostly hydrogen gas to collapse in on itself.

一颗恒星的诞生是由于重力迫使大量的氢气坍缩。

The Moon holds that tilt steady. The various gravity forces that are exerted tend to act in a way that holds that steady.

月球一直保持着一个倾角。在万有引力作用下始终保持在一个固定倾角。

The gravity force pulls down the runoff from the highest point to the lowest point.

重力将径流从最高点拉到最低点。