时差N小时 地心引力为何有益于人类?(上)（在线收听）

When astronauts return from long missions in space, their exit from the spacecraft is usually hidden from public view.

当宇航员结束太空的长期任务返回地球时，他们走出太空飞船的画面往往不会曝光。

This is because after several months in weightless conditions, things we usually take for granted,

这是因为在失重状态下生活了数月，一些我们认为理所当然能够做到的动作，

such as walking and keeping our balance, present great difficulties for the astronauts.

如走路或保持平衡对宇航员来说十分困难。

Forces Around Us

环绕四周的力量

The culprit is gravity, or the virtual lack of it in space.

罪魁祸首是地心引力，或者说太空没有地心引力。

Gravity is the force with which particles of matter attract each other.

地心引力是指物质粒子相互吸引的力量。

The greater its mass, the stronger an object’s gravitational force.

一个物体质量越大，其地心引力越大。

That’s why we normally stay rooted to the surface of the earth instead of floating off; the earth’s mass is much greater than ours.

由于地球的质量远大于我们，所以我们可以一直站在地球表面而不会漂浮在空中。