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Zero Gravity, Does It Exist?
零重力，是否真的存在？

This Moment of Science: Is there such a thing a zero gravity?
本期科学一刻：是否真的存在零重力？

No Zero Gravity
零重力，无稽之谈

Contrary to popular belief, there’s no such thing as zero gravity. Weightlessness and zero gravity are two different things. The earth’s gravity keeps the moon in orbit¹. And astronauts are generally much closer to earth than the moon is, which means that the earth’s pull on them has to be much stronger.
与人们通常的说法不同，零重力纯属无稽之谈。失重和零重力其实是两种完全不同的概念。地球的重力使月球绕轨运行。通常，宇航员其实比月球更为接近地球，这就意味着地球作用于他们的地心引力更强。

While we’re on Earth, we feel our weight because, as the Earth’s gravity pulls us into its center, the ground pushes back against our feet.
我们站立于地面上可以感受到重力，是因为地球本身的引力将我们拉向地心，但同时力又反作用于我们的双脚。宇航员绕地飞行时，他们仍然受到重力的作用。

When astronauts orbit the earth, they’re still subject to gravity, but they’re moving sideways so quickly that even though they’re being pulled towards the earth, they’re not getting any closer to the center of the planet². In other words, they’re basically³ in a state of constant free fall, and that’s why they’re weightless.
但是由于他们始终侧向移动，而且速度如此之快，所以即使他们受到地心引力的影响，他们也并没有更接近地心。换而言之，他们基本上是处于恒定的自由落体运动中，也就是他们失重的原因。

Weaker Gravity
弱重力

Keep in mind that gravity never disappears entirely⁴. It just gets weaker. And every object with mass, including you and me, has what’s known as gravitational attraction. Although you’re subject to all those forces, the Earth’s gravity masks their pull because the Earth is so large and so close.
但是由于他们始终侧向移动，而且速度如此之快，所以即使他们受到地心引力的影响，他们也并没有更接近地心。换而言之，他们基本上是处于恒定的自由落体运动中，也就是他们失重的原因。

So moving away won’t free you of the Earth’s gravity. All that would happen is that the Earth’s pull would be masked by the pull of some other object, like Mars⁵.
因而，离开地球并不能让你脱离地心引力的影响。但有可能会是这样的情况，地球的引力会被诸如火星的物体所掩盖。