听力文章：零重力是否真的存在
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Zero Gravity, Does It Exist?
This Moment of Science: Is there such a thing a zero gravity?
No Zero Gravity
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 orbit. 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.
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.
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 planet. In other words, they’re basically in a state of constant free fall, and that’s why they’re weightless.
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