A Weighty Issue Gravity’s kind of tricky. Depending on where you are, it can seem weak or strong. For example, if you visit the moon, your mass there will be the same as on Earth. But your weight will be only 1/6
what it is on Earth. T at’s because the moon’s gravity is weaker than Earth’s. So gravity pulls you down less. Why? T e moon has less mass than Earth.
Gravity’s Long Reach Gravity is the reason there’s a moon for us to look at. T ink about it. What keeps the moon going around Earth? T e pull of Earth’s gravity! And the moon’s gravity also pulls on Earth. T e moon’s gravity causes the ocean tides on Earth. T e sun has more mass than either
Earth or the moon. Or any object in the solar system. So the sun’s gravity forces all the planets to go around it. Why doesn’t the sun’s gravity pull
the planets to it? Because they move around the sun quickly. T e closer a planet is to the sun,
the more the sun’s gravity pulls on it. T e planets have to move fast to keep from being pulled in. So Mercury, the planet closest to the sun, moves faster than Venus, the next planet out.
14 NATIONAL GEOGRAPHIC EXPLORER
An Important Force Gravity is the most important force in space. It is also a matter of life and death. Imagine turning off all gravity in the universe. Earth’s gravity no longer pulls you down. You start to fl oat. T at’s fun—at fi rst. But then the air you breathe starts
getting thinner. It’s fl oating away, into space. Without gravity, Earth loses its air. We have nothing to breathe. T e sun’s gravity no longer holds
on to Earth. So Earth fl ies off into the deep freeze of space. T is isn’t going well. We’d better
turn gravity back on again. T at way, we have air to breathe and the sun to keep us warm. Me, I’m a big fan of gravity. I think we need to keep gravity around. Even though I fall down sometimes.
WORDWISE
force: a push or pull that can make something move
gravity: a force that pulls one object toward another object
mass: the amount of material in an object
weight: how heavy something is
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