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A Weighty Issue Gravity’s also 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? Because the moon has less mass than Earth. What about Mars? It’s less massive than


Earth but more massive than the moon. You weigh less on Mars than on Earth but more than on the moon.


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 at’s right! It’s the pull of Earth’s gravity. And the moon’s gravity also pulls on Earth. For example, the moon’s gravity causes ocean tides on Earth. T e sun is much more massive than


either Earth or the moon. It’s the most massive 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 suck all the


planets into it? Because they zip around it. If they stopped moving, they would fall into the sun. T e closer a planet is to the sun, the more


the sun’s gravity pulls on it. So the closer the planet, the faster it must move. Mercury moves faster than Venus, Venus moves faster than Earth, and so on. Gravity goes way beyond our solar


system. Gravity makes the sun go around the center of our galaxy, the Milky Way, every 230,000,000 years. 14 NATIONAL GEOGRAPHIC EXPLORER


An Important Force Gravity is the most important force in space. Why? First, it operates over a distance. Second, gravity always attracts. It never repels. If you think about it, gravity is also a


matter of life and death. Imagine turning off all gravity in the universe. Earth’s gravity no longer pulls you down, so you start to fl oat. T at’s fun—at fi rst. But then you notice that the air you


breathe is getting thinner. It’s fl oating away into space. Without gravity, Earth loses its air, and we have nothing to breathe. Other bad things happen. T e sun’s


gravity no longer holds on to Earth. So Earth fl ies off into the deep freeze of space. As a result, Earth gets colder and colder. 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 it around.


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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