Gravity — Astronomy, 4–6
Gravity is the pull that keeps our feet on Earth and brings thrown objects back down. It also acts between planets, moons and stars.
What gravity does
Gravity is an invisible pull between things that have mass, such as Earth, you and a ball. Earth is very large, so its pull is easy to notice: it keeps you on the ground. Gravity does not need a rope or a hand that we can see, but its effects are all around us.
Why gravity matters
Without gravity, things would not stay together on Earth, and people, air and oceans could drift away. Gravity also helps keep the Moon near Earth and Earth near the Sun. Long ago, people noticed that dropped objects always came down; studying that puzzle helped scientists understand how the universe moves.
Dropping a ball
Hold a 200-gram ball one metre above the ground. First, your hand supports it, so it stays still. Then open your hand: Earth’s gravity pulls the ball downward, and it falls until the ground pushes back. If you throw it upwards, gravity slows it, stops it for an instant, and pulls it down again.
A reasonable mix-up
A falling feather seems to show that heavy things fall faster. That is a reasonable idea because air pushes against a feather much more than against a ball. In a place with almost no air, a feather and a hammer fall together. Near Earth, air resistance changes what we see, but gravity pulls both down.
Gravity around us
Gravity helps us walk, pour water and build towers that stand on the ground. Engineers include it when designing bridges, playgrounds and spacecraft. Astronauts in orbit seem to float because they and their spacecraft are falling around Earth together, not because gravity has disappeared. Gravity is still pulling them.
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