What is gravity?
Why things fall, why you stay on the ground, and why a feather and a hammer fall together on the Moon.
Drop a pencil. It falls. Always. You have never seen a pencil float upward. Why?
The answer is gravity: a pull between things.
Gravity is a pull
Every object pulls on every other object. This pull is called gravity. The more mass something has, the stronger its pull.
You are pulling on this page right now, and the page is pulling on you. But you and a page are tiny, so the pull is far too small to notice.
Earth is enormous, with a huge amount of mass. So Earth's pull is big enough to feel. It pulls everything toward its centre:
This is why people on the other side of the world do not fall off. For them, "down" is also toward the middle of the Earth. Down is just the direction of gravity.
Weight and gravity
Your weight is how hard Earth's gravity pulls on you. If gravity were weaker, you would weigh less. That is exactly what happens on other worlds:
| World | Gravity | A 40 kg person would feel like |
|---|---|---|
| Moon | 1.6 m/s² | about 7 kg |
| Mars | 3.7 m/s² | about 15 kg |
| Earth | 9.8 m/s² | 40 kg |
| Jupiter | 24.8 m/s² | about 101 kg |
These are the same values you see in Launch Lab. On the Moon, a ball flies six times as far, because gravity takes so much longer to bring it back down.
Things fall together
Which falls faster, a heavy ball or a light one? Many people guess the heavy one. But there is a surprise.
If there is no air, both fall at exactly the same rate. After 1 second, both have fallen 4.9 metres. After 2 seconds, both have fallen 19.6 metres.
Why? A heavy ball is pulled harder by gravity. But it is also harder to get moving. The two effects cancel out perfectly.
So why does a feather fall slowly?
On Earth, a feather falls slowly because of air. The air pushes up against it as it falls. This is called air resistance. A feather catches a lot of air for its tiny weight. A hammer does not.
In 1971, an Apollo astronaut called David Scott tested this on the Moon, where there is no air. He dropped a hammer and a feather at the same time. They hit the ground together!
Gravity keeps things in orbit
Gravity does not only pull things down. The Moon goes around the Earth because gravity keeps pulling it. The planets go around the Sun for the same reason.
A satellite is the same. It is always falling toward Earth, but it is also moving sideways so fast that it keeps missing the ground. That is an orbit, and you can try it in Orbit Keeper.
There is a famous story that Isaac Newton saw an apple fall and wondered: "If gravity pulls the apple, does it also pull the Moon?" He worked out that the same force makes apples fall and keeps the Moon in its path. One idea explained both.
Words to know
- Gravity
- The pull between all things that have mass.
- Mass
- How much stuff something is made of.
- Weight
- How hard gravity pulls on something.
- Air resistance
- Air pushing against something that moves through it.
- Orbit
- The path something takes when it keeps falling around a planet.
Check yourself
Which way is "down"?
Show the answer
Toward the centre of the Earth. Down is the direction gravity pulls.
Would a bag of sugar have a different mass on the Moon?
Show the answer
No. Its mass stays the same. Its weight would be less, because the Moon pulls less.
Without air, which falls faster, a heavy ball or a light ball?
Show the answer
They fall at the same rate.
Why does a feather fall slowly on Earth?
Show the answer
Because of air resistance. The air pushes up against it.