What is gravity?

The pull you cannot see

Why do you land back on the ground when you jump? Why does a dropped ball fall down, and not sideways or up? The answer is gravity. Gravity is an invisible force that pulls objects towards each other. Earth's gravity pulls everything towards the centre of the Earth. It keeps you on the ground, and it makes things fall.

Here is a question that people sometimes ask: if gravity pulls things "down", why don't Australians fall off the bottom of the world? The answer is that "down" just means towards the centre of the Earth. Everyone on the planet, in Perth, Paris or Peru, is pulled towards the same centre, so everyone feels "down" under their own feet.

Gravity does much more than keep your feet on the floor. It holds the air around Earth, so that we can breathe. It keeps the Moon going around Earth, and Earth going around the Sun. It even helps to make stars and planets, by pulling clouds of gas and dust together.

More mass, more pull

Everything that has mass has gravity. Mass is how much stuff something is made of. The more mass an object has, the stronger its pull. Earth has a huge mass, so its pull is strong enough for us to feel. You have gravity too, and you pull on Earth just as hard as it pulls on you. But Earth is so much more massive than you that your pull makes no difference to it.

Distance matters as well. The further apart two objects are, the weaker the pull between them. Double the distance, and the pull falls to one quarter. Triple it, and the pull falls to one ninth. Isaac Newton worked this out in the 1600s. The story goes that in the late summer of 1666, he was in his garden at Woolsthorpe Manor in England. He saw an apple fall from a tree, and it made him wonder why things fall.

So gravity depends on two things: how much mass there is, and how far apart things are. That one idea explains why the planets go around the Sun, and why the Moon goes around Earth.

Falling together

Which falls faster, a heavy hammer or a light feather? Here on Earth, the feather floats down slowly. But that is because of the air. Air pushes up against a feather much more than against a hammer. The scientist Galileo worked out, hundreds of years ago, that without air all objects fall at the same rate, however heavy they are.

In 1971, astronaut David Scott tested this on the Moon, where there is almost no air. At the end of the last Apollo 15 moonwalk, he held out a hammer and a falcon feather. He let go of both at the same moment, in front of the TV cameras. They hit the ground together.

On Earth, if air did not get in the way, a falling object would speed up by about 9.8 metres per second, every second. After one second of falling it would be moving at about 9.8 metres per second, and after two seconds at about 19.6 metres per second. That number tells us how strong Earth's gravity is at the surface.

Your weight on other worlds

Gravity is not the same everywhere. A planet or moon with less mass has weaker gravity at its surface. Your weight is the pull of gravity on you, so you weigh less where gravity is weaker. Your mass, the amount of stuff you are made of, stays the same wherever you go.

On the Moon, gravity is about one-sixth as strong as on Earth. Someone who weighs 30 kilograms on Earth would weigh about 5 kilograms on the Moon. That is why astronauts seem to bounce across the surface in videos of moonwalks. On Mars, gravity is about 38 per cent of Earth's, so that same person would weigh about 11 kilograms. At the cloud tops of Jupiter, gravity is about two and a half times as strong as on Earth, and they would weigh about 75 kilograms.

What about astronauts on the International Space Station? Many people think there is no gravity in space. In fact, at the height of the station, about 400 kilometres up, Earth's gravity is still nearly nine-tenths as strong as it is on the ground. The astronauts float because they and the station are falling all the time. They are also moving sideways so fast that they keep missing the ground. So they are falling around Earth, not towards it. Everything inside falls at the same rate, so it all floats.

Gravity across Australia

Gravity is a tiny bit different from place to place on Earth. It is slightly stronger in a valley than on a hilltop. That is because a valley is closer to the centre of the Earth. It is slightly weaker near the equator, where Earth bulges out. And it is slightly stronger over places with more mass underground. You cannot feel the differences, but Geoscience Australia can measure them with very sensitive instruments. Cape York, in far north Queensland, is close to the equator. So you would weigh a tiny bit less there than you would in Tasmania.

Why measure it? Gravity is a little stronger over heavy rock and a little weaker over lighter rock. With very sensitive instruments, scientists can use gravity to map what is hidden under the ground. This helps them look for minerals, energy and groundwater. Geoscience Australia has looked after Australia's gravity records since the 1940s. Its maps use readings taken on the ground, and now from aircraft too.

Gravity is also helping us explore the universe. In 2015, scientists detected tiny ripples in space called gravitational waves. They were made when two huge black holes crashed together about 1.3 billion years ago. Australian scientists from a team called OzGrav helped make that first detection. In 2020, four of the team's scientists won the Prime Minister's Prize for Science.

📚 Words to know

Gravity

The invisible force that pulls objects towards each other. Anything with mass has gravity.

Mass

How much stuff an object is made of. Your mass is the same on Earth, on the Moon and in space.

Weight

The pull of gravity on an object. You weigh less on the Moon than on Earth because the Moon's gravity is weaker.

Orbit

The curved path an object takes around a planet or star, held there by gravity. An object in orbit is always falling, but it keeps missing the ground.

Microgravity

The feeling of floating when you and everything around you are falling together, as on the space station.

🧪 Try this: race a book and a sheet of paper

1. You need a paperback book, a sheet of paper and a clear space with a soft floor, such as carpet or grass. Make sure that nobody's feet are under the drop zone.
2. Hold the book in one hand and the flat sheet of paper in the other, at the same height. Let go of both at the same moment. Which lands first? Why do you think that happens?
3. Now lay the sheet of paper on top of the book, so that it sits on the cover, and drop them together. What does the paper do? The book pushes the air out of the way, so the paper falls along with it.
4. Scrunch the paper into a tight ball and drop it beside the book. Now they land at almost the same time. The air was slowing the flat sheet, not a difference in gravity. On the Moon there is hardly any air, which is why the hammer and the feather landed together.
5. Challenge: make a parachute from a handkerchief and four pieces of string tied to a small toy. Drop the toy with and without the parachute, from the same height. Which one lands later? What is slowing it down?