Gravity is the force that pulls objects with mass toward each other. It is why an apple falls, why the Moon circles Earth and why planets orbit the Sun. Every object with mass attracts every other object, but the pull is only noticeable when at least one object is very large, such as a planet or star.
Newton’s description
In the 1680s, Isaac Newton showed that the same law explains falling objects on Earth and the motion of the Moon. The force grows with the masses of the two objects and falls off with the square of the distance between them. Double the distance, and the pull becomes a quarter as strong.
Einstein’s description
In 1915, Albert Einstein’s general theory of relativity described gravity as the curvature of space and time caused by mass and energy. Objects follow the curved paths in this geometry. Newton’s law still gives accurate results for most everyday situations, and Einstein’s theory is needed for very strong gravity, such as near black holes, and for GPS corrections.
Why gravity matters in the universe
Gravity pulls gas together to form stars and planets, holds galaxies in shape and keeps planets in orbit. Without it, matter would spread out and never form the structures we see.
Why gravity feels weak
Gravity is by far the weakest of the four fundamental forces. A small magnet can lift a paperclip against the pull of the whole Earth. Gravity seems strong only because the planet has an enormous amount of mass, and its effects add up.
Weight and mass
Mass is the amount of matter in an object and stays the same everywhere. Weight is the force gravity exerts on that mass, so it changes with location. On the Moon, a person weighs about one sixth as much as on Earth, but their mass is unchanged.
Frequently asked questions
Does gravity need air to work?
No. Gravity acts through empty space, which is why the Sun’s pull reaches Earth across 150 million kilometres of vacuum.
Why don’t we feel the gravity of other people?
Their mass is tiny compared with Earth’s, so the pull between you and another person is far too small to notice.
