Introduction For Primary

If you lift an object, for example your pencil, then let it go, the pencil falls down towards the ground. The force which pulls your pencil towards the ground is gravity. Earth pulls everything towards it using gravity, including pencils, people and even huge objects like the Moon. Earth pulls on the Moon with gravity, which causes the Moon to orbit around Earth, so that we see the Moon each night. Similarly, all the planets in the Solar system (including Earth) orbit around the Sun because the Sun is pulling the planets with gravity.

Just imagine if you could switch gravity off. The smallest push upwards with your legs or with some other force would take you up, up and away, never to come back down to the ground. The Moon would be able to move away from the Earth. It would appear smaller and smaller each night as it drifted further and further away, and would eventually disappear into distant space. The Earth would be able to escape its orbit around the Sun, and move away from it. Each day the Sun would appear smaller and dimmer as we moved further away, becoming colder as we move further from the Sun’s heat. Without gravity, the Earth would be a very different, darker, colder place.

When gravity pulls you down on the Earth, it’s trying to take you down into its centre. Thankfully there are other forces that stop us from going through the surface of the Earth! The pull of gravity you experience depends on the object you are standing on. The Earth’s gravity is much stronger than the Moon’s because it has more mass. The Apollo Astronauts had to find a different way to move on the Moon because the pull is much less (they jumped around like kangaroos). In general, the pull of gravity acts between objects. In the solar system the Sun by far has the largest mass and its pull of gravity dominates. However each of the planets will also pull on the Sun and the strength depends on the planet’s mass. The end result it that the Sun wobbles slightly. It has been recognised that if other stars have planets around them, then gravity will again cause this wobble. The hunt is now on to use gravity to find extra solar planets, or exoplanets, that are orbiting other stars.

With thanks to Dr Gordon Robb, Strathclyde University. Additional material (last paragraph) by Laura Thomas.