First look
The idea, in plain words
Drop a ball and it falls. The same thing that pulls the ball down holds the Moon near the Earth and the Earth near the Sun. It is called gravity.
Everything pulls on everything else. You pull on the Earth and the Earth pulls on you. Two things decide how strong the pull is.
The first is how much stuff there is. The Earth is huge, so its pull is strong. The Moon is smaller, so on the Moon you would weigh only a sixth as much, and could jump six times as high.
The second is distance. The further away you go, the weaker the pull gets. But it never stops completely.
Astronauts in a space station float, yet gravity up there is nearly as strong as on the ground. They float because they and the station are falling together, all the way round the Earth.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Gravity
- The pull that every mass has on every other mass.
- Mass
- How much matter a thing holds. It is the same wherever you take it.
- Weight
- The force of gravity on a mass. It changes from world to world while the mass stays the same.
- Free fall
- Moving under gravity alone, with nothing pushing back.
- Inverse square law
- The rule that light, and gravity, spread out and weaken with the square of the distance.
- Barycentre
- The balance point that two bodies both go round.
- Orbit
- The path one body follows around another, held there by gravity.
Where this comes from
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.