From experimental data it can be shown that:
1. There is a gravitational pull between all bodies with mass. They attract each other.
2. Bodies with large masses have large pull. So big planets have high gravity.
3. The further they are from each other the smaller the gravitational pull between them.
4. Results show that at twice the distance the gravitational pull between the bodies is ¼. At 3 times the distance, the pull is one ninth. This is called an 'INVERSE SQUARE' law.
Gravity attraction image courtesy of science-resources.co.uk
The Moon is smaller than the Earth, so its gravitational pull is less than the Earth's. A person on the Moon can lift 5 times as much more weight as on Earth. The same weight will crush him on Jupiter.
Gravity canon image courtesy of science-resources.co.uk
If an object is fired at 8Km/s, 25 times the speed of sound, then it would still fall towards Earth, but it would stay the same height above the earth's surface, because of the curving of the Earth. This then becomes a satellite, in orbit around the Earth.
Satellite orbits image courtesy of science-resources.co.uk
In order to stay in orbit at a certain altitude (height) , the satellite must maintain the right speed to balance the pull of gravity. Lower the altitude of the satellite faster it would have to travel to stay up. Those satellites which are high in the orbit travel at slower speeds, but take longer to complete 1 orbit of the Earth.
Although satellites travel very fast, some of them seem to be standing in one place, when viewed from the ground! This happens when a satellite is put into an orbit at just the right altitude (36,000 Km above equator) and speed so that it takes 24 hours to circle round the Earth once. This is the same time as the Earth, hence the satellite appears to float over one point. These type of satellites are known to have a geo-stationary or synchronous orbit.