Rights: University of Waikato Published 27 March 2013 Download

Dr Allan McInnes and Dr Adrian McDonald, from the University of Canterbury, explain how gravity and projectile motion keep natural and artificial satellites in an orbital path.


Gravity is important to keep satellites in orbit. Well, I guess not just important, it’s crucial – it’s what keeps the satellite in orbit. If we didn’t have gravity, the satellite wouldn’t orbit the Earth, it would just go flying off into space.

What is gravity? The simple answer is the old one that Isaac Newton came up with – gravity is a force that’s an attraction between anything that’s got mass. So I have mass, you have mass, the Earth has mass, all that mass attracts other mass.

So gravity’s a fundamental force – you wouldn’t have orbiting satellites or the Moon orbiting the Earth or the Earth orbiting the Sun without gravity.

When we hear the story about Newton and the apple falling off the tree onto his head, Newton was clever enough to work out the Earth attracted the apple towards the Earth. But the apple also had a force pulling the Earth towards the apple. Now because the mass of the Earth is so much bigger than the apple, you can’t actually see the effect or measure the effect of the Earth moving towards the apple but there is opposite and equal forces pulling the apple and the Earth towards each other.

Another way you can think about it is if you tied a tennis ball to the end of a string and swing the tennis ball around over your head. The string is like gravity, and if you let go of the string, well the tennis ball isn’t held by a force spinning around you anymore, it just flies off in a straight line. In the same way, if we had a satellite going around the Earth and for whatever reason the Earth’s gravity switched off, the satellite would just shoot off into space.

Aaron Fung
Arpan Jolly