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orbiting the Earth


With all this gravity pulling things down towards the centre of the Earth, how is it ever possible to put something into orbit around the Earth? Well, it's a clever trick that was first worked out by Newton, even though we had to wait nearly 300 years to put his ideas into action.


Image of Sir Issac Newton with a thought bubble saying 'What would happen if someone took a cannon to the top of a high mountain and fired a cannon ball horizontally?Newton thought a lot about this and, because he had no technology to experiment with, he had to settle for a 'thought experiment'.


Newton probably chose a cannon because that was the most powerful thing around. But we can think about the idea with anything that could be thrown, kicked or fired fast enough. Let's try hitting a tennis ball.


Imagine you could get to the top of a mountain about 30 times higher than Everest (that's about 300 km high) and you serve a tennis ball. If it's a weak service the ball will just fall down to Earth. Hit it a bit harder and it will go further, but still land on Earth. If the service is really fierce, the ball will travel fast enough to go into orbit around the Earth. That's the principle of how satellites are put into orbit.


Notice that the speed required to send the ball into orbit at this height is about 7.7 km/s (over 17,000 mph) - that's some tennis serve!

Remember: 2 things are needed to make a satellite orbit the Earth. 1. Gravity - keeps pulling the satellite back towards the Earth and stops it shooting off into space. 2. A high sideways speed - Stopa the satellite hitting Earth when gravity pulls it back.


The moral of this tale is that if you hit the ball hard enough, at the right sideways speed, the ball will go into orbit and stay there!  


Notice also that once a satellite is in orbit it doesn't need any power to keep it up there - gravity does all the work.


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