Could you simplify gravity for me?

05 July 2016


Could you simplify gravity for me? My problem is if the ISS has microgravity only 400km above Earth. How does the Sun have an effect on us and even more unbelivelby have a gravitional effect on the outer planets (keeping the planets in orbit) when it losses its affect so quickly?


We put this question to professor Andrew Norton, astronomer from the University of Cambridge...

Andrew - Right. Well, the thing is on the space station, people talk about zero gravity, but of course, it's not. Gravity is there just as it is everywhere else around the Solar System. It's better to think of being on the space station as being in free-fall. If you were to let's say, be in a lift and someone carelessly cut the lift cable so that the lift plummeted down to the Earth. Whilst you were falling, you would be in free-fall and you would feel weightless because when we say, you have weight, what we mean by weight is the force pushing up from the Earth into your feet. So if you're standing on the Earth, the weight that you feel is gravity is pulling your body down but the Earth is, if you like, pushing you up. There's a reaction force, a contact force pushing up through your feet and that's what we experience as weight. Now, if you're in the space station going around the Earth in orbit, you've still got the force of gravity pulling you down. In fact, that's what's holding the space station in orbit. The space station if you like is constantly falling, but it's moving sideways. So, it's falling around the Earth and that's what the orbit is. So, the space station is going round and round the Earth, constantly falling, the astronaut is continuously in free-fall and so, they're not experiencing weight. But they are experiencing gravity.

Chris - Isaac Newton had a beautiful way of getting his head around this which he wrote up in his principia. His point was, if I had a gun and I fired it, and I fired it quite hard, the bullet would come out and under the influence of gravity, eventually drop to Earth. If I fired it harder, the bullet will go further before it dropped to Earth. If I fire it really hard then the bullet will actually keep dropping towards the Earth. But because the planet is curved, the curve of the planet falls away beneath the bullet so the bullet never touches the ground and you are in what we call an orbit. And so the bullet is not weightless. It's feeling gravity pulling it down but it's always falling towards the Earth and missing.

Andrew - And just the same with Tim Peak.

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