Science Questions

Could you simplify gravity for me?

Tue, 5th Jul 2016

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Dan asked:

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 Universitydark matter of Cambridge...

Andrew - Right. Well, the thing is on the space station, people talk about zero gravity, but of course, itís not. Gravity is there just as it is everywhere else around the Solar System. Itís better to think of being on the space station as being in free-fall. If you were to letís 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 thatís what we experience as weight. Now, if you're in the space station going around the Earth in orbit, youíve still got the force of gravity pulling you down. In fact, thatís what's holding the space station in orbit. The space station if you like is constantly falling, but itís moving sideways. So, itís falling around the Earth and thatís 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. Itís feeling gravity pulling it down but itís always falling towards the Earth and missing.

Andrew - And just the same with Tim Peak.


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The forces we can exert on objects are not uniformly distributed throughout those objects. In the case of gravitation the force is evenly distributed throughout the objects under its influence. This means that gravity is more efficient since it operates at the microscopic quantum level. I hope this helps. jeffreyH, Mon, 14th Mar 2016

The ISS (and everyone on it) is accelerated by Earth's gravity, at pretty much the same acceleration as someone standing on Earth's surface.

The Earth's surface has a radius of around 6,370 km, and gravitational acceleration is around 9.8m/s2.
The ISS orbit has a radius of around 6,370+400 = 6,770km, or 6% more.

The acceleration due to gravity at the ISS orbit is 88% of the value at the Earth's surface, or 8.7m/s2 (by the "inverse square law"). This is enough to keep the ISS in orbit, to keep bending its path in a circle so it doesn't go flying off into space. And it bends the path of the astronauts by the same amount, so they don't get noticeably closer or farther away from the walls of the ISS. ie it looks like microgravity, even though gravity is still quite strong.

So gravity would not be noticeably lower if you were standing on a 400km high extension of the Eiffel tower.

The Earth's gravity is still quite strong at the distance of the Moon (around 300,000km) - enough to keep the Moon in orbit.

The Sun's gravitational field also obeys the inverse square law. So Mercury orbits more quickly than Earth, which orbits more quickly than Jupiter. The Sun even plays a significant part in the motion of cometary objects in the Oort cloud.

In theory, the Earth's gravitational field extends to the stars, but at large distances, the Earth's gravitational field can be ignored in the presence of the much larger mass of Jupiter and the Sun. evan_au, Mon, 14th Mar 2016

Microgravity is a misleading term; the astronauts inside the ISS are floating around not because gravity there is appreciably weaker but because they are weightless, which is not the same as "zero/low gravity". They are weightless because an orbiting spacecraft is in a state of perpetual freefall - it is constantly falling towards Earth but missing.  As they are inside the spacecraft, they too are in a similar state of freefall; hence, relative to their surroundings they are weightless and float about. The effect would be the same if you were inside a lift at the top of a very tall building and somehow the cable was cut. The lift would fall and so would you, leaving you weightless inside the lift... chris, Mon, 14th Mar 2016

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