One day Isaac Newton sat under a tree and watched an apple fall (or probably he didn’t, but let’s not dwell on that just now). He thought to himself “I wonder if an apple at the bottom of the tree feels a stronger pull towards the earth than an apple at the top of the tree?” – so perhaps the force something feels due to Earth’s gravity changes depending on how far away it is from the Earth. “Now I come to think of it,” he continued, “I’m hungry for apple pie,” and he went on his way, for all we know.
If you draw a line from the lower apple, to the higher apple, then continue it out into the universe, the strength of the pull from Earth’s gravity gets weaker and weaker in a very well-defined way that Newton is credited with describing in a lovely little equation.
So let’s draw this line up past the higher apple, up past our atmosphere, forward a few hundred years in time, and let it hit the International Space Station. The apple at the bottom of the tree, when it drops, gets pulled down by gravity and ends up plummeting towards the surface of the Earth with an acceleration of 9.81 metres per second every second. If we go to the top of our line, onto the ISS, and drop an apple, how much weaker would the force of gravity be?
Actually, not a whole lot weaker. The apple on the ISS would fall with an acceleration of about 9 metres per second every second. So the force the ISS feels from gravity 400km above our heads isn’t hugely different then the gravity it would feel if it was in Newton’s back garden.
But you may have noticed that the people in the ISS float about in a fun way, almost as if there’s no gravity up there.
But, as we just saw, there’s loads of gravity up there. So how does that work?
Well, it’s because the people (and everything else) on the ISS aren’t free from gravity – they’re in free fall. To help get our heads around this, one thing to bear in mind is that gravity pulls everything towards the centre. It’s mostly why the Earth is the shape it is – if you pull equally in every direction you end up with a sphere (although the Earth isn’t a perfect sphere).
So, keep that in mind while we think about a cannon on top of a hill. It shoots a cannonball, which goes a certain distance and hits the ground.
Now let’s give the cannonball a bit more oopmh – it goes a bit further.
Now let’s give it a lot more oomph – and it misses the surface of the Earth completely. And with the right amount of oomph, it will come right back around to where it started and just keep going around and around and around.
We call this an orbit, and from the point of view of the people inside they’ll float because they’re constantly falling towards the Earth and missing. However, sometimes the people in the ISS wish one of their member would manage to actually hit the ground…