Why is Earth’s gravity strong enough to keep the Moon in orbit but not strong enough to prevent astronauts in space from floating?
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It’s not strong enough to stop you lifting up a pencil either! Gravity is actually a surprisingly weak force, the weakest of all the fundamental forces!
Lifting millions of pencils would still be hard, and a rocket is similarly heavy, so it still takes a lot of fuel and aerodynamic ingenuity to achieve orbit.
As to why something the size of the moon is locked into orbit, well that’s actually gravity’s one advantage: it’s incredibly long range! The other forces just don’t have the reach to move planets and moons around (unless you include supernovae and such), but gravity can reach far enough out that it’s basically the only player, and so it’s the deciding factor for orbits.
Because just like the moon and whatever station or ship they’re in, the astronauts are also in orbit. And they have to go REALLY fast to stay there. 25 times the speed of sound fast.
That’s why putting things in orbit is so difficult and expensive, BTW. It’s not the part where you go straight up, that’s only a few hundred kilometres. It’s the mach 25 speed to have to make it go sideways.
If you send anything straight up and not at all sideways (like in a balloon or an elevator), it will fall down exactly like you’d expect.
The ISS is moving sideways at around five miles per second. It, and the astronauts inside it, are in orbit. They only look like they’re floating because when we film astronauts, we film them from inside the ISS, so the camera is moving along with the astronauts. If we looked from the ground, we would see they are orbiting the Earth just like the moon is.
The astronauts in space ARE in orbit, they’re only floating stationary relative to their station/ship because it’s in the same orbit they are.
If you had two skydivers at the same speed and altitude and took away the air, they would look like they were floating relative to each other, because they’re both feeling the same acceleration from gravity.
Gravity in low earth orbit is almost as strong as it is at sea level, you’re just moving fast enough sideways that by the time you fall back down you miss the planet.
Because these astronauts, just like the moon, aren’t actually floating but falling sideways around earth the exact speed it takes for them to stay at the same distance i.e *float*