There is no air resistance, I get that. And the space craft/ISS is in orbit, I *kind* of get that. But if astronauts still experience acceleration in space, why do they not experience deceleration when they are no longer getting “pulled” by their vehicle?
BONUS QUESTION: at what point does acceleration forces stop? A space ship accelerates in space, all crew are pushed back into their seats, but when does that feeling dissipate if they remain at that new speed?
In: Physics
>But if astronauts still experience acceleration in space, why do they not experience deceleration when they are no longer getting “pulled” by their vehicle?
I think the crucial thing to understand here is that the ISS is not undergoing any acceleration. It doesn’t have rockets propelling it around the earth, it’s simply in orbit – so whether the astronaut is sitting/touching/attached to the ISS doesn’t actually matter as neither the astronaut nor the ISS is changing speed. They are both zipping around the earth every 90 or so minutes, and will continue to do so forever*.
Consider the fact that the earth spins such that – at the equator – a person is zipping around relative to the center of the earth at around 1,000 miles per hour. Not only that, but the whole earth is zipping around the sun at about 20 miles *every second*. Relative to the sun, every time you blink you’ve moved the distance to your local supermarket and back, yet you can happily jump up into the air, blinking away with frankly reckless abandon, and you don’t land 50 miles away. Why? Because…
>at what point does acceleration forces stop? A space ship accelerates in space, all crew are pushed back into their seats, but when does that feeling dissipate if they remain at that new speed?
Another important thing to understand is that acceleration is simply a change in speed. It’s not a force itself, it’s just the consequence of a force acting upon something. So it’s actually no different to a car – if you floor it as you join a fast-moving road full of traffic, you might get thrust back into your seat a little – because the car is moving faster than you are, and you’re essentially being ‘bumped’ forwards by it, like a tractor pushing a broken down car. But once you hit 70mph and stay at 70mph, this feeling goes away because you’re now travelling the same speed as the car – it doesn’t need to push you forwards. Slam on the breaks and the opposite happens; The car slows down but you don’t (which is why seat belts are very important – that make sure you slow down at the same rate as the car, rather than fly through the wind screen and then suddenly stop when you hit a wall or lamp-post or another car). The feeling of accelerating (or decelerating) is just the feeling of being pushed forwards or pulled back by something travelling a different speed to you.
It’s no different in space. In fact, it’s easier to do in space, because the lack of wind resistance means that when something is travelling at X speed, it stays at that speed unless another force acts upon it (though it’s worth remembering that ‘speed’ is relative to *something* – as mentioned above, everyone is travelling very, very fast relative to the sun, even when they’re asleep – yet two people asleep next to each other are considered to have a speed of 0mph relative to one another).
On earth, our thick, dense atmosphere creates so much air resistance (and the floor creates friction, too) so anything travelling at any speed that isn’t just 0mph (relative to the ground) needs constant force acting on it keeping it there – jet engines in a plane, wheels turning in a car, leg muscles whilst walking etc. This is also how docking space craft is, without intending to understate its difficulty, relatively easy. Once you have matched speeds between two crafts (or a human doing a space walk and a craft), you will stay the exact same distance apart. You can then slooowwwlllyyy inch towards your target and, despite the fact you’re flying around the earth at about 4.5 miles per second, your speed relative to each other only changes as much as your thrust.
(* The ISS is in low enough orbit that, actually, there *is* a small amount of air resistance which means the ISS is constantly losing a little altitude. Every now and then the ships that are docked to it will ‘boost’ it back up a little, wherein it’ll start very slowly falling again and they’ll boost it back up again, the cycle repeating. This is an unavoidable part of being at the orbital level they’re at, but it’s also handy for satellites of any kind to have this sort of actively maintained trajectory so that when they reach the end of their life or otherwise we lose control of it, it’ll slowly descend and burn up in earth’s atmosphere rather than stay up there for thousands of years. The GPS satellites, by comparison, are much higher up and their orbits won’t naturally decay for millenia – if that – but then, being so much further away, are less likely to actually cause a problem even if we do lose control of them.)
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