You know how in realistic science-fiction (realistic as in obeying the accepted laws of physics), books and films such as 2001: A Space Odyssey, space stations simulate gravity by rotating? Basically they
are shaped like a bicycle wheel, and they rotate, so that the outerpart
(the tyre, if you follow the wheel analogy) has a similar pull to gravity, so that people in the space station experience a pull towards the outer-most part of the wheel, and in effect they are pulled towards the “floor” by “gravity”?
Well, how does this work? I know that if you put, say, a brick into a carrier bag and swing it around in a circle, then the brick is pulled “outwards” of the circle, and so won’t fall out of the bag, when the bag is directly above you and it’s opening is facing down. But this is due to the brick first pressing to the side of the bag, not the bottom, when you begin to swing the bag around, and the brick is never trying to fall out of the bag by the bottom of the bag, it is pushed forwards by one side of the bag, and is trying (according to it’s aquired momentum, when you’re swinging the bag around) to go forward, through the other side of the bag, in a straight line. This it cannot do due to the continual swinging of the bag, right? This is centrafugal force, as I understand it.
But when the space station is swinging around, people are not forced by any pressure to move “forward”, and since that forward movement is not present, then it cannot be converted to downward movement by centrafugal force, so how does artifical gravity on a rotating space station occur?
Because an object in motion tends to stay in motion, the energy needed to deflect that path into a circle is felt as an acceleration identical to gravity.
There is not really any such thing as centrifugal force, what there is is centripetal acceleration. The pull towards the center feels like a push towards the outside.
The floor and the air inside the space station are spinning with the station. So the friction with the air and the floor will insure that everything in the space station is spinning around at the same rate. Unless there are other forces that overcome it (e.g. walking).
Just as friction keeps everything on Earth spinning at the same rate as the Earth, unless there are other forces on it.
Yes, but if say you are in a space station (which for the purposes of this discussion is one room, basically a huge metal drum of say 100 feet circumference, full of oxygen and one human), in space, and the space station is not moving at all, so it stays in the same place and doesn’t rotate, then there’s no gravity, right?
So the human is inside the space station, and weightless. Now, according to films and science fiction novels, if the space station starts to rotate along it’s central drum axis, then gravity will appear (or at least some force(s) that will seem like gravity to the human will come into action), and the human will be pulled down to the ‘floor’ (actually whichever part of the curved drum’s wall that he’s closest to.
But why? I doubt that the atmosphere in the drum alone would be powerful enough to do it, and surely the closer you get to the centre of the drum, the slowe the atmosphere would be rotating anyway.
Edit: Scr4, are you saying that the atmosphere inside the drum (the gases, oxygen + whatever) is responsible for the force that would (since they assume the same spinning motion as the drum, which works out as a forward motion in effect) is responsible for the ‘gravity’ effect? If so, then would that mean that if there were no atmosphere (just a vacuum) in the drum, then there would be no artificial gravity, no matter how fast the drum rotated?
Atmosphere can transmit tremendous force. Ever seen a hurricane?
And it is true that the gravity would be lower at lower “latitudes” (and zero along the axis of rotation).
True; in the absence of atmosphere you would only feel gravity if you were standing on the “floor”.
If the films and novels you’re reading say that, then they’re wrong. But I don’t think I’ve ever seen a film where someone was on a large space station that wasn’t spinning, then it started spinning while they were in the air. And I’ve never read a SF novel that had a space station start to spin up while people were around in it that wasn’t trying to get the physics right. Someone in the air while the space station started moving probably would not pick up the velocity of the station, and would need to worry about getting smacked by a wall or support beam if there is one, or making his way to a ‘floor’ or ‘ceiling’ and not getting hurt by the speed difference.
IIRC Ender’s Game and a couple of the Man-Kzin War stories have scenes with people fighting in a rotating environment and using the non-simple characteristics of the pseudo-gravity tactically to win the fight. Babylon 5 has a train running down the center of the station that requires passengers to wear harnesses because it’s got practically no pseudo-gravity, and has a scene in season 3 where someone has to jump out of a train and floats in the air for a time instead of just falling to the outside.
You have to bear in mind that most TV SF uses rotating stations for gravity as a quick plot device to avoid having to do zero-gravity effects for every scene without having to give magical tech like gravity control. They’re not likely to delve into the specifics of how it really works, since they’re just using it to make filming scenes on earth much easier.
No, that’s not what happens. When the space station is initially spun up, the occupants need to be on the “floor” and holding on to something fixed to the floor, so they are pulled forward and move with the station. Once the station spins up, there’s no longer any need to hold on to things - momentum keeps them moving, and friction on their feet keeps them moving at the same speed.
(Which films and novels are you thinking of, by the way? Space stations are usually constantly rotating, and I can’t think of any scenes where a stationary station starts spinning and gravity suddenly appears. )
I only mentioned the atmosphere because that’s how small things floating in the air (if any) will eventually fall to the floor. But you’re right, friction with the floor is far greater than friction with air.
If you have a spinning space station that is vacuum inside, then you can “float” an inch above the floor indefinitely. (Assuming there are no walls that come around and hit you.) You’d only get the “artificial gravity” if you are in physical contact with the floor (or something moving with the floor). But if the station is filled with air, eventually the air (wind) will push you forward until you hit the floor.
For the 100 foot circumference station, the speed of the “floor” would be about 20 miles an hour. A wind of that magnitude against a person “floating” inside the space station would be noticeable - and it would move the person closer to the floor, where the wind would be even stronger.
A good fictional account of this concept occurs in Clarke’s Rendezvous with Rama, when someone flies a lightweight human powered plane down the center of a huge space station - he’s fine until he drifts out of the center.
Forget about the gasses. They are not important for the artificial gravity - just for breathing.
Maybe think of it this way.
A moving object wants to keep going in a straight line. Same as a person attached to surface of the rotating drum. The person ‘wants’ to go in a straight line (a tangent) but the drum always changes the direction of the person inward, as if it is pushing up on the person. This feels the same as gravity - assuming a large enough drum.
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Remove the atmosphere. No need for it. If you’re floating inside a drum, and the drum starts rotating, you will not feel any “gravity”. You will still float inside the drum.
If you contact the drum, though, and grab onto it, then you will feel “gravity” pulling you onto the inner surface of the drum. Look at it from the outside. Your body wants to go in straight line. The drum forces it to go in a curve, thus creating a force which you will perceive as gravity.
If you are in that airless drum, and you jump backwards, against the movement of the drum, with exactly sufficient speed (think “escape velocity”), you will float above the surface of the drum as it rotates under you, just like the initial condition at the beginning of my post.
The bag ripped open and the brick flew in a straight line through my neighbor’s window … opps … “for every action there’s an equal and opposite reaction” … the neighbor threw the same brick through my window …
Just to be clear - once the space station is rotating at a constant speed, someone standing on the floor does not need any forward force to keep moving with the floor. There’s just the force perpendicular to the force (centripital force) making sure the person moves in a circle rather than a straight line. From that person’s standpoint, centrifugal force pushes him/her down against the floor, and that’s all. There are no lateral forces.
There’s an amusement park ride that’s a pretty exact analogy. You get in a metal can, it starts spinning, then the floor drops away. You are pressed against the wall because of the rotation. That’s what you want to happen on the space station.
Now, imagine someone climbing down a rope into the center of the ride. Not touching anything so they aren’t spinning, and don’t feel any force. If they swing over and touch the spinning surface, they get pushed by the wall until they’re at the same speed as the wall. Violently.
If you are on the spinning surface and manage to jump off, you are still moving parallel to the wall at some speed, so when you come back in contact with it you won’t get hurt very much.
In the space station, the walls and floors are moving, so if you are touching them they get you moving. If you at the axis and slowly drift out, you won’t feel any force until you touch the ground. Then you get a lot of force applied (neglecting air). etc
That ride also gives you a vigorous demonstration of another “fictitious” force, the Coriolis Force. When you push your hands forward (toward the center, or axis, of the drum) your hands jerk off to one side; when you pull them back, they wrench away to the other side.
(I have a friend who is so slight and slender, she wasn’t sufficiently pressed to the wall, and slid downward, toward where the floor wasn’t any longer!)
When you get in this contraption, it is horizontal, and not spinning. You are strapped against the cylinder for safety. After it is spun up, the center pivot, which is mounted on a long arm, will rise, raising the entire cylinder and tilting it to near-vertical. Newton’s First Law presses you against the cylinder so you don’t fall out (usually).
This is artificial gravity. Atmosphere has nothing to do with it.
I believe this is the ride that ** Chopsticks** means. It starts with the riders standing up, pressed against the vertical wall. The rotating drum never changes orientation. The principle is similar, though.
