I read a lot of sci-fi, and simulated gravity is a frequent feature of space sci-fi. I am interested in real physics, not hand-wavy magic artificial gravity. In the Expanse, highly efficient nuclear rockets create the experience of gravity by way of constant acceleration. This is easy for me to understand – on the vessel, the deck you are standing on is accelerating and pushing against your feet.
What I do not under is how centrifugal force acts in a way similar to gravity. I have a scenario in my imagination that illustrates my confusion. I imagine that a torus-shaped station in space has stopped rotating and everyone evacuated. A repair person in a space suit has floated into the station to repair it. He or she travels in microgravity in a space between the inner walls and outer walls of the station, almost like someone floating inside of a holo donut. While inside the immobile torus, and suspended between the walls in such a way that he or she isn’t touching either the innermost or outermost walls of the donut, s/he completes the repair needed to spin up the station again.
What happens next? The sci-fi I have read would lead me to believe that as the torus begins to spin up, the repair person will experience an effect similar to gravity and will coast toward the outermost wall until the inside of that wall becomes like a floor to them. If that is true, why? I would think that a person suspended within the boundaries of a torus would continue to float while the innermost and outermost walls in front of and behind them spun in their cycle. I don’t get why this spinning will make the repair person drift to one side and create a “down” to their perspective. Can anyone enlighten me?
Tl;dr: centrifugal force confuses me, and I don’t know why it works in microgravity(or just fundamentally misunderstand it)
In: Physics
gravity is an acceleration
an acceleration is a change of velocity
velocity is a speed in a direction
so acceleration can be a change in direction you are moving, not just a change in speed.
in a spinning ring, if you are in contact with the ring, your direction of motion is constantly changing, so you are experiencing an acceleration, just like gravity
If your not in contact with the ring (the repair person), this has no effect on you.
If you were in contact with the ring and jumped, you would move in a straight line through space, but since you are inside a ring, you will eventually hit the side of the ring again, so it will “feel” kinda like you fell.
Its not a perfect gravitational stand-in, and some of the physics gets super weird (like how bullets would do different things depending on the direction they were shot), but it should work well enough for people sitting on the object
If you aren’t touching the walls in a vaccuum and are stationary, the revolving reference frame doesn’t affect you. You’ll just sit there with the wall spinning near you. But as you start to come up to speed (either because the air is also spinning and taking you with it, or because you’ve touched the wall and are being dragged with it), then your body will try to continue moving in a straight line and be frustrated by the wall of the torus, which will result in a pseudo-force that appears (to you) similar to gravity. Once you’re up to speed, you can walk and jump around normally.
Your thinking is correct. They would initially just float there. The air would be touching the walls though and would probably reach some kind of equilibrium with the walls (like when you stir a pot of water or some liquid with bubbles and particulates on top and after you remove the spoon it looks like the surface is rotating as a disc). That moving air is going to then blow around with it.
Imagine you’re riding the teapot rides in Disney world, and you’re spinning really fast. Essentially, your body will want to be flung off the teacup but it can’t, because the seat is applying a force keeping you in the teacup.
Now imagine instead of being seated against the chair of the tea cup, you were standing up (your feet are where your back would be). The teacup is now simulating a force holding you in the teacup; the same way we feel the earth pushing against us to avoid gravity.
There also are the amusement park rides where they spin really fast and then drop the ground – same basic principle. If you stood up perpendicular to the wall, it would be the exact same thing as spinning in space (except spinning in space attempts to simulate gravity, whereas those rides overcome the force applied by gravity, so centripetal force in space does not require as much effort)
Worth adding: all of this requires you to be touching the walls. Once you’re touching the walls, you’d feel a sudden force to get you up to speed with the centripetal force (imagine jumping onto a moving train), but once you’re up to speed you’d only feel the force pushing you away from the center of the circle. If you jumped into the air, you’d be floating, because you’re no longer feeling that force pushing you towards the center of the circle.
If the area the person is floating in is a vacuum, you’re right, they would “stay still” while the station rotated around them. If there was still air in the station, however, when it started to spin, friction or internal walls would cause the air within to start moving with it. The air would blow on the repair person, which would eventually make them start to move slightly along with the air. But since objects like to move in straight lines, and the inside of the station is round, the repair person will eventually bump into the outer wall. (the direction the “wind” pushes them in will change when they’re in different parts of the ring, but it will never directly push them towards or away from the wall).
If the wall drags them along, they will continue to speed up along with it. But, since they constantly “want” to move in a straight line and the wall is curved, they’ll find themselves being pressed against the wall. If the station is spinning fast enough, they’ll be able to stand “up” on the wall, using their legs to support themselves against it. Their legs would be pushing the rest of their body as well, which would feel (about) the same as standing on a stationary surface in a gravitational field.
There isn’t actually any field pushing objects inside the torus. Things are just spinning along with the ring and colliding with the outer wall because the wall isn’t straight. But if the ring is large enough, and everything inside the ring is spinning with it, the differences between that and “real” gravity for someone on the outer wall are very hard to detect.
If there is air inside the corridors, then there is resistance between that and the walls and any objects in there. So the air is going to get spun up. If the astronaut is stationary, then this is going to be felt as wind. This would be enough to accelerate them sideways, taking them out into the wall/floor.
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