Artificial gravity in space

1. Apr 15, 2013

I hope this is the right place for this question.

How would spinning a ship in space create artificial gravity?

I've long wondered if it actually would, or if you'd just have people floating in a spinning ship. I did a minimum of searching and found that there have been actual proposals involving the principle, so I guess in theory it would work. I just can't wrap my head around why, lacking some physical connection to an object, making it spin would create artificial gravity. The image that comes to mind is spinning an object, say a rock, in circles on a string, then placing something inside the orbit of the rock unattached to the string. Would the object "gravitate" toward the rock, and if so why?

2. Apr 15, 2013

Staff: Mentor

A physical connection is required.

For a spinning spaceship, you could hover at the rotation axis indefinitely. You could slowly move outward from there, but not for long: air friction would cause you to start rotating with the ship, which would cause you to accelerate toward the side of the ship (in the rotating frame). If you were not up to the speed of rotation of the ship when you hit it, friction between you and the side of the ship would quickly accelerate you to that speed.

3. Apr 15, 2013

Thank you! Every time I see or read the sci-fi "artificial gravity" through spinning device, I wonder why people and objects in contact with the interior of the spinning "thing" in space don't just bounce along, and why things not in contact with the interior would be drawn to it.

4. Apr 15, 2013

One more thing - I assume that although friction would bring you up to speed, you wouldn't be held against the interior?

5. Apr 15, 2013

bahamagreen

It is a little peculiar. Take a standard example of a rotating hollow disc shaped structure with you standing on the inner wall of the outer edge (so when you look "straight up", you are looking at the center of the structure). Let the diameter and rotation speed of the structure be such that you experience one G.

If you release a ball, the ball will have a momentum from the rotation, but not be subject to any other forces. If you release the ball at waist level, its speed will be that of your waist, but since your legs and feet are going faster, and since the trajectory of the ball as viewed from outside the rotating structure will be a straight line that intersects the floor of the structure in front of you (in the direction you are rotating)... the ball will appear to you to fall "to the floor" but also to fall slightly behind you, with respect to your direction of travel around the center.

If you toss the ball straight up, gently, you will also be swinging around up in front of it to intercept it, so the illusion of it traversing an arc will be there. If you toss it hard enough to approach the center, the Coriolis effects will get pretty weird from your tossing perspective.

As long as things are in contact, the normal activities like stacking things, balancing things, rolling things, sliding things... these will appear to be fairly natural to the degree that there is plenty of friction. As soon as you lose contact with something, (releasing or tossing objects, pouring liquids, spraying gases, etc.) the distinction between these objects' proper linear motion (as viewed from outside the structure) and your interpretation of it from within you rotating perspective will start to show itself as curious motions, lopsided parabolic motion curves, liquids that don't "stream", clouds of gas or vapor that don't make density gradients, ...etc; it won't look like behavior in a natural 1 G Earth surface environment.

6. Apr 15, 2013

cepheid

Staff Emeritus
What do you mean?

I mean, if the ship is a giant spinning ring, the outside wall is certainly holding you in, in the sense that it's the only thing keeping you from flying out into space in a straight line. So you feel this normal force (the contact force between your feet and the floor/wall), just as you do on the surface of the Earth, under gravity, where it's what holds you up. This normal force is also a centripetal force, keeping you in circular motion.

7. Apr 15, 2013

Staff: Mentor

When you are moving along with the rotation of the ship you are held against the side of the ship.

8. Apr 15, 2013

cepheid

Staff Emeritus
Question: presumably from the point of view of a non-rotating (inertial) frame outside the ship, what this looks like is that you sort of "spiral" outward from your initial location until you hit the outer wall, right?

Also, what you said about having some initial component of "radially" outward motion before you start getting spun up is essential for this to happen, right? I mean, if you started at distance r from the centre and you weren't initially moving outward, wouldn't air friction just accelerate you until you were moving in a circular orbit around the centre at the same speed as everything else at that radius?

9. Apr 15, 2013

cepheid

Staff Emeritus
Hmm, no, maybe not. Because air drag is not the same thing as having a rigid wall constraining motion in certain directions. So, even if you started with no radially outward motion, as soon as the air starts to push on you, you will start to move tangentially to the circular orbit at that radius, and that takes you to larger radii where you get pushed on even harder. Hmm..

10. Apr 16, 2013

So in simple terms, the released ball has a linear momentum, but the curved surface moves in a way that intercepts the ball in its trajectory, making it appear to the person releasing the ball that it "fell" to the "floor?"

It sounds like this WASN'T such a silly question after all.

11. Apr 16, 2013

Staff: Mentor

Yes.
I'm not quite understanding/that doesn't sound like what I said: I said if you are exactly at the center (at the axis of rotation) you will remain there. If you then start to move from the center, you'll move in that spiral you describe.

I'm not sure how one could start the problem stationary in a place other than the center, but if you did, yes, air would start to make you move with the rotation of the hull of the ship and you'd start to spiral toward the side again.

The key here is that the air is rotating with the ship.

12. Apr 16, 2013

It now seems obvious to me that I was failing to factor in the atmosphere that would need to be maintained, rather than the inside of the "thing" being a vacuum.