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### Artifical gravity

Posted: Sun Jun 07, 2009 4:23 am UTC
This has been bothering me for a while now and I can't think of a way that artifical gravity is feasible at all so I'm open to suggetions.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 4:28 am UTC
Negative mass planets?

For smaller scale, maybe something that stores an absolutely titanic concentration of energy right about your head, giving the illusion of antigravity? Problem: can't lift itself, will eat birds.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 4:45 am UTC
some experiments have found interesting gravitational anomalies from spinning super-conductors (or something like that) I'll see if I can find an article on it

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 10:01 am UTC
There is no known mechanism at this point for artificially creating gravity and other than having a suitably large mass at hand.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 10:23 am UTC
Well, since gravity for all intents and purposes is identical to acceleration, standing on the inside of a rotating ring would effectivly create artificial gravity. An other less feasible way to do it would be to stand in an forever accelerating space-craft.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 2:49 pm UTC
Adding energy to an object increases it's mass (if it's the spin your increasing). So could you just keep pumping energy into a really fast spinning particle/object?
The problem being, it would have an upper limit that it can spin at, and you'd need a ton of energy.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 2:56 pm UTC
defaultusername wrote:Well, since gravity for all intents and purposes is identical to acceleration, standing on the inside of a rotating ring would effectivly create artificial gravity. An other less feasible way to do it would be to stand in an forever accelerating space-craft.

Aren't they the same thing?
I know that is how the flat earth society explains gravity, where they got the energy for that, well, you'll have to ask them, but I wouldn't hold your breath for a well reasoned answer!

But a question I have with the rotating space craft, being a ring, are you always walking slightly "uphill"? Because as soon as you change direction, uphill is whichever way you're walking. Can you ever walk "downhill" on it. Is the same correct in reverse on a perfectly spherical planet.

I know on a planet the effect would be too small to matter, but one could feasibly imagine a small rotating spaceship, with say radius 7 feet, where this apparent paradox (apologies if there isn't one here), would be apparent to large proportions?

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 4:20 pm UTC
Pastinator wrote:But a question I have with the rotating space craft, being a ring, are you always walking slightly "uphill"? Because as soon as you change direction, uphill is whichever way you're walking. Can you ever walk "downhill" on it. Is the same correct in reverse on a perfectly spherical planet.

I know on a planet the effect would be too small to matter, but one could feasibly imagine a small rotating spaceship, with say radius 7 feet, where this apparent paradox (apologies if there isn't one here), would be apparent to large proportions?

It would depend on the size of the ship you may never notice the curve. but on something like the ship from 2001 a space odyssey the ship spins vertically and the ship isn't curved, you would probably not even notice you are spinning.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 4:42 pm UTC
ajbleck wrote:but on something like the ship from 2001 a space odyssey the ship spins vertically and the ship isn't curved, you would probably not even notice you are spinning.

What? I remember two main ships from that, one of which was a large ring and so obviously was curved, and one that didn't spin at all in any direction. What are you talking about?

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 6:33 pm UTC
defaultusername wrote:Well, since gravity for all intents and purposes is identical to acceleration, standing on the inside of a rotating ring would effectivly create artificial gravity.

That's not actually why a rotating ring can be used for a sort of 'artificial gravity'.

Because objects have a tendency to move in a straight line, what happens in a rotating inertial frame is that you start to move in a straight line and stop up against the hull which carries you around in a curved line so your natural straight-line moving tendency forces you up against the hull. This is fundamnetally different from an acceleration frame such as from gravity or constant acceleration. If the ring is small enough, or rotating fast enough, you get a strong gradient between your head and your feet, which can be pretty awkward and uncomfortable.

Also, falling objects will show centripetal motion in straight line towards the edge of the spacecraft, which because you are rotating, will apear as a curved path instead of appearing as a straight path in a constant acceleration frame.

Here's an article on the gravitational weirdness of rotating superconductors, enjoy.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 6:33 pm UTC
i remember wathing the movie and i swear i saw this ship spin

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 6:53 pm UTC
Discovery One has a centrifuge inside it:

I'm not completely sure, but I think it is rotating around a axis that is vertical in the picture ajbleck just posted.

The entire ship was spinning in 2010 because without maintenance the kinetic energy of the centrifuge was somehow transferred or "spread" to the entire ship.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 7:48 pm UTC
EdgarJPublius wrote:
Here's an article on the gravitational weirdness of rotating superconductors, enjoy.

Wow! If this actually works, and it can be done to above a 10th of a G, then the future is bright and Star-Trekey!

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 8:46 pm UTC
EdgarJPublius wrote:Because objects have a tendency to move in a straight line, what happens in a rotating inertial frame is that you start to move in a straight line and stop up against the hull which carries you around in a curved line so your natural straight-line moving tendency forces you up against the hull. This is fundamnetally different from an acceleration frame such as from gravity or constant acceleration.

I don't see why it is. It is my understanding that this is exactly what causes the centripetal acceleration. Sure, it's not constant, but it will always be directed toward the center of the motion, much like gravity is always directed toward the center of mass.

Also, falling objects will show centripetal motion in straight line towards the edge of the spacecraft, which because you are rotating, will apear as a curved path instead of appearing as a straight path in a constant acceleration frame.

Well, sure, but the same is true on a planet (assuming rotation, which AFAIK they all do). I don't think there is any experiment you could perform to discern standing inside a rotating ring from standing on a massive object (that's rotating).

Edit: Oh, standing on the poles is cheating. OK, so I suppose there is an experiment then .

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 9:30 pm UTC
Well, you could look up. If you see sky, you're on a planet. If you see the inside of a spinning object, you're in a spinning object.

Ceilings complicate this procedure.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 9:40 pm UTC
Kow wrote:Well, you could look up. If you see sky, you're on a planet. If you see the inside of a spinning object, you're in a spinning object.
This also clearly qualifies as cheating.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 11:34 pm UTC
From what i can remember, the big problem with synthesising gravity by rotation is that, for a sufficiently small rotating ship, when you walk your head bobs up and down, and this induces motion to the side (because of the corriolis effect) which can cause pretty intense nausea if you move too fast (inner ears tend to not like jerky side to side motions). To reduce the effect you either have to make the ship much bigger (think Halo sized) or you have to 'lighten up' on the gravity.

### Re: Artifical gravity

Posted: Sun Jun 07, 2009 11:37 pm UTC
Ubik wrote:Discovery One has a centrifuge inside it

No, it had a circular floor inside it. Any artificial gravity was created by running, though, not by the thing actually spinning.

### Re: Artifical gravity

Posted: Mon Jun 08, 2009 1:35 am UTC
gmalivuk wrote:
Ubik wrote:Discovery One has a centrifuge inside it

No, it had a circular floor inside it. Any artificial gravity was created by running, though, not by the thing actually spinning.

Nope, most of the interior shots of this area rather cleverly show that it spins with respect to the rest of the ship.

EdgarJPublius wrote:Because objects have a tendency to move in a straight line, what happens in a rotating inertial frame is that you start to move in a straight line and stop up against the hull which carries you around in a curved line so your natural straight-line moving tendency forces you up against the hull. This is fundamnetally different from an acceleration frame such as from gravity or constant acceleration.

I don't see why it is. It is my understanding that this is exactly what causes the centripetal acceleration. Sure, it's not constant, but it will always be directed toward the center of the motion, much like gravity is always directed toward the center of mass.

Also, falling objects will show centripetal motion in straight line towards the edge of the spacecraft, which because you are rotating, will apear as a curved path instead of appearing as a straight path in a constant acceleration frame.

Well, sure, but the same is true on a planet (assuming rotation, which AFAIK they all do). I don't think there is any experiment you could perform to discern standing inside a rotating ring from standing on a massive object (that's rotating).

Edit: Oh, standing on the poles is cheating. OK, so I suppose there is an experiment then .

Right, Centripetal acceleration causes the perception of centrifugal force (a so-called 'fictional force'), but it's just an artifact of inertia being redirected by the presence of a spaceship hull.

The two biggest consequences of this are, again, a non-inertial gradient as you approach the center of rotation, and Coriolis force (I was incorrect earlier when describing this effect on falling objects) which actually tends to curve an object against the motion of the spinning ring (rather than preserving it's straight line, which would be present in any rotating frame)

using either of these it's actually pretty easy to tell if you are in a true inertial frame or a rotating centripetal frame.

### Re: Artifical gravity

Posted: Mon Jun 08, 2009 1:43 am UTC
EdgarJPublius wrote:
gmalivuk wrote:
Ubik wrote:Discovery One has a centrifuge inside it

No, it had a circular floor inside it. Any artificial gravity was created by running, though, not by the thing actually spinning.

Nope, most of the interior shots of this area rather cleverly show that it spins with respect to the rest of the ship.

Hm. I guess that's what i get for not having seen the movie in at least a decade.

### Re: Artifical gravity

Posted: Mon Jun 08, 2009 5:19 am UTC
Maybe you were thinking of Skylab and got it mixed up with the movie?

### Re: Artifical gravity

Posted: Mon Jun 08, 2009 11:00 pm UTC
Hi,
Just to get this straight, would the spinning "habitat" ring on discovery realy work or is it too small or too slow?

I can see the space station with it's twin huge wheels working as it wouldn't have to spin as fast being a bigger wheel.

The discovery would surely have to spin the habitat ring so fast that things would drop sideways at great speed, all HAL had to do to kill them was spin it faster or throw some junk ae35's in with them

As a small scale model It's not so much like a bucket being spun round your head but more like lots of ball bearings in a spinning inflated bike tyre. Would the air not stay still while the floor moves?

### Re: Artifical gravity

Posted: Mon Jun 08, 2009 11:18 pm UTC
gmalivuk wrote:
EdgarJPublius wrote:
gmalivuk wrote:
Ubik wrote:Discovery One has a centrifuge inside it

No, it had a circular floor inside it. Any artificial gravity was created by running, though, not by the thing actually spinning.

Nope, most of the interior shots of this area rather cleverly show that it spins with respect to the rest of the ship.

Hm. I guess that's what i get for not having seen the movie in at least a decade.

The thing that's always bugged me about that is that in the sequel book Clarke writes that the centrifuge had shut down due to... whatever, wear and tear, power failure, I can't remember, and transferred the momentum of its spin to the ship. Now, I'm no physicist but so far that seems logical enough to me, however, in the movie (and this is fairly accurate to what it says in the book, I think) Discovery is spinning around its own centre. The middle of the ship, which has the big antenna dish that fails in the first book (and movie) is where the spin is centred, and there is no way that's where the centrifuge was. How's that work?

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 1:49 pm UTC
The thing that's always bugged me about that is that in the sequel book Clarke writes that the centrifuge had shut down due to... whatever, wear and tear, power failure, I can't remember, and transferred the momentum of its spin to the ship. Now, I'm no physicist but so far that seems logical enough to me, however, in the movie (and this is fairly accurate to what it says in the book, I think) Discovery is spinning around its own centre. The middle of the ship, which has the big antenna dish that fails in the first book (and movie) is where the spin is centred, and there is no way that's where the centrifuge was. How's that work?

Let me get this straight... the centrifuge was the only part of the ship that was actually spinning? Wouldn't the rest of the ship already be spinning in the other direction, thanks to Newton's Third? Seeing as the ship is in space, there would have to be another way to prevent the rest of the ship from spinning...

In either case, if the centrifuge stopped working, it would just keep spinning until the friction with the rest of the ship causes it to "stop". Now, this could mean two things.

If the ship is spinning in the opposite direction, because of what I said above, then both parts would slow down and eventually both be rotating slightly in the direction of whichever part is more massive.
If the ship isn't spinning in the opposite direction, and is somehow stopped, then yes; the centrifuge would cause the ship to start spinning with it. As the centrifuge slowed down and the ship sped up, they would eventually come to a point where they're both rotating at the same speed, and would stay at that speed.

Unless the connection between the ship and centrifuge is frictionless, I don't see why this wouldn't apply.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 4:11 pm UTC
Presumably the ship has attitude control, which could be used occasionally to kill whatever spin was added by friction with the centrifuge.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 4:12 pm UTC
Drax wrote:If the ship is spinning in the opposite direction, because of what I said above, then both parts would slow down and eventually both be rotating slightly in the direction of whichever part is more massive.

The only way for the total rotational momentum of the system to increase is to apply an off-center force. That is... If the centrifuge is spun by a motor in the ship, then the ship will immediately start spinning in the opposite direction, and when it spins down (due to friction or whatever when the torque is no longer applied) both will stop spinning. If, on the other hand, it is spun by thrusters located on the outside of the centrifuge, this gives the system a net angular momentum and when friction brings the centrifuge to the same speed as the rest of the ship it will be spinning (at a slower speed)

(Ninja'd: Right, attitude control. That would be in the form of off-center thrusters located on the main body of the ship. Same basic concept.)

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 6:33 pm UTC
I've dug out some quotes:

"The equatorial region of the pressure sphere- the slice, as it were, from Capricorn to Cancer- enclosed a slowly rotating drum, thirty-five feet in diameter. As it made one revolution every ten seconds, this carrousel or centrifuge produced an artificial gravity equal to that of the moon."

"The spin of the carrousel could be stopped if necessary; when this happened, its angular momentum had to be stored in a flywheel, and switched back again when rotation was restarted."

"but he did not really feel it in his bones until he saw the entire hundred-meter length of Discovery turning end-over-end, while Leonov kept at a safe distance. Years ago, friction had braked the spin of Discovery's carousel, thus transferring its angular momentum to the rest of the structure. Now, like a drum majorette's baton at the height of its trajectory, the abandoned ship was slowly tumbling along its orbit."

In all depictions of the ship, it never revolves, apart from after the carrousel breaks down and the ship is stuck in a decaying orbit around Io, when as I said, it is spinning around its centre, end over end. This is the only thing that has ever bugged me about the physics in the Odyssey-books, even though I say that without much knowledge of physics. The carrousel would transfer its momentum to the rest of the ship, fine, yes- I buy that, but why would it start rotating around its center? And end-over-end? Surely it should start spinning in the same direction as the carrousel? As far as I can tell, the carrousel can only stand in two positions inside the sphere: Upwards, and along the "sides" of the sphere, like so: -----( l ) or, like it appears in the first quote, like an equatorial band: -----(-), but the spin in 2010 is described and depicted as going end-over-end, which confuses me, since in order for the carrousel to be going like a slice down the middle, it would interfere with the cockpit and the space garage. Also, in any case, shouldn't the spin be centred around the sphere where the carrousel was?

Image for reference:
Spoiler:

Holy smokes, there's even a youtube video with weird sounds to illustrate my point:

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 7:22 pm UTC
Actually when you think a little deeper about it the ship would initially spin centered near the carousel but once as equilibrium was reached it would spin at the ship's center of gravity.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 7:27 pm UTC
Even so, angular momentum is a vector. If it's rotating about a different axis, momentum hasn't been conserved.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 7:30 pm UTC
I think the differences in the rotation speed account for it.
Gah, I can see it in my head but don't know how to word it right.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 8:15 pm UTC
Sockmonkey wrote:I think the differences in the rotation speed account for it.

No, they don't, because the axis is still changing and so the angular momentum vector is not being conserved.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 8:22 pm UTC
Yes! I knew there was something wrong with it!

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 8:31 pm UTC
gmalivuk wrote:
Sockmonkey wrote:I think the differences in the rotation speed account for it.

No, they don't, because the axis is still changing and so the angular momentum vector is not being conserved.

No, if the _direction_ of the axis is the same, that's the only component that matters for the angular momentum vector. There's a different moment of inertia (angular "mass") when measured about a different point, but - again, the difference in rotation speed accounts for that. A vector has a direction and a magnitude only, it doesn't also have a "starting point". And the direction of the vector is the direction of the axis (oriented so it points towards you if the object appears to be rotating counterclockwise as you look at it, and away from you if it is clockwise). "The position of the axis" has nothing to do with it.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 9:41 pm UTC
There's one thing that doesn't quite fit with this rotating spaceship wheel: In a situation of zero gravity, what force is keeping you down on the ship's hull when you're not in contact with it? I see how a spinning motion combined with the friction of your feet with the floor would simulate the force of gravity, but if you stopped spinning and lifted your feet, you would still be floating, right? (Provided that your momentum is identical to the ship's.) Then it wouldn't entirely be able to simulate gravity.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 9:54 pm UTC
Well no, but it's generally going to be hard to stop yourself spinning, since there's a lot of momentum to kill for that to happen. At 10m radius you've got to go 10m/s to get 10m/s2, I believe. Which is a fast sprinting speed. So sure, if you can sprint fast enough, you can become weightless in that situation, but it's not going to be terribly easy.

And with a bigger ring, you've got to go faster. At 100m, it's 31m/s to get 1g, and so on.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 11:12 pm UTC
that reminds me when i was watching the science of star wars or something like that on the discovery channel. They said that for the Death Star to have artificial gravity it would have to be spinning at an absurdly fast speed. Although it seems to me like it would have enough mass to have a gravitational field big enough to create some gravity without having to spin.

### Re: Artifical gravity

Posted: Tue Jun 09, 2009 11:47 pm UTC
Concerning 2001, when the Discovery entered orbit, it likely had to change it's orientation which could cause it to start spinning on a different axis as it would encounter gyroscopic effects. Then, as Sockmonkey said, I assume the ship would start revolving around it's centre of mass. Also note that the ring in Discovery was probably simulating less than 1g.

As for stopping while rotating in the ring, I agree that might be kind of hard, but it would depend on the size of the ring as they spin at different speeds.

As I know there are a lot of mathy people here, here's some maths stolen from Atomic Rockets:

How fast will the ship have to spin in order to provide acceptable gravity?

Ca = 0.011 * Cr2 * Cl
Cl = Ca / (0.011 * Cr2)
Cr = sqrt( Ca / (0.011 * Cl))

where
Ca = centrifugal artificial gravity acceleration at point X (m/s2)
Cl = distance from point X to the center of rotation (m)
Cr = rotation rate at point X (rotations per minute)
Remember that 1.0 g is 9.81 m/s

### Re: Artifical gravity

Posted: Wed Jun 10, 2009 6:35 pm UTC
ajbleck wrote:that reminds me when i was watching the science of star wars or something like that on the discovery channel. They said that for the Death Star to have artificial gravity it would have to be spinning at an absurdly fast speed. Although it seems to me like it would have enough mass to have a gravitational field big enough to create some gravity without having to spin.

frustratingly ( for darth vader), the moonlike gravity of the death star would in fact counteract any inertial artificial gravity. And bear in mind it's mostly hollow and filled with airy bits for people to hang about in, and isn't a huge sphere of solid rock, like the actual moon.

When darth vader is choking that guy, which way is his head facing? Into the middle of the death star or outwards? The Star Wars universe seems to have conquered artifical gravity, though. "Real" artificial gravity. They stand around and talk in ships that clearly don't have round centrifuges.

### Re: Artifical gravity

Posted: Fri Jun 12, 2009 7:17 am UTC
gmalivuk wrote:Well no, but it's generally going to be hard to stop yourself spinning, since there's a lot of momentum to kill for that to happen. At 10m radius you've got to go 10m/s to get 10m/s2, I believe. Which is a fast sprinting speed. So sure, if you can sprint fast enough, you can become weightless in that situation, but it's not going to be terribly easy.

Also, the air is the room will also be spinning, so even if you manage to get going fast enough to pick up your feet and just float there (and you have to be precise or you will float back down to the floor, which will be like jumping out of a car that was moving at exactly the speed you were sprinting) the moving air will pretty quickly get you moving again and you will touch down shortly thereafter (although if you can lay yourself out flat, you may be able to gain enough momentum from the air that touchdown is not quite as violent as in the former case).

Zhatt wrote:it likely had to change it's orientation which could cause it to start spinning on a different axis as it would encounter gyroscopic effects.

If it were rotating along its long axis, gyroscopic effects should resist the axis changing to the end over end rotation axis rather than promote it. The only thing I can think of is that the end over end rotation is unrelated to the absorption of the drum's momentum (a small meteoroid hit it or the attitude thrusters got fired or something like that).

### Re: Artifical gravity

Posted: Fri Jun 12, 2009 8:26 pm UTC
quintopia wrote:If it were rotating along its long axis, gyroscopic effects should resist the axis changing to the end over end rotation axis rather than promote it. The only thing I can think of is that the end over end rotation is unrelated to the absorption of the drum's momentum (a small meteoroid hit it or the attitude thrusters got fired or something like that).

The rotation along the long axis wouldn't "resist" a directional change, but a turn to the left or right would transmit the forces to cause a up or down rotation on the ship. A standard gyroscope that resists directional changes works by having multiple rotating masses in different axis all resisting each other. If you take a toy gyroscope (the kind with the one rotating disc) and hang it sideways by a string, the force of the gravity trying to pull it downwards causes it to rotate around the string left or right. Check out some videos of gyroscopes and you might get the idea (I'd post links, but I'm at work and don't want to be surfing youtube).