Artificial gravity in spinning space ship conumdrum

[Atomsk]

When talking about the person dropping a ball dropped inside the "artificial gravity" in reference to the person inside the ring the ball would look like it fell straight down. The ball in question will already have the tangential velocity of the spinning ring that both person and ball are in motion with.

I already asked what your point was, that using rotational acceleration to simulate gravity for one that is in sync with the object that is rotationally accelerating is not artificial gravity, or was your point that rotational acceleration will not even simulate gravity. I think we all agree that it's not real gravity like what we see as a result of mass being present but it's the best approximation we can currently realize.

The problem being, you are stating that it's not artificial gravity but you aren't stating any evidence to back up your claim and then continue to avoid clarifying your statement.

 

[Shooting Star]

I just think "artificial gravity" (walking around in space) is just a myth, and pure science fiction/speculation.

Oh, good lord, NUBY, what exactly do you mean by space walking? We hear that astronauts sometimes go for space walks, and that means nothing more than that they are tethered to the ship and have gone out do some work on the ship (EVA). They are essentially just "floating" around in space, though actually they are in free fall around Earth if that vehicle happens to be orbiting earth.

Do you mean that by "artificial gravity"? Also, let me say once more that there is nothing called artificial gravity.

Science fiction and speculation do not share the same status, as you seem to imply in the above post.

If you do want to discuss space walks using artificial gravity, at least open a new thread as I had requested you before.

 

[nuby]

Atomsk,

I'm thinking that centrifugal force might behave differently in space than it does on earth, and that 'artificial gravity' (ie 2001: A Space Odyssey) would never work due to the coriolis effect. I'm not arguing the terminology, just the theory.

 

[Doc Al]

I'm thinking that centrifugal force might behave differently in space than it does on earth...

Be advised that personal speculation is not permitted here.

 

[TVP45]

Artificial gravity (that is the correct term and needs not to be set off in quotes) is very real, is practical, and is being widely studied (JSC, Cleveland Clinic, Univ. of Texas, Mt. Sinai, etc.). It is not about tossing balls in the air; it is about the physiological effects on humans of living for long periods in microgravity.

In a very large radius ship, as in 2001, one could create an artificial gravity that closely mimics real gravity and a ball would fall almost correctly. A ship that large is out of the question now; it may be built, perhaps within your lifetime, and you'll be able to see demonstrations of that gravity. In the meantime, we all have to be satisfied with short-arm centrifuges.

I earlier posted several links to the work being done at JSC. A centrifuge has been flown, and I mentioned that. There are a ton of links to papers by respected scientists on the sites I gave you. Have you looked at those?

YouTube is fun but not a citation.

 

[nuby]

It wouldn't be hard for NASA to demonstrate artificial gravity on a small scale, and there are tons of videos out there with other experiments. Where is the one for artificial gravity?

(27 seconds in) shows a coriolis affect

 

[TVP45]

It wouldn't be hard for NASA to demonstrate artificial gravity on a small scale, and there are tons of videos out there with other experiments. Where is the one for artificial gravity?

(27 seconds in) shows a coriolis affect

Are you following any of the links I've suggested? NASA has created artificial gravity hundreds (thousands?) of times. Of course there is a coriolis effect; again, if you look at the sites I've suggested, you'll find a number of papers referencing the problems that creates.

 

[Shooting Star]

Artificial gravity (that is the correct term and needs not to be set off in quotes) is very real, is practical, and is being widely studied (JSC, Cleveland Clinic, Univ. of Texas, Mt. Sinai, etc.).

(I think you and I have gone through this before.)

I wanted to avoid that term since there is a lot of confusion going around here. How can somebody confuse "space walks" with "walking in artificial gravity"? Also, "artificial gravity" may suggest something like Star Trek gravity (which would be truly artificial gravity), so I thought I would clarify matters.

The artificial gravity we are talking about here is the effect of inertial forces in any non-inertial frame. I sincerely hope it is clear now.

Let me sum up with what we are dealing here:

Whenever there is an accelerating frame, there are inertial forces, whose effects on a small region and over a short time can be approximated by a uniform gravitational field. (This sounds like the equivalence principle, which it is actually, but all discussions here are within the Newtonian scenario.)

These may be identified as the centrifugal force or the Coriolis force in a uniformly rotating frame. In a uniformly accelerating frame in a straight line, it will be an effective gravitational field in the opposite direction, as in elevators/lifts. If the acceleration of the frame is arbitrary, the inertial forces will be different from these familiar examples. It may be very difficult to calculate and also to predict what effects will be felt by a human being under such arbitrary forces. Also, remember that for a human being to feel anything, he should not be in free fall in that frame, which would just be equivalent to moving with constant velocity wrt some IFR, but has to be stationary wrt the non-IFR. Then only he can feel the effect of inertial forces.

In a very large radius ship, as in 2001, one could create an artificial gravity that closely mimics real gravity and a ball would fall almost correctly.

That is not correct. If you push something from the centre toward the rim, the trajectory will not seem like a straight line in the frame of the ship. We have already discussed this https://www.physicsforums.com/showpost.php?p=1546281&postcount=29".

Similarly, in a small centrifuge, the variations are much more over a short distance, than say, over the Earth for the same distance (due to Earth's rotation).

I earlier posted several links to the work being done at JSC. A centrifuge has been flown, and I mentioned that. There are a ton of links to papers by respected scientists on the sites I gave you. Have you looked at those?

I fail to understand why we have to give an example of something that NASA has built to prove the existence of inertial forces. I know that you are interested in microgravity, but that has nothing to do with the discussion here. (Remember, you can take the horse to the water but you cannot make it drink.)

I don't think you and I have any argument, just some confusion over terminologies. Arguing here about centrifugal forces is just adding fuel to the fire. If you really feel you have to disagree, PM me, as I want to unsubscribe from this thread.

 

[TVP45]

I think unsubscribing is a wonderful idea. I'll join you.

 

[Shooting Star]

Atomsk,

I'm thinking that centrifugal force might behave differently in space than it does on earth, and that 'artificial gravity' (ie 2001: A Space Odyssey) would never work due to the coriolis effect. QUOTE]

Hold on for a moment. (I was about to unsubscribe when I read this again.) Do I see a glimmer of hope yet?

Nuby, why don't you just explain exactly what you meant when you wrote the above sentence? Did you mean that if I let a ball drop in a big rotating space ship, it will not fall straight toward the rim? Feel free to explain in detail, but just stick to this point.

 

[HallsofIvy]

As long as the astronaut was connected to the rim, the ball would have a curved trajectory toward the ground/rim ..

If the answer is "C" and the ball would drop straight towards his feet. What force would keep the ball adjacent to the floor below?

The ball would not drop straight toward his feet. "Coriolis" force would make its trajectory curve a little bit. The ball's initial rotational speed would be less than that at the astronaut's feet. That's one difference between "gravity" simulated by spinning and true gravity. That's only noticable in a very large volume.

It is, of course, the "centrifugal force" that would keep it on the floor. Yes, I know that centrifugal force is "ficticious". It is a convenient way of saying that the ball, with its instantaneous velocity parallel to the direction of rotation would attempt to continue in that straight line and is prevented from doing so by the "centripetal" force the floor exerts on it.

There would be no force toward the floor if the astronaut were not on the floor and were not rotating around the axis with the rest of the ship, then there would be no "fake gravity" force. But how would he get there? If he jumped up from the floor, he would still have the floors rotational velocity.

Oh, and to answer the question that was posed in the very first post, "If the ship were very far from any star, what would it be rotating with respect to?", with respect to its own axis, of course. Velocity is relative. Rotation involves an acceleration and is not relative.

 

[LURCH]

If it sheds any new light, remember that there is Coriolis effect on Earth as well. So, if you drop a ball while standing on the ground, it will not really fall in a perfectly straight line downward, will it? Its path will be slightly curved due to Coriolis, and the fact that your hand is slightly farther from the center of rotation than the ground toward which the ball falls. Even on Earth we are experiencing gravity in a rotating frame, and we have certain effects acting upon us as a result of that rotation. But for the most part, we can ignore these effects because they are vanishingly small.

Likewise, artificial gravity generated by rotation would have some Coriolis effects involved. And, these effects would be greater than we exerience on Earth (because the rotating frame is much smaller). But, for the most part, the effect could still be ignored (just like it is here on Earth). Artificial gravity is, after all, only an approximation of real gravity and, for every-day use, it works perfecty well. You could put your feet on what others would consider the "wall" of the ship, and walk around and feel perfectly normal calling it the "floor." You could put a meal on a plate and it would stay there, and someone outside the ship would say it is stuck to the wall by centrifugal force, but to you it would seem to be sitting on the plate in a perfectly normal way. You could drop a ball and it would fall to the floor at your feet. If you had precise enough instruments, you could measure some curve to that drop, but you could do that on Earth, too.

As for centrifugal force behaving differently in space than it does on Earth, it just doesn't. The scientists on the ISS have been conducting experiments using a centrifuge pretty much since the staion went operational. Centrifugal force works just the same up there as it does down here.

 

[Shooting Star]

It is, of course, the "centrifugal force" that would keep it on the floor. Yes, I know that centrifugal force is "ficticious". It is a convenient way of saying that the ball, with its instantaneous velocity parallel to the direction of rotation would attempt to continue in that straight line and is prevented from doing so by the "centripetal" force the floor exerts on it.

It would be extremely difficult to keep a ball on the outer "floor" of a rotating frame just by centrifugal force alone, as that would be a very unstable situation. It is generally the friction in conjunction with the centrifugal force which will keep the ball stationary wrt the floor, or else the ball would have to be clamped down. As on a horizontal surface of earth, things stay in their places because of friction and gravity, otherwise the rotation of the Earth would cause them to slip. Think of that the next time you are typing and your computer is not sliding off.

Oh, and to answer the question that was posed in the very first post, "If the ship were very far from any star, what would it be rotating with respect to?", with respect to its own axis, of course. Velocity is relative. Rotation involves an acceleration and is not relative.

Read posts #2 and #18.
--------------------------------------------------------------------
Hey NUBY,

Anticipating an answer from you is the only thing that's keeping me here.

 

[nuby]

Shooting Star, Like other have explained, I think the coriolis effect will be at play in a rotating spaceship (and on earth). But, I'm wondering if the magnitude of the effect is a greater in space, and how it is calculated in space or on land (moon, earth, etc).

 

[Shooting Star]

Shooting Star, Like other have explained, I think the coriolis effect will be at play in a rotating spaceship (and on earth). But, I'm wondering if the magnitude of the effect is a greater in space, and how it is calculated in space or on land (moon, earth, etc).

(Nuby, if I have ever seen hijacking, this takes the cake.)

I have this morbid curiosity to know why you think that the Coriolis effect is different in what you call "space"?

You did not answer the question I had asked you in my last post. I am asking you two more.

1. What is "space", according to you?

2. Can we explain all phenomenon happening in the rotating ships without invoking centrifugal or Coriolis force, from the point of view of an IFR?

You can read https://www.physicsforums.com/showpost.php?p=1546281&postcount=29" discussion, which I had mentioned earlier. (It is a simple and very short description of the two points of view.)

Do answer the question in my last post by just saying yes or no.

 

[nuby]

I read that discussion, and almost everything makes sense to me. The thing I see making artifical gravity impractical are the issues astronauts would have with nausea, disorientation, etc. One thing confusing to me is the coriolis effect the astronauts would experience. If an astronaut was standing still in the rotating ship, why would he not experience a coriolis (not sure if this the best term) affect?

It seems since his head and feet would be moving at two different speeds (from an outside reference point), he might experience some sort of (rotating) force from that, even while not moving.

Shooting Star, if you have any more questions for me, send a private message. Since I can't say what I think here.

 

[Shooting Star]

I read that discussion, and almost everything makes sense to me.

HEAR THAT, FELLAS? One up for PF.

The thing I see making artifical gravity impractical are the issues astronauts would have with nausea, disorientation, etc.

That has nothing to do with rotating frames and inertial forces, and need not be discussed here. Many people throw up when traveling by car.

One thing confusing to me is the coriolis effect the astronauts would experience. If an astronaut was standing still in the rotating ship, why would he not experience a coriolis (not sure if this the best term) affect? It seems since his head and feet would be moving at two different speeds (from an outside reference point), he might experience some sort of (rotating) force from that, even while not moving.

The Coriolis force is given by -2wXv. This v is wrt the rotating frame, not the "stationary" inertial frame of reference. So, if an astronaut is standing still in the rotating frame, the velocities of all mass points on him is zero wrt the rotating frame. So, v is 0 for all points on his body and the Coriolis force on him is 0.

As you said, his head and feet has two different velocities wrt the IFR, but that has nothing to do with the Coriolis effect. You have to measure velocities wrt the rotating frame.

But the centrifugal force on his head and feet would be different, because they are at different distances from the rotating centre. It may not be noticeable if his distance from the centre is large compared to his height. Even on earth, out head and feet experiences different gravity.

I won't discuss anymore for fear of confusing you with something. Try to understand the Coriolis force. Read that discussion again if you have confusion. Think of something moving with uniform velocity inside a rotating shell, and what you see as st line motion becomes a spiral trajectory wrt the shell. That is what the Coriolis force basically is all about.

Shooting Star, if you have any more questions for me, send a private message. Since I can't say what I think here.

You can PM me any time.

If anybody asks you a question, and you answer honestly without expressing your own opinion, then I believe and hope that you won't get any points any more. I did not unsubscribe because at the last moment I thought you were not able to communicate properly, and also lacked some knowledge. Others will understand that too. But no more comments like in space things are different. Space is the same everywhere, at least for the purpose of our discussion.

And, remember, psychological or even technological issues are not very relevant in theoretical discussions.

 

[linton]

I read that discussion, and almost everything makes sense to me. The thing I see making artifical gravity impractical are the issues astronauts would have with nausea, disorientation, etc. One thing confusing to me is the coriolis effect the astronauts would experience. If an astronaut was standing still in the rotating ship, why would he not experience a coriolis (not sure if this the best term) affect?

It would have to be a pretty tiny spaceship if nausea was a major problem. Specifically because if they actually employed this method in the design they would increase the ring diameter so that the rotational speed would be less, to create the same centripetal force.

 

[LURCH]

Maybe this image will chaneg the way things look;

NASA's "Mars Express" plan, to get astronauts to Mars, includes a plan to spool out the capsule, in which they will spend most of the trip, on the end of a long cable. On the other end will be the final booster stage of the rocket. The two objects will be set spinning around each other (the cenetr of thwe cable will eb nearyl staionary). This will provide a downward push to keep the astronauts pressed against the floor, keeping them healthy and making many ordinary tasks easier.

Nuby, when you think of it that way, with the center of rotation outside of the capsule, does it make any difference?

 

[DaveC426913]

It would have to be a pretty tiny spaceship if nausea was a major problem. Specifically because if they actually employed this method in the design they would increase the ring diameter so that the rotational speed would be less, to create the same centripetal force.

Even in a medium-sized spaceship, the astronaut is still spinning about an axis - it just happens to be an axis that is an arbitrary number of metres over his head. You're going to feel it.

 

[Shooting Star]

It would have to be a pretty tiny spaceship if nausea was a major problem. Specifically because if they actually employed this method in the design they would increase the ring diameter so that the rotational speed would be less, to create the same centripetal force.

That is one of the most sensible comments I’ve encountered in this thread.

Even in a medium-sized spaceship, the astronaut is still spinning about an axis - it just happens to be an axis that is an arbitrary number of metres over his head. You're going to feel it.

Is this your feeling or have you got any numbers to show?

Let us take some conservative estimates and do the math.

1. Radius of the rotating space station is 100 m.
2. The angular speed w is such that the centrifugal acceleration equals 9.8 m/s² at the rim.
3. A typical astronaut is, say, 2m tall.
4. He walks at a pace of 5 km/hr = (25/18) m/s.

The difference between the centrifugal acceleration at his feet and head is 0.02 g. This is the “tidal force” when he is not moving wrt the station. I don’t think the human body can sense this variation in g between the head and the foot.

More concern has been shown here about the Coriolis effect. It is maximum when the velocity is perpendicular to the axis of rotation, that is, when he walks along a latitude of the cylinder, and the value turns out to be 0.089 g, which is less than 9/100 than g at earth’s surface. Generally, it will be less than this.

So, when he walks, he may have to compensate for this force. Remember, this is a small space station compared to what people have in minds for the future. (The ISS is around 120 m across, just for the purpose of comparison.) For bigger stations, the Coriolis force will be much less, and people will not notice it or will get habituated to it if it is small but appreciable.

Do not compare the effects of rotations in small or even big centrifuges with effects on a rotating space station.

 

[DaveC426913]

Let us take some conservative estimates and do the math.

1. Radius of the rotating space station is 100 m.

Well we were talking about a spaceship, and a spaceship of 100m diameter is anything but conservative.

That being said, there's no need to belabour it; we'll talk about space stations.

 

[blunt187]

Shooting star is the Man! or woman?

ok I'm just a college dropout who found this webpage on a humbug. Artificial graivty, or should I say mechanically produced gravity? would definitely work if the station is spining around fast enough at a steady space, as long as it is a large enough, and on a constant axis(how large I don't know, but i looked at the video link and sure enough, it seemed to be working pretty well!) . It doesn't really matter if gravity is affecting it or not becuase it is going to affect the whole thing including the people equally (unless it's like a black hole or some weird outer space thing I don't know about). Think of the carnival ride once again (I believe there is a link to it up above in this thread). Imagine that, on a giant robotic arm, it doesn't matter if they turn it upside down or sideways those people are still going to stick to that wall. Unless the giant arm turned em upside down and shook em like it was trying to get the last drop of ketchup out of a glass bottle, which i think would be cool, especially if nuby was on it. j/k :-)

 

[KenJackson]

would definitely work if the station is spining ... It doesn't really matter if gravity is affecting it or not ... Think of the carnival ride once again (I believe there is a link to it up above in this thread). Imagine that, on a giant robotic arm, it doesn't matter if they turn it upside down or sideways those people are still going to stick to that wall.

Spinning? Who says the carnival ride is spinning and not the Earth? Who nominated the Earth to be the center of the universe?

It's easy to slip into classic absolute space thinking. Though the question isn't absolutely answered yet, I'm still pleased enough by what was said earlier.

I was pleased that someone mentioned Mach's Principle, which caused me to find the wikipedia discussion on the same. I am content to learn that Einstein grappled with the issue and decided that inertia originates in a kind of interaction between bodies. That is, (as I understand it) the presence of other matter (I guess all matter in the universe) determines what is and is not spinning.

 

[atyy]

The Earth is a spinning spaceship - with artificial gravity - in the "wrong" direction! See D H's comment on how the centrifugal force affects pendulums at the equator (post #3 pf this thread):

https://www.physicsforums.com/showthread.php?p=1868564#post1868564

 

[atyy]

I was pleased that someone mentioned Mach's Principle, which caused me to find the wikipedia discussion on the same. I am content to learn that Einstein grappled with the issue and decided that inertia originates in a kind of interaction between bodies. That is, (as I understand it) the presence of other matter (I guess all matter in the universe) determines what is and is not spinning.

I think even in General Relativity, rotation is absolute, in the sense that it produces a gravitational field without a matter source. The other common absolute motion in GR texts is the uniformly accelerating rocket, which also produces a gravitational field without a matter source.

 

[Nick666]

For anyone planning to check in at the Centrifugal Space Station, remember to sit your *** and put your furniture on the far ends of the space station.

 

[Shooting Star]

Spinning? Who says the carnival ride is spinning and not the Earth? Who nominated the Earth to be the center of the universe?

It's easy to slip into classic absolute space thinking. Though the question isn't absolutely answered yet, I'm still pleased enough by what was said earlier.

It is also easy to simplify the matter to an extent where metaphysics takes precedence over Physics. Mach’s Principle, though a guiding light for Einstein, need not be last word. In addition, the principle itself indicates that nearby masses should have an appreciable effect on the motion of a body. Taking the view that the carnival ride can be described completely by considering relative rotations would be too simplistic.

I think even in General Relativity, rotation is absolute, in the sense that it produces a gravitational field without a matter source. The other common absolute motion in GR texts is the uniformly accelerating rocket, which also produces a gravitational field without a matter source.

Please remember that all these results apply to our universe, where there is a background of matter, distant or otherwise. Otherwise, what would we measure the rotation against? (Please don't say with accelerometers.) That is the whole spirit behind Mach’s Principle. We have to find a different universe and conduct some experiments before conclusively stating how much of an effect the distant or nearby matter have on the motion with respect to any frame of reference.

(As far as I remember vaguely, there are matter-free solutions to the GR Equations, though I must admit that I am not familiar with their status in accepted science.)

[About "Shooting star is the Man! or woman?", please look up my profile, which contains a single entry. However, due to the way language and society have evolved, "name_of_woman is the the woman!" doesn't quite deliver the original connotation...]

 

[atyy]

Please remember that all these results apply to our universe, where there is a background of matter, distant or otherwise. Otherwise, what would we measure the rotation against? (Please don't say with accelerometers.) That is the whole spirit behind Mach’s Principle. We have to find a different universe and conduct some experiments before conclusively stating how much of an effect the distant or nearby matter have on the motion with respect to any frame of reference.

Well, it's a bit unclear what Mach's Principle is. General Relativity respects some form of Mach's Principle in the sense that the local gravitational acceleration of a test particle is inertial (ie. an accelerometer measures zero). I'm not sure I'm recalling correctly, but I believe "General Relativity" by Hobson, Efstathiou, and Lasenby actually exclude rotated frames from being considered inertial, and call this "Mach's Principle"!

 

[schroder]

Shooting Star, Like other have explained, I think the coriolis effect will be at play in a rotating spaceship (and on earth). But, I'm wondering if the magnitude of the effect is a greater in space, and how it is calculated in space or on land (moon, earth, etc).

From reading this one cannot avoid the conclusion that there is a lot of confusion concerning gravity and centripetal acceleration. This is understandable since gravity is a centripetal acceleration, but generally when we speak of centripetal acceleration we are not speaking about gravity! Gravity, as far as we know, is not related to rotation, but is directly proportional to mass and indirectly proportional to the square of the distance. Centripetal acceleration is directly proportional to the velocity of rotation and the inversely proportional to the distance from the axis of rotation. The idea of utilizing centripetal acceleration to simulate the force of gravity does certainly have it’s merits. Since the earliest days of orbital space flight it became apparent that humans who are removed from the force of gravity as well as electromagnetic fields suffer biological consequences. The excuses which were used in the 1960’s, such as, “He slipped and fell in the shower”, for returning astronauts, are no longer offered or accepted. But a great deal more research needs to be done to determine if this form of “gravity substitute” is indeed viable, as it may well have serious long-term health consequences. The first thing to realize is that the force of centripetal acceleration due to rotation, while generating a similar force magnitude, is not the same as the force of gravity. On the surface of the earth, the force of gravity can be calculated from the equation: g = G x M / r^2 Where G is the gravitational constant of 6.67 x 10^-11, M is the mass of the Earth 5.98 x 10^24 kilograms and r is the radius of the Earth 6.37 x 10^6 meters. This results in a gravitational acceleration of ~ 9.8 m/s^2. Now, for an astronaut, or any human, who has the height of 2 meters, this gravitational force will vary along his height, from head to toe, by a factor of only one part in ten million. So essentially, there is no variation in gravity along the height/length of a human on the surface of the earth. However, the centripetal acceleration, and thus centripetal force, exerted on this same astronaut who is inside a spacecraft which employs rotational centripetal acceleration to simulate the force of gravity will experience quite a bit of variation along his 2 meter height. If we assume a spacecraft of one kilometer in length, from the axis of rotation, it will need to have an angular velocity of 0.099 rad per second to achieve a centripetal acceleration which is the same as gravity, 9.8 m/sec^2. Due to the astonauts’ height of 2 meters, the radius from the axis is now 998 meters and with the same angular velocity the centripetal acceleration will be only 9.78 m/sec^2. This may not seem to be much of a variation, but it certainly is not a trivial consideration! This represents a variation of 2 parts in one thousand, compared to the one part in ten million on the surface of the earth. No one can say what the possible biological consequences may be of this increased variation of about five orders of magnitude over a long length of time. Obviously, much more research needs to be done in this area.