help me understand the rotating ring artificial gravity thing

[hellokeith]

The Wikipedia artificial gravity article says that the rotation causes objects to move towards the outer hull.

My question is why would objects move towards the hull? It has something to do with movement of the air and/or friction against the air, correct?

Does it matter which direction the ring is rotating in respect to the movement of the spaceship/space station?

According to the article, the ring would have to be fairly large in order for the simulated 1g to extend many feet "off the ground" such that houses and buildings could be built without the top floor having a much-reduced g-force compared to the bottom floor. So would transportation/modes of travel, like bicycle, motorcycle, car, helicopter, airplane be viable? Safe?

 

[Pulsar]

There's nothing magic here, and you're thinking along the right lines.

The idea is, that if I'm standing on the floor, I'm moving at the same speed as the floor. Naturally I will continue trying to move in a straight line. However, I am traveling in a circle. To travel in a circle, there must be a force applied to me to cause me to curve through space - supplied by the floor.

Anything I "let go" in mid air would immediately assume a straight line of travel, however, I would continue to curve and the object would appear to fall to the floor.

http://regentsprep.org/Regents...6/bcentrif/default.htm

This explains it pretty well. Look at the centripetal force formula to understand why you have to have a really really big radius. Essentially, any distance you move up, say in a house, would have to be negligible compared to the radius. So if your 3 story house is 30 feet up, your radius would have to be:

a = V^2 / r

We want a to be 9.8 m/s^2 so, if our velocity is a sedate 5 m/s the radius would have be about 2.7 m. At 20 m/s, we'd get a radius of 40 meters. (I'm approximating because I don't have a calculator handy. The problem is, 30 feet up, or 10 meters, is 25% less acceleration! Big problem!

So, if we're trying to make a 3 story house be reasonable, then lets say 10 meters has to vary the acceleration 1%. Then 10 meters is 1% of the radius. So 1000 meters of radius, or 1 kilometer, would get you within a reasonable area for it to be negligible. Assuming of course, that you don't have a basement......

Put the formula in excel and play with the numbers.

 

[Throckmorton]

It's centrifugal force. Tie something to a string and spin it around. The object wants to keep going straight, but the string prevents that. The object feels like it's continuously being pulled outward by a force.

 

[KingGheedora]

A few more sentences into the wikipedia article you linked, you'll find your answer.

Objects in motion will tend to stay in motion, the objects will try to travel in a straight line, but the ship's hull continually provides normal and friction forces to change the objects' paths to match the hull's rotation. Objects in contact with the hull will experience this gravity, but I wonder if the air friction would be enough to make objects not in contact with the hull but near the hull experience the pseudo-gravity.

 

[Throckmorton]

Originally posted by: KingGheedora
A few more sentences into the wikipedia article you linked, you'll find your answer.

Objects in motion will tend to stay in motion, the objects will try to travel in a straight line, but the ship's hull continually provides normal and friction forces to change the objects' paths to match the hull's rotation. Objects in contact with the hull will experience this gravity, but I wonder if the air friction would be enough to make objects not in contact with the hull but near the hull experience the pseudo-gravity.

An object has to be rotating with the hull to feel the effect. The air would slowly accelerate a floating object and it would float (fall) into the hull.

 

[DrPizza]

Originally posted by: Throckmorton
It's centrifugal force. Tie something to a string and spin it around. The object wants to keep going straight, but the string prevents that. The object feels like it's continuously being pulled outward by a force.

There is no force pulling the object outward. This is a misconception. The force is the sting pulling the object toward the center. (Or, in the case of the rotating ring, the force is the floor pushing up against the person's feet - again, this force is directed toward the center.) I won't go so far as to say there's no such thing as a "centrifugal" force - Newton would say that for every force, there is an equal and opposite force. Thus, if the floor is pushing against your feet (or the string is pulling on you) then your feet are pushing against the floor (and you're pulling on the string.)

 

[hellokeith]

Originally posted by: KingGheedora
Objects in contact with the hull will experience this gravity, but I wonder if the air friction would be enough to make objects not in contact with the hull but near the hull experience the pseudo-gravity.

This is my main question as well. I don't think you can count on the air (as a whole) to just magically push things towards the hull, because air in this environment would be a very dynamic system. We are talking about an artificial environment where the air must be contantly circulated in order to scrub the CO2 and refresh any depleted O2.

 

[TuxDave]

Originally posted by: hellokeith

Originally posted by: KingGheedora
Objects in contact with the hull will experience this gravity, but I wonder if the air friction would be enough to make objects not in contact with the hull but near the hull experience the pseudo-gravity.

This is my main question as well. I don't think you can count on the air (as a whole) to just magically push things towards the hull, because air in this environment would be a very dynamic system. We are talking about an artificial environment where the air must be contantly circulated in order to scrub the CO2 and refresh any depleted O2.

I think the assumption is that you have to be accelerated to match the hull prior to entering the enviroment. Otherwise if you just jump into space, the world will be flying past you and it'll look and feel like you're in orbit around your virtual earth.

 

[frostedflakes]

I think you're right, as soon as a person breaks contact with the surface acceleration should be reduced (significantly, I'd think). Never really thought about it before but it's an interesting problem. Has artificial gravity via this method ever been tested in space?

EDIT: Well actually now that I think about it more I'm not sure. If you were to jump off the surface you should still have a lot of sideways momentum, air drag force wouldn't reduce it significantly. And I'd assume before drag force would slow you down enough for you to start floating away, you'd collide again with the rotating hull and be back under full "gravity."

This is why I hate physics, lol. Even the simplest physics concepts can really make my head hurt.

 

[Throckmorton]

Originally posted by: DrPizza

Originally posted by: Throckmorton
It's centrifugal force. Tie something to a string and spin it around. The object wants to keep going straight, but the string prevents that. The object feels like it's continuously being pulled outward by a force.

There is no force pulling the object outward. This is a misconception. The force is the sting pulling the object toward the center. (Or, in the case of the rotating ring, the force is the floor pushing up against the person's feet - again, this force is directed toward the center.) I won't go so far as to say there's no such thing as a "centrifugal" force - Newton would say that for every force, there is an equal and opposite force. Thus, if the floor is pushing against your feet (or the string is pulling on you) then your feet are pushing against the floor (and you're pulling on the string.)

Well I said the object feels like it's being pulled outward. And obviously the floor exerts a force on the object. None of these are the same as real gravity

 

[Throckmorton]

Originally posted by: frostedflakes
I think you're right, as soon as a person breaks contact with the surface acceleration should be reduced (significantly, I'd think). Never really thought about it before but it's an interesting problem. Has artificial gravity via this method ever been tested in space?

EDIT: Well actually now that I think about it more I'm not sure. If you were to jump off the surface you should still have a lot of sideways momentum, air drag force wouldn't reduce it significantly. And I'd assume before drag force would slow you down enough for you to start floating away, you'd collide again with the rotating hull and be back under full "gravity."

This is why I hate physics, lol. Even the simplest physics concepts can really make my head hurt.

If you jump off the surface, you'll keep moving in the direction tangent to the side of the circle. IE, you'd fall back into the outside of the ring.

 

[DrPizza]

Originally posted by: Throckmorton

Originally posted by: DrPizza

Originally posted by: Throckmorton
It's centrifugal force. Tie something to a string and spin it around. The object wants to keep going straight, but the string prevents that. The object feels like it's continuously being pulled outward by a force.

There is no force pulling the object outward. This is a misconception. The force is the sting pulling the object toward the center. (Or, in the case of the rotating ring, the force is the floor pushing up against the person's feet - again, this force is directed toward the center.) I won't go so far as to say there's no such thing as a "centrifugal" force - Newton would say that for every force, there is an equal and opposite force. Thus, if the floor is pushing against your feet (or the string is pulling on you) then your feet are pushing against the floor (and you're pulling on the string.)

Well I said the object feels like it's being pulled outward. And obviously the floor exerts a force on the object. None of these are the same as real gravity

I was addressing the first 3 words of your post.

 

[KIAman]

Nobody's ever seen a motorcycle in the sphere going round and round and up and down, seemingly defying the laws of gravity?!?

Gravity is just acceleration. To feel 1G of gravity you just need to accelerate at 9.8m/s^2.

Do that by going in a rocket going in 1 direction or be inside a massive donut that "lifts" you up at relative 9.8m/s^2 (or your fall against the edge of the donut at 9.8m/s^2 doesn't matter, different perspective, same thing).

As Throckmorton has stated, the word "artificial" is misleading as what you feel is not gravity at all but a force equal to gravity. Nothing is being created other than the acceleration.

 

[TuxDave]

Originally posted by: KIAman
Nobody's ever seen a motorcycle in the sphere going round and round and up and down, seemingly defying the laws of gravity?!?

Gravity is just acceleration. To feel 1G of gravity you just need to accelerate at 9.8m/s^2.

Do that by going in a rocket going in 1 direction or be inside a massive donut that "lifts" you up at relative 9.8m/s^2 (or your fall against the edge of the donut at 9.8m/s^2 doesn't matter, different perspective, same thing).

As Throckmorton has stated, the word "artificial" is misleading as what you feel is not gravity at all but a force equal to gravity. Nothing is being created other than the acceleration.

So in terms of acceleration, how does that apply to gravity being created from a large mass?

<--- barely remembers the answer from high school.

 

[hellokeith]

Originally posted by: Throckmorton

If you jump off the surface, you'll keep moving in the direction tangent to the side of the circle. IE, you'd fall back into the outside of the ring.

Remember there is no downward force like gravity in this situation. You jump away from the "floor", the only thing acting against that upward motion is very minor air friction. Yes you will still have tangent intertia perpendicular to the floor, but that only means you will track horizontally more or less the spot on the floor where you left, it says nothing of how far away from the floor you will or won't get.

It comes down to the Y/X, Y being your upward jump force and X being your perpendicular inertia. If you jump upward harder than the ring is pushing you sideways, then you'll end up hitting the "ceiling". The fun thing to do would be jump with just enough force that you barely miss the ceiling and end up in the same spot where you left like 30 seconds ago.

An interesting outcome of this is that since your body does not have angular momentum, after a long/high jump you would approach the floor at an angle.

 

[PolymerTim]

I guess it depends on how high your ceiling is

But it seems to me, with a high enough ceiling, the floor will always catch up to you. Once you jump off the floor, you are now travelling tangent to the circle, on the inside of the floor, so the floor is actually coming up to meet you. I'm not sure about this, but it seems to me, due to te shape of the circle, that from the point of view of the person jumping in the giant ring, that the floor would appear to be accelerating back towards you. And inside a sealed system, this would probably be indistinguishable from standing on earth and jumping, except maybe for this angle thing.

I can see how in the spinning ring, the floor would be approaching at an angle (although depending on the size of the ring, the angle may be very small) and I can't imagin how that would happen in true gravity like on earth. Anyone have thoughts on this?

-Tim

 

[TuxDave]

I think the whole idea of jumping inside a rotating ring is just going to make people sick. If you attempt to jump just straight up, you'll suddenly feel like you just took a turn in the air and land somewhere else. If you tried jumping in the direction of rotation, you first feel like you're jumping foward only to land back where you started. If you jump in the opposite direction you'll find yourself jump hella far.

 

[PolymerTim]

Not necessarily. I think your first statement might be true for small rings since, as hellokeith points out, the floor would approach you at an angle. But for this to be true, the height of your jump has to be of a similar order of magnitude as the ring radius. As the ring radius gets large (and assuming there is enough angular momentum for some decent acceleration) then the rotation of the floor relative to you during jump is very small.

As for being able to jump farther in one direction than another you are either forgetting the jumpers inertia or are creating wind resistance that isn't there. A good example is being on a moving train and jumping. Since you and the train are moving at the same velocity, a jump straight up will land you right were you started and you won't be able to jump any farther relative to the train in one direction than another. I realize that is on earth with real gravitiy, but I believe the principle holds the same in a relatively large rotating ring. Once you jump straight up in the ring, you will continue to move in a tangent to the ring at the same linear velocity you were travelling before you jumped. The floor will continue to move in a circle and come up to meet you and you will end up landing on the same spot on the floor you jumped from, albeit at a small angle rotated. I think similarly, you would not be able to jump farther in one directino than another.

Maybe I'm confused on this, but it seems to make sense to me.

-Tim

 

[frostedflakes]

Originally posted by: hellokeith

Originally posted by: Throckmorton

If you jump off the surface, you'll keep moving in the direction tangent to the side of the circle. IE, you'd fall back into the outside of the ring.

Remember there is no downward force like gravity in this situation. You jump away from the "floor", the only thing acting against that upward motion is very minor air friction. Yes you will still have tangent intertia perpendicular to the floor, but that only means you will track horizontally more or less the spot on the floor where you left, it says nothing of how far away from the floor you will or won't get.

It comes down to the Y/X, Y being your upward jump force and X being your perpendicular inertia. If you jump upward harder than the ring is pushing you sideways, then you'll end up hitting the "ceiling". The fun thing to do would be jump with just enough force that you barely miss the ceiling and end up in the same spot where you left like 30 seconds ago.

An interesting outcome of this is that since your body does not have angular momentum, after a long/high jump you would approach the floor at an angle.

What would be fun is hitting the ceiling, then walking around on it.

EDIT: And I think you're right PolymerTim, a ring of very large radius should be able to mimic gravity pretty well.

 

[PolymerTim]

Originally posted by: frostedflakes
What would be fun is hitting the ceiling, then walking around on it.

Sounds like fun. I also wonder if you could run in the opposite direction of the rotation and at an equal linear velocity to the floor and then jump and float clear to the other side.

Actually, that does make me think of a pretty significant difference between this artificial gravity and that of earth. In the ring, no matter how large, as you travel at a velocity relative to the floor in the direction of rotation, you will actually alter the magnitude of your acceleration and thus your apparent gravity. So if your running in the opposite direction of spin, apparent gravity will get smaller and reach zero as you reach the speed of the floor. Then, if you kept going faster, it would pick up again. Likewise, if you were running in the same direction as the spin, the apparent gravity would go up with speed.

Maybe someone who actually knows what they're talking about (I really am just conjecturing) can confirm some of this.

-Tim

 

[KIAman]

At a diameter of 1.1 miles rotating at 1RPM, you'd have to run 66mph to counter the acceleration effects. This is highly unlikely as you would have no friction to continue moving as the acceleration wore off. You would probably get to around 60mph then slip and slide all over as you become 1/10th of your weight and the moving air mass (which is rotating as well) causes too much air friction.

The real interesting thing to me would be running in the direction of rotation. That'd be a good workout.

 

[Mark R]

I wonder how hard it would be to play simple ball games like tennis or perhaps even basketball. On a ring even 1 mile in diameter, the coriolis effect will be substantial and will make thrown objects appear to take bizarre trajectories.

 

[Throckmorton]

Originally posted by: PolymerTim
Not necessarily. I think your first statement might be true for small rings since, as hellokeith points out, the floor would approach you at an angle. But for this to be true, the height of your jump has to be of a similar order of magnitude as the ring radius. As the ring radius gets large (and assuming there is enough angular momentum for some decent acceleration) then the rotation of the floor relative to you during jump is very small.

As for being able to jump farther in one direction than another you are either forgetting the jumpers inertia or are creating wind resistance that isn't there. A good example is being on a moving train and jumping. Since you and the train are moving at the same velocity, a jump straight up will land you right were you started and you won't be able to jump any farther relative to the train in one direction than another. I realize that is on earth with real gravitiy, but I believe the principle holds the same in a relatively large rotating ring. Once you jump straight up in the ring, you will continue to move in a tangent to the ring at the same linear velocity you were travelling before you jumped. The floor will continue to move in a circle and come up to meet you and you will end up landing on the same spot on the floor you jumped from, albeit at a small angle rotated. I think similarly, you would not be able to jump farther in one directino than another.

Maybe I'm confused on this, but it seems to make sense to me.

-Tim

I think you're right

 

[hellokeith]

Originally posted by: Mark R
I wonder how hard it would be to play simple ball games like tennis or perhaps even basketball. On a ring even 1 mile in diameter, the coriolis effect will be substantial and will make thrown objects appear to take bizarre trajectories.

Frisbee would be awesome.

I just thought of something, I wonder if having windows would be a good idea? Depending on how close to another planet or moon is the ring, it could be very disorienting to see stars/moon/planet go by the window quickly.

 

[TuxDave]

So I'm not one who likes to make a statement without backing it up so here goes. So to prove my first statement I produce the below.

Assuming the initial vertical jump velocity of a human is 2m/s (I made up this number)
Assuming the ring is simulating Earth's gravity (9.8 m/s^2)

Ring's radius (m) : How far ahead you land in the direction of rotation (m)
8.00000 : 0.11579
16.00000 : 0.08433
32.00000 : 0.06054
64.00000 : 0.04313
128.00000 : 0.03061

I'll figure how to translate my paper scratches to a digital form so you'll have to take my word for the results until then. And yes, I know it results in centimeters results but hey, at least it's not zero distance.


And to prove my 2nd statement of jumping further in one direction, I reason if jumping up results in a forward motion, jumping in that same direction versus jumping in the opposite directions should net different results.