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H. Humbert
SFN Die Hard
USA
4574 Posts |
 Posted - 02/28/2007 : 15:37:45 [Permalink]
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quote:
Originally posted by Dave W.
Once on the "floor" and standing, if you were to jump against the spin motion, at the correct angle and speed to counteract your inertia, you would, indeed, start just floating around within the rotating portion until the air moving past you (because it's rotating with the hull, too) pushed you back to the "floor."
Wow, Dave, you even anticipated what my follow-up question was going to be! 
Ok, thanks for both replies. I think I have a much better understanding now. The only thing I'm not 100% clear on is why the air rotates at the same speed as the cylinder. Why isn't the air movement independent of the ship movement? Or why doesn't the ship twist the the outer edges of the air faster than the center, creating a whirlpool or tornado effect? Would it be possible to rotate a cylinder fast enough to create an air vaccum at its center?
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"A man is his own easiest dupe, for what he wishes to be true he generally believes to be true." --Demosthenes
"The first principle is that you must not fool yourself - and you are the easiest person to fool." --Richard P. Feynman
"Face facts with dignity." --found inside a fortune cookie |
| Edited by - H. Humbert on 02/28/2007 15:41:40 |
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Dave W.
Info Junkie
USA
26022 Posts |
Posted - 02/28/2007 : 16:06:58 [Permalink]
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The air moves because it bumps into furniture. Or just friction with the walls. Tornados or whirlpools, I'm not sure. But if you managed a 100-G rotation, the air would probably be a lot thinner in the middle than at the "floor."
For fun, put a helium-filled round ballon in your car, and drive around. With the windows rolled up and the A/C off, take some sharp turns and watch what the inertia of both the balloon and the air in your car do to the balloon's motion. |
- Dave W. (Private Msg, EMail)
Evidently, I rock!
Why not question something for a change?
Visit Dave's Psoriasis Info, too. |
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Neurosis
SFN Regular
USA
675 Posts |
Posted - 02/28/2007 : 17:01:50 [Permalink]
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quote:
Originally posted by H. Humbert
quote:
Originally posted by Dave W.
Once on the "floor" and standing, if you were to jump against the spin motion, at the correct angle and speed to counteract your inertia, you would, indeed, start just floating around within the rotating portion until the air moving past you (because it's rotating with the hull, too) pushed you back to the "floor."
Wow, Dave, you even anticipated what my follow-up question was going to be!
Ok, thanks for both replies. I think I have a much better understanding now. The only thing I'm not 100% clear on is why the air rotates at the same speed as the cylinder. Why isn't the air movement independent of the ship movement? Or why doesn't the ship twist the the outer edges of the air faster than the center, creating a whirlpool or tornado effect? Would it be possible to rotate a cylinder fast enough to create an air vaccum at its center?
If you imagine all the molecules in the room being pulled down, the ones already down will be forced over. The same as cold water and hot water in a circular tube. When the heated water replaces the cold and so forth you get a spinning motion. A similar thing happens on Earth to generate wind, but there is no driving force to insure it moves in one direction as it would be here. A circular force is actually the composite of two single-directional forces, making a vector which keeps becoming calculated with the previous vector against the two forces until it comes full circle, so, a circular force has two directions, away from the center and lateral to the center (up and left and down and right (these are the same) or up and right and down and left (these are the same).
A tornado vacuum is definitely possible. It would be no different then the air becoming thinner toward outer space, however this would generate more pressure in that area and the air would tend to press back equally. |
Facts! Pssh, you can prove anything even remotely true with facts.
- Homer Simpson
[God] is an infinite nothing from nowhere with less power over our universe than the secretary of agriculture.
- Prof. Frink
Lisa: Yes, but wouldn't you rather know the truth than to delude yourself for happiness?
Marge: Well... um.... [goes outside to jump on tampoline with Homer.] |
| Edited by - Neurosis on 02/28/2007 17:06:03 |
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McQ
Skeptic Friend
USA
258 Posts |
Posted - 02/28/2007 : 19:42:08 [Permalink]
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quote:
Originally posted by Neurosis
quote:
Originally posted by H. Humbert
Ok, this has bugged me for a long time.
I don't quite understand how rotational gravity is supposed to work in spaceships. From what I understand, a ship outfitted with a large rotating cylinder would simulate gravity for the people inside, since centrifugal force would keep them pressed against the hull of the ship.
But wouldn't they have to be strapped to the hull? I mean, if not, wouldn't they just tumble around inside like a sock in a dryer? Is there a warm up period? Like if you strap yourself to the hull and wait until it gets up to speed, can you then get up and walk around, since now you're already stuck?
I've just never quite understood how this principle is supposed to work.
I don't know about the warm up period thing, but centripital force(not centrifugal force) pulls the people to the ground. The centripital force acts on any body in a centrifuge pushing it away from the center. This is the principal that seperates things by their density in a centrifuge. If the dryer were spinning fast enough, it would lock the sock to the side with a constant force. It could easily be calibrated to equal gravity and in space, it would never slow down.
Centripetal Force doesn't push things away from the center. It is the force which pulls objects toward the center. It is the "center seeking" force. The force is always directed toward the center of the circle, in the case of a rotating space station, that would be toward the "hub" of the station.
A great description on the web can be found here:
http://regentsprep.org/Regents/physics/phys06/bcentrif/default.htm
I keep this page bookmarked because I love it's simplicity and use of pictures!  |
Elvis didn't do no drugs!
--Penn Gillette |
| Edited by - McQ on 02/28/2007 19:43:40 |
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Vegeta
Skeptic Friend
United Kingdom
238 Posts |
Posted - 03/05/2007 : 08:36:10 [Permalink]
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| in a dryer, the rotational force makes the sock stick to the side of the drum, then if it is not spinning fast enough, the force of gravity makes the sock tumble from the top of the drum to the bottom. in space, there is no significant gravity to make this happen |
What are you looking at? Haven't you ever seen a pink shirt before?
"I was asked if I would do a similar sketch but focusing on the shortcomings of Islam rather than Christianity. I said, 'No, no I wouldn't. I may be an atheist but I'm not stupid.'" - Steward Lee |
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H. Humbert
SFN Die Hard
USA
4574 Posts |
Posted - 03/05/2007 : 11:11:07 [Permalink]
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quote:
Originally posted by Vegeta
in a dryer, the rotational force makes the sock stick to the side of the drum, then if it is not spinning fast enough, the force of gravity makes the sock tumble from the top of the drum to the bottom. in space, there is no significant gravity to make this happen
Of course. But gravity also sticks the socks to the bottom of the dryer when it starts spinning. In space, the socks would be floating in the center of the dryer when it started spinning. I didn't understand why the socks would stick there if gravity wasn't responsible for maintaining the initial contact.
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"A man is his own easiest dupe, for what he wishes to be true he generally believes to be true." --Demosthenes
"The first principle is that you must not fool yourself - and you are the easiest person to fool." --Richard P. Feynman
"Face facts with dignity." --found inside a fortune cookie |
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Dave W.
Info Junkie
USA
26022 Posts |
Posted - 03/05/2007 : 11:32:19 [Permalink]
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| Eventually, H., the air in the dryer in zero G would be rotating with the tumbler. Any force imparted to the floating socks by the "wind" would eventually result in them making contact with the sides of the tumbler itself, which would impart even more momentum to the socks, which would then bump into the tumbler more quickly, which would add momentum to the socks... etc. Eventually, the socks would be moving at the same speed as the tumbler, pasted in place by their own inertia (after all, if the tumbler were to suddenly vanish, the socks would fly off in straight lines tangent to where the tumbler had been). |
- Dave W. (Private Msg, EMail)
Evidently, I rock!
Why not question something for a change?
Visit Dave's Psoriasis Info, too. |
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H. Humbert
SFN Die Hard
USA
4574 Posts |
Posted - 03/05/2007 : 12:25:03 [Permalink]
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quote:
Originally posted by Dave W.
Eventually, H., the air in the dryer in zero G would be rotating with the tumbler. Any force imparted to the floating socks by the "wind" would eventually result in them making contact with the sides of the tumbler itself, which would impart even more momentum to the socks, which would then bump into the tumbler more quickly, which would add momentum to the socks... etc. Eventually, the socks would be moving at the same speed as the tumbler, pasted in place by their own inertia (after all, if the tumbler were to suddenly vanish, the socks would fly off in straight lines tangent to where the tumbler had been).
Yep, it was the air I wasn't taking into consideration. If the dryer was in an airless part of the ship, then I guess the socks would just stay stationary (assuming no contact). If they did have a slight initial movement, then the socks would bump around quite a bit, although given enough time and enough bumps, they'd eventually be moving the same speed and direction as the tumbler, as you said.
I think I get it now.
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"A man is his own easiest dupe, for what he wishes to be true he generally believes to be true." --Demosthenes
"The first principle is that you must not fool yourself - and you are the easiest person to fool." --Richard P. Feynman
"Face facts with dignity." --found inside a fortune cookie |
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HalfMooner
Dingaling
Philippines
15831 Posts |
Posted - 03/05/2007 : 12:32:54 [Permalink]
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Please excuse the hijack. Apropos of nothing, but with the thought brought on by reading the above, here's something I once read about.
A very large planetoid is discovered. It does not rotate. It is artificially created, apparently. It is spherical, and made of thick, dense metal. It's massive enough to have considerable gravity on its surface. It is hollow, with a uniform thickness to its massive shell. It has a huge spherical interior volume. Astronauts open a hatch and rappel down and enter the sphere.
What sort of gravity conditions would they find in its various areas?
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“Biology is just physics that has begun to smell bad.” —HalfMooner
Here's a link to Moonscape News, and one to its Archive. |
| Edited by - HalfMooner on 03/05/2007 12:41:03 |
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JohnOAS
SFN Regular
Australia
800 Posts |
Posted - 03/05/2007 : 13:45:53 [Permalink]
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quote:
Originally posted by HalfMooner
A very large planetoid is discovered. It does not rotate. It is artificially created, apparently. It is spherical, and made of thick, dense metal. It's massive enough to have considerable gravity on its surface. It is hollow, with a uniform thickness to its massive shell. It has a huge spherical interior volume. Astronauts open a hatch and rappel down and enter the sphere.
What sort of gravity conditions would they find in its various areas?
That's a good one Halfmoner.
On the outside of the sphere, it's exactly the same as if it were a uniform, solid sphere with the same total mass as in the shell.
Anywhere inside the shell, the integral says that the net gravitational field is zero. This is because the net mass "inside" the imaginary sphere around by the person and centred on the sphere's centre is also zero. (Ignoring the fact that a person is not in fact a perfect point mass) Intuitively, this makes sense if you imaging being right at the centre. It may not seem quite so intuitive elsewhere inside the shell, but this is still the case. I think I could explain it better with electric fields rather than gravitational fields, I'll have to think about it.
What is really interesting is that if the thickness of the shell was significantly thinner than the height of the astronaut, while halfway through the "hatch", the top part of the astronaut would be in a positive gravity field,while their legs (assuming they go feet-first) could be in a gravity free zone. Let go halfway through and you continue to be forced towards the centre (and intertia helps too), but would feel compression internally as the force is not uniformly distributed.
I haven't done any of the math yet, this one has made me think I should really do so.
Edited to add: Note, this may seem kind of like the "hole through the centre of the earth" problem, but it's not the same, as once you get past the surface, there is no longer any mass inside the sphere to provide an even reduced attraction towards the centre.
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John's just this guy, you know. |
| Edited by - JohnOAS on 03/05/2007 13:56:16 |
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Vegeta
Skeptic Friend
United Kingdom
238 Posts |
Posted - 03/05/2007 : 13:56:25 [Permalink]
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Anywhere inside the shell, divergence theory says that the net gravitational field is zero. Intuitively, this makes sense if you imaging being right at the centre. It may not seem quite so intuitive elsewhere inside the shell, but this is still the case.
weird, i dont get it
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What are you looking at? Haven't you ever seen a pink shirt before?
"I was asked if I would do a similar sketch but focusing on the shortcomings of Islam rather than Christianity. I said, 'No, no I wouldn't. I may be an atheist but I'm not stupid.'" - Steward Lee |
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HalfMooner
Dingaling
Philippines
15831 Posts |
Posted - 03/05/2007 : 14:24:05 [Permalink]
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quote:
Originally posted by JohnOAS
quote:
Originally posted by HalfMooner
A very large planetoid is discovered. It does not rotate. It is artificially created, apparently. It is spherical, and made of thick, dense metal. It's massive enough to have considerable gravity on its surface. It is hollow, with a uniform thickness to its massive shell. It has a huge spherical interior volume. Astronauts open a hatch and rappel down and enter the sphere.
What sort of gravity conditions would they find in its various areas?
That's a good one Halfmoner.
On the outside of the sphere, it's exactly the same as if it were a uniform, solid sphere with the same total mass as in the shell.
Anywhere inside the shell, the integral says that the net gravitational field is zero. This is because the net mass "inside" the imaginary sphere around by the person and centred on the sphere's centre is also zero. (Ignoring the fact that a person is not in fact a perfect point mass) Intuitively, this makes sense if you imaging being right at the centre. It may not seem quite so intuitive elsewhere inside the shell, but this is still the case. I think I could explain it better with electric fields rather than gravitational fields, I'll have to think about it.
What is really interesting is that if the thickness of the shell was significantly thinner than the height of the astronaut, while halfway through the "hatch", the top part of the astronaut would be in a positive gravity field,while their legs (assuming they go feet-first) could be in a gravity free zone. Let go halfway through and you continue to be forced towards the centre (and intertia helps too), but would feel compression internally as the force is not uniformly distributed.
I haven't done any of the math yet, this one has made me think I should really do so.
Edited to add: Note, this may seem kind of like the "hole through the centre of the earth" problem, but it's not the same, as once you get past the surface, there is no longer any mass inside the sphere to provide an even reduced attraction towards the centre.
Thanks, John. That was the answer I'd read, too. Though it was not my own intuition on the subject. I would have thought, wrongly, that within the shell, there'd be gravity towards the outside, at least close to the shell.
It would be very disorienting to enter that planetoid, as you point out. There'd be a sudden and unnatural-seeming loss of gravity. If the astronaut had simply jumped into the hole, and there was no atmosphere within the shell, he or she would "fall" toward and past the center, to the opposite side, smashing into it.
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“Biology is just physics that has begun to smell bad.” —HalfMooner
Here's a link to Moonscape News, and one to its Archive. |
| Edited by - HalfMooner on 03/05/2007 14:28:19 |
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 03/05/2007 : 14:26:17 [Permalink]
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quote:
Originally posted by Neurosis
If you imagine all the molecules in the room being pulled down, the ones already down will be forced over.
Eh... No, not really. They ones already down will still occupy the down region. But they will now have to compete for the space: hence the pressure will increase. Only the lighter molecules, and hotter molecules will rise due to having less mass per unit volume.
quote:
The same as cold water and hot water in a circular tube. When the heated water replaces the cold and so forth you get a spinning motion.
Hot water is less dense than cold water, cold water being heavier push away the lighter hot water, making it float on the cold.
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 03/05/2007 : 14:28:59 [Permalink]
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| It's kind of like a Faraday's Cage, but with gravitational force instead of electricity. |
Dr. Mabuse - "When the going gets tough, the tough get Duct-tape..."
Dr. Mabuse whisper.mp3
"Equivocation is not just a job, for a creationist it's a way of life..." Dr. Mabuse
Support American Troops in Iraq:
Send them unarmed civilians for target practice..
Collateralmurder. |
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 03/05/2007 : 14:49:21 [Permalink]
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quote:
Originally posted by Vegeta
Anywhere inside the shell, divergence theory says that the net gravitational field is zero. Intuitively, this makes sense if you imaging being right at the centre. It may not seem quite so intuitive elsewhere inside the shell, but this is still the case.
weird, i dont get it
If you are immediately inside the sphere (figure yourself standing upside-down with magnetic boots on the inside), the mass closest to you is the square-meter you're standing on.
But the rest of the mass of the sphere that should be pulling you toward the center is far away. Remember, the gravitationan force is proportional to the inverse square of the distance (1/rē) so, the mass closest to you will pull at you hardest. But the mass farther away (the rest of the sphere) is so much more distant that it loses much of the pull. The net effect will be you experiencing zero gravitation. |
Dr. Mabuse - "When the going gets tough, the tough get Duct-tape..."
Dr. Mabuse whisper.mp3
"Equivocation is not just a job, for a creationist it's a way of life..." Dr. Mabuse
Support American Troops in Iraq:
Send them unarmed civilians for target practice..
Collateralmurder. |
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Trivial Questions
