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Rendezvous with Rama
I'm reading this book by Arthur C. Clarke. It involves a spaceship of
sorts, which is a cylinder that rotates about its long axis. The idea is that a person standing on the inside of the cylinder will experience a pseudo-'gravity' in the form of a centripetal force that pushes his feet up. I have a few concerns with this, which are probably relevant to any such device engineered to create 'gravity' this way. 1. If I'm standing on the inside surface of the cylinder and I jump up, there is no true 'pull' on me that accelerates me toward the 'floor' like there is with real gravity. Consequently, if I jump, I will simply float up and through the long axis, to the other side of the cylinder. A natural consequence of this is that I can't run on the floor, since this involves both my feet leaving the floor and not touching again till I hit the other side. If I fell asleep an area with no roof, I could float away if I were not strapped down. 2. Also, imagine I were standing on the floor of this cylinder facing the direction in which it turns. The ground and I are travelling in that same direction, so I appear to be stationary. However, the ground also slowly moves 'up' in front of me and also rotates my body 'backwards' slightly. This would mean that every time I jump I would appear to somersault slightly in the direction of turn, or alternatively if I turned around 180 degrees, I would appear to somersault slightly backwards when I jump (or even just standing there). This would mean that whatever angle I stand at, my postural reflexes will have to alter slightly on two sides of my body and I will easily tell which way the ground am turning. 3. Arthur C Clarke supposes that the air is denser toward the 'floor' than toward the middle of the cylinder. This doesn't seem right at all. There is nothing attracting the air to the floor. Surely the air might be caused to rotate slightly because of the friction between the floor and the outermost laminar layer, but that would not be transferred to all the other air molecules. Air is 'slippery' enough that even two metres above the floor I'd imagine that it would be relatively motionless and that the gas would evenly distribute itself throughout the rest of the cylinder. This means that if I'm standing on the floor, I will feel a breeze. Am I right in all this? |
#2
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Rendezvous with Rama
Testing wrote:
I'm reading this book by Arthur C. Clarke. It involves a spaceship of sorts, which is a cylinder that rotates about its long axis. The idea is that a person standing on the inside of the cylinder will experience a pseudo-'gravity' in the form of a centripetal force that pushes his feet up. I have a few concerns with this, which are probably relevant to any such device engineered to create 'gravity' this way. 1. If I'm standing on the inside surface of the cylinder and I jump up, there is no true 'pull' on me that accelerates me toward the 'floor' like there is with real gravity. Consequently, if I jump, I will simply float up and through the long axis, to the other side of the cylinder. No, that's wrong. Let's choose our coordinate system so that the point you are standing is at the bottom of the cylinder, and the cylinder rotates clockwise. Because of this rotationally movement, you have a speed to the left at this point (seen by an outside observer - you yourself would say that you are simply standing there and are not moving at all). If you now jump upwards, you *still* have this speed to the left - hence you will not go directly "upwards" (towards the center of the cylinder), but "upwards" *and* to the left (again, as seen by an outside observer). In other words, you will land on the cylinder again - and to you, this looks like a force of gravity is pulling you down. A natural consequence of this is that I can't run on the floor, since this involves both my feet leaving the floor and not touching again till I hit the other side. If I fell asleep an area with no roof, I could float away if I were not strapped down. Wrong for the reason explained above. 2. Also, imagine I were standing on the floor of this cylinder facing the direction in which it turns. The ground and I are travelling in that same direction, so I appear to be stationary. Right. However, the ground also slowly moves 'up' in front of me and also rotates my body 'backwards' slightly. Huh? No. This would mean that every time I jump I would appear to somersault slightly in the direction of turn, or alternatively if I turned around 180 degrees, I would appear to somersault slightly backwards when I jump (or even just standing there). This would mean that whatever angle I stand at, my postural reflexes will have to alter slightly on two sides of my body and I will easily tell which way the ground am turning. False premise, false conclusion. 3. Arthur C Clarke supposes that the air is denser toward the 'floor' than toward the middle of the cylinder. This doesn't seem right at all. There is nothing attracting the air to the floor. See point 1. Surely the air might be caused to rotate slightly because of the friction between the floor and the outermost laminar layer, but that would not be transferred to all the other air molecules. Why not? Air molecules collide often. Air is 'slippery' enough that even two metres above the floor I'd imagine that it would be relatively motionless and that the gas would evenly distribute itself throughout the rest of the cylinder. I don't think so. This means that if I'm standing on the floor, I will feel a breeze. Am I right in all this? No, sorry. Bye, Bjoern |
#3
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Rendezvous with Rama
Dear Testing:
"Testing" wrote in message om.au... I'm reading this book by Arthur C. Clarke. It involves a spaceship of sorts, which is a cylinder that rotates about its long axis. The idea is that a person standing on the inside of the cylinder will experience a pseudo-'gravity' in the form of a centripetal force that pushes his feet up. I have a few concerns with this, which are probably relevant to any such device engineered to create 'gravity' this way. A very bright fellow... I'll only add a little to Bjoern's comments. .... 3. Arthur C Clarke supposes that the air is denser toward the 'floor' than toward the middle of the cylinder. This doesn't seem right at all. There is nothing attracting the air to the floor. "Attract" is not the right word. The air at the outer cylinder is in contact with the outer cylinder. There are features on the outer cylinder that tend to keep the air there at similar velocity. Therefore, the bulk of the air is compressed since it is also being forced to follow a curved path. Since this air is compressed, the air just "above" (being defined as closer to tha axis of symmetry) it flows down, and the air above this, and the air above this... The lowest pressure air in a tornado is the air in the center. Surely the air might be caused to rotate slightly because of the friction between the floor and the outermost laminar layer, but that would not be transferred to all the other air molecules. Blow air out of your mouth onto some scattered flour. You'll see that the effects of your breath are much wider than your mouth. Air is 'slippery' enough that even two metres above the floor I'd imagine that it would be relatively motionless and that the gas would evenly distribute itself throughout the rest of the cylinder. You are thinking that air is imbedded in something that it can transfer energy/momentum to. The only thing that can serve this purpose is the walls of the ship. If the ship is spinning, the air will spin also. When it spins it is "slung to the outside", just as if it were water. Water also is 'slippery'. Spin a bucket of water (like it were sitting on a lazy susan), keep it spinning for a while, and see what shape its upper surface forms. This shape is indicative of the pressure distribution. It happens to be a parabola, which is how some mirrors are cast. The pressure is higher near the outer walls of the container. This means that if I'm standing on the floor, I will feel a breeze. No. Am I right in all this? You are right to question. Clarke did his homework. David A. Smith |
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Rendezvous with Rama
I'll follow up here owing to newsgroup server problems. Thanks to all
replies. However, the ground also slowly moves 'up' in front of me and also rotates my body 'backwards' slightly. Huh? No. - I think this is the thing that had me tricked. I forgot that the person standing on the inside of the cylinder is himself rotating at the same speed as the cylinder. - Have experiments confirmed the behaviour of air inside the cylinder? |
#5
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Rendezvous with Rama
Dear Testing:
"Testing" wrote in message om.au... I'll follow up here owing to newsgroup server problems. Thanks to all replies. However, the ground also slowly moves 'up' in front of me and also rotates my body 'backwards' slightly. Huh? No. - I think this is the thing that had me tricked. I forgot that the person standing on the inside of the cylinder is himself rotating at the same speed as the cylinder. - Have experiments confirmed the behaviour of air inside the cylinder? Yes. David A. Smith |
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Rendezvous with Rama
On Sat, 27 Mar 2004 13:33:13 +1100, "Testing"
wrote: I'll follow up here owing to newsgroup server problems. Thanks to all replies. However, the ground also slowly moves 'up' in front of me and also rotates my body 'backwards' slightly. Huh? No. - I think this is the thing that had me tricked. I forgot that the person standing on the inside of the cylinder is himself rotating at the same speed as the cylinder. - Have experiments confirmed the behaviour of air inside the cylinder? Yes. That's how a centrifuge works. The only requirement for this is that air have a non-zero viscosity, which it does. Al Moore |
#7
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Rendezvous with Rama
On Fri, 26 Mar 2004 23:49:23 +1100, "Testing"
wrote: I'm reading this book by Arthur C. Clarke. It involves a spaceship of sorts, which is a cylinder that rotates about its long axis. The idea is that a person standing on the inside of the cylinder will experience a pseudo-'gravity' in the form of a centripetal force that pushes his feet up. I have a few concerns with this, which are probably relevant to any such device engineered to create 'gravity' this way. 1. If I'm standing on the inside surface of the cylinder and I jump up, there is no true 'pull' on me that accelerates me toward the 'floor' like there is with real gravity. Consequently, if I jump, I will simply float up and through the long axis, to the other side of the cylinder. No, instead, you go straight, and the floor comes up to meet you. A natural consequence of this is that I can't run on the floor, since this involves both my feet leaving the floor and not touching again till I hit the other side. If I fell asleep an area with no roof, I could float away if I were not strapped down. Once again, you are mistaken. In the rotating cylinder you are subjected to a continuous acceleration that provides a good simulation for weight. The larger the cylinder, the better the simulation. 2. Also, imagine I were standing on the floor of this cylinder facing the direction in which it turns. The ground and I are travelling in that same direction, so I appear to be stationary. However, the ground also slowly moves 'up' in front of me and also rotates my body 'backwards' slightly. This would mean that every time I jump I would appear to somersault slightly in the direction of turn, or alternatively if I turned around 180 degrees, I would appear to somersault slightly backwards when I jump (or even just standing there). That's correct. This is referred to as "coriolis force" and was covered in the novel. This would mean that whatever angle I stand at, my postural reflexes will have to alter slightly on two sides of my body and I will easily tell which way the ground am turning. How easy it is to tell depends on how large the cylinder is, and how fast it's spinning. 3. Arthur C Clarke supposes that the air is denser toward the 'floor' than toward the middle of the cylinder. This doesn't seem right at all. There is nothing attracting the air to the floor. Surely the air might be caused to rotate slightly because of the friction between the floor and the outermost laminar layer, but that would not be transferred to all the other air molecules. Air is 'slippery' enough that even two metres above the floor I'd imagine that it would be relatively motionless and that the gas would evenly distribute itself throughout the rest of the cylinder. Once again, Clarke is correct and you are mistaken. Air has viscosity, and so rotates with the cylinder, and, in consequence becomes denser at the periphery, and less dense along the axis. This means that if I'm standing on the floor, I will feel a breeze. Am I right in all this? Well, one out of three isn't totally wrong. Al Moore |
#8
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Rendezvous with Rama
Anyone want to comment on Larry Niven's 'Ringworld'?
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