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Rockets
Hi,
if there is no Atmosphere, where do rockets that go in Space get thrust from |
#2
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In article ,
"George Kinley" wrote: if there is no Atmosphere, where do rockets that go in Space get thrust from They expell mass in one direction, which causes them to go in the other direction. The atmosphere has very little to do with how rockets work; it's just a simple law of physics: every action (force) causes an equal and opposite reaction (counterforce). The rocket "pushes" on the exhaust gas, causing it to accelerate to a very high velocity; the exhaust gas pushes back on the rocket, causing it to accelerate in the opposite direction (but more slowly, since the rocket has a lot more mass than the exhaust gas -- but that involves a different law). When you get into high school physics, you'll learn all about Newton's laws and how things like rockets work. It's really cool stuff, and it's neat to see you taking an interest in it already -- you'll probably be way ahead of most of the class! Best, - Joe ,------------------------------------------------------------------. | Joseph J. Strout Check out the Mac Web Directory: | | http://www.macwebdir.com | `------------------------------------------------------------------' |
#3
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On Thu, 10 Jul 2003, George Kinley wrote:
if there is no Atmosphere, where do rockets that go in Space get thrust from Stand on a skateboard on a smooth flat surface, with a bag of rocks. Throw the rocks in one direction, and you'll go in the other direction (except for the inconvenient effects of friction - use bigger rocks and you should see an effect). The rocket works exactly the same way. Throw mass away in one direction, and you go in the other direction. "Conservation of momentum" is what it's all about. -- Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/ Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html |
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In article Pine.LNX.4.50.0307110958020.29738-100000
@kolmogorov.physics.uq.edu.au, says... On Thu, 10 Jul 2003, George Kinley wrote: if there is no Atmosphere, where do rockets that go in Space get thrust from Stand on a skateboard on a smooth flat surface, with a bag of rocks. Throw the rocks in one direction, and you'll go in the other direction (except for the inconvenient effects of friction - use bigger rocks and you should see an effect). Or increase the speed. Stand on the skateboard and fire a shotgun, you'll move. Marc |
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Timo Nieminen wrote in message ...
On Thu, 10 Jul 2003, George Kinley wrote: if there is no Atmosphere, where do rockets that go in Space get thrust from Stand on a skateboard on a smooth flat surface, with a bag of rocks. Throw the rocks in one direction, and you'll go in the other direction (except for the inconvenient effects of friction - use bigger rocks and you should see an effect). The rocket works exactly the same way. Throw mass away in one direction, and you go in the other direction. "Conservation of momentum" is what it's all about. The direction of the rockets acceleration is irrespective of the direction of mass explusion. I could build a rocket that expels its mass upwards and it would still accelerate upwards. It is the explosion in the combustion chamber that pushes out in ALL directions. The upwards push makes contact with the top of the combustion chamber transferring a net upwards momentum to the rocket. However, the downward push from the explosion does not make contact with any structure from the rocket and escapes - so the net momentum is upwards. It should be noted that the actual upwards acceleration has nothing to do with the output direction of the expelled mass (I could very well route the mass from the explosion pushing downards via the top or side, although this is both extremely difficult and very inefficient). Also note that you don't even need mass to escape from a rocket. Inertial propulsion is not prohibited in physics if you think about it. All that needs to be done is to make the upwards push a greater impulse than the downwards push - the rocket would essential jerk its way upwards - after all there is no "conservation of displacement" with such an inertial system. |
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John Schoenfeld wrote:
Timo Nieminen wrote in message ... On Thu, 10 Jul 2003, George Kinley wrote: if there is no Atmosphere, where do rockets that go in Space get thrust from Stand on a skateboard on a smooth flat surface, with a bag of rocks. Throw the rocks in one direction, and you'll go in the other direction (except for the inconvenient effects of friction - use bigger rocks and you should see an effect). The rocket works exactly the same way. Throw mass away in one direction, and you go in the other direction. "Conservation of momentum" is what it's all about. The direction of the rockets acceleration is irrespective of the direction of mass explusion. I could build a rocket that expels its mass upwards and it would still accelerate upwards. How? Won't that break conservation of momentum? ----------------------------------------------------------------------- To reply to me directly: Replace the text after the@symbol with: totalise DOT co DOT uk |
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Joe Strout wrote in message ...
In article , (John Schoenfeld) wrote: The direction of the rockets acceleration is irrespective of the direction of mass explusion. I could build a rocket that expels its mass upwards and it would still accelerate upwards. No you couldn't; that would violate conservation of momentum. In the system you've described, the center of mass of the entire system would (through some trickery of pipes and combustion chambers) accelerate itself off in one direction. That's impossible -- you might as well propose a perpetual motion machine. It is the explosion in the combustion chamber that pushes out in ALL directions. The upwards push makes contact with the top of the combustion chamber transferring a net upwards momentum to the rocket. However, the downward push from the explosion does not make contact with any structure from the rocket and escapes - so the net momentum is upwards. It should be noted that the actual upwards acceleration has nothing to do with the output direction of the expelled mass (I could very well route the mass from the explosion pushing downards via the top or side, although this is both extremely difficult and very inefficient). Only if by "extremely difficult" you mean "impossible." Certainly not. You could add holes along the side of the rocket so that the air-pressure can let the explosion exit without affecting the overall net upwards momentum. Although this would be inefficient, it demonstrates that the CAUSE of upwards acceleration is IRRESPECTIVE of the exit direction of expelled mass. Also note that you don't even need mass to escape from a rocket. Yes, you do (if only by definition of a rocket). Of course there are other ways to accelerate something, e.g. by pushing it with an external force (like a sail) or by pushing against an external medium (like an airplane). But then it's not a rocket. Inertial propulsion is not prohibited in physics if you think about it. Um, yes, it is. No it is not. Imagine a stationary black-box floating in space. One wall of the box is hard iron and the opposite side is ellastic. If a ball is thrown from the middle at the hard iron wall there will be a high-impulse transfer of momentum from the ball to the box. Relative from the center of the box (which at this point is moving), the ball now approaches the opposite ellastic wall in which it inevitably collides with and transfers the same momentum but in the opposite direction bringing the box to rest again. However, the elastic wall collision was low-impulse and took longer for the momentum to be conservered. Irrespective of momentum conservation, there is an overall displacement. At this point we have the box at rest yet it is displaced from its original position, however in future time this same effect will occur but in the opposite direction and thus the overal motion of this contraption would be to OSCILLATE about the original position. So technically speaking, its not inertial propulsion yet as the center of mass is constant. So the third and final requirement would to have a constant stream of balls colliding just as the first one thus always staying one step ahead of the "backwards oscillation phase". All that needs to be done is to make the upwards push a greater impulse than the downwards push - the rocket would essential jerk its way upwards - after all there is no "conservation of displacement" with such an inertial system. There is conservation of momentum, and you just proposed to violate it. This is a stiction (static friction) drive, and works only when in contact with some other body. The movement comes from the difference between static and sliding friction. And, BTW, if you want a drive that only works when in contact wiith a larger body, there are much better ones (the wheel comes to mind). Such a drive is utterly useless in space, however. Jerk your rocket around all you want, it'll never make any net progress at all. Sure you can, read above. |
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"John Schoenfeld" wrote in message om... Certainly not. You could add holes along the side of the rocket so that the air-pressure can let the explosion exit without affecting the overall net upwards momentum. Although this would be inefficient, it demonstrates that the CAUSE of upwards acceleration is IRRESPECTIVE of the exit direction of expelled mass. No you can't since the "explosion" exiting would now push against the rocket from all sides and the rocket would not move. No it is not. Imagine a stationary black-box floating in space. One wall of the box is hard iron and the opposite side is ellastic. If a ball is thrown from the middle at the hard iron wall there will be a high-impulse transfer of momentum from the ball to the box. First, you've already ignored, "what throws the ball." Hint, if it's attached to the box (saw a cannon) the box will move backwards until the ball hits the front wall. At that point the box will move forward and end up where it started. No net movement. If the cannon is not attached, it will be propelled backwards, hitting the back wall, forcing the box backwards. The ball hitting the front will then push the box forwards the same amount. No net movement. Relative from the center of the box (which at this point is moving), the ball now approaches the opposite ellastic wall in which it inevitably collides with and transfers the same momentum but in the opposite direction bringing the box to rest again. However, the elastic wall collision was low-impulse and took longer for the momentum to be conservered. Irrespective of momentum conservation, there is an overall displacement. No, there isn't. The box returns to its original position. At this point we have the box at rest yet it is displaced from its original position, however in future time this same effect will occur but in the opposite direction and thus the overal motion of this contraption would be to OSCILLATE about the original position. So technically speaking, its not inertial propulsion yet as the center of mass is constant. THat part is right. All it will do is oscillate. So the third and final requirement would to have a constant stream of balls colliding just as the first one thus always staying one step ahead of the "backwards oscillation phase". Again, this simple doesn't work. All that needs to be done is to make the upwards push a greater impulse than the downwards push - the rocket would essential jerk its way upwards - after all there is no "conservation of displacement" with such an inertial system. There is conservation of momentum, and you just proposed to violate it. This is a stiction (static friction) drive, and works only when in contact with some other body. The movement comes from the difference between static and sliding friction. And, BTW, if you want a drive that only works when in contact wiith a larger body, there are much better ones (the wheel comes to mind). Such a drive is utterly useless in space, however. Jerk your rocket around all you want, it'll never make any net progress at all. Sure you can, read above. |
#10
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In article ,
John Schoenfeld wrote: ... ... ellastic. ... ... ... ellastic ... ... I have often wondered why idiosyncratic misspelling so often coincides with crackpotism. Anyone know? (It is certainly a helpful shibboleth.) |
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