EPTOn September 27 2008 15:40 evanthebouncy! wrote:
don't u tell Micronesia what to do ;(
don't u tell Micronesia what to do ;(
Was this directed at Sonuvbob? If so, don't worry and just reread what he said hehe.
Thinking Physics Question 2 - Page 3
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micronesia
United States24799 Posts
Was this directed at Sonuvbob? If so, don't worry and just reread what he said hehe. | ||
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Purind
Canada3562 Posts
If the question is unclear and it turns out it makes assumptions different from yours, you can complain in the next topic and keep talking about how right you were | ||
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Klockan3
Sweden2866 Posts
If someone finds anything wrong with this please poke it, but for now I believe that most in this thread are wrong: + Show Spoiler + Actually, I am pretty sure that we would have a net force on the can during the fillup process, and once it is full we would have an equilibrium again and there is no way air resistance would stop it in an instant. This do not break any of newtons laws since when it is absorbing air it will reflect less molecules and thus while it will be moving towards the hole the closed system would start moving in the other direction due to the air pressure getting slightly lower on one end. (Or the average of the gas will be moving in the direction the can is not moving) The easiest way to picture this is to imagine a square. Remove one of the walls and imagine that its interior gets filled by gas molecules, until the molecules who entered fills the whole square up the air pressure on the other side of it will create a net force and thus speed it up, and the moment it is full it will have a net force of 0 on both sides if we are neglecting the air resistance(And we can do that since air resistance will not slow it to a dead stop in such a limited time interval). The net force on the square gets exactly evened out by the net loss of reflected air from the removal of one of the walls. If you want an open system just imagine that we have an artificial air around it, all with a speed directed towards the can and only in a volume to have exactly enough air to hit the can untill it has reached the critical point were we stop observing it. In such a system we would get a gas sphere around the can except were the hole is, there we would also get an air hole in the sphere, and that is were the momentum gained by the can is taken from. In the end, the answer is B: It is moving to the right. I can not believe that so many missed this... Also yes even if the hole is extremely small, the fact that there is no air resistance unless the object is moving, and that until the last of the air have moved in there is always a net force working on the can from the higher pressure from the wall wo a hole it got to be moving at the time when equilibrium is reached. | ||
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LaLuSh
Sweden2358 Posts
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betaben
681 Posts
On September 28 2008 10:25 Klockan3 wrote: Much wrong use of the laws of Newton in this thread. If someone finds anything wrong with this please poke it, but for now I believe that most in this thread are wrong: + Show Spoiler + Actually, I am pretty sure that we would have a net force on the can during the fillup process, and once it is full we would have an equilibrium again and there is no way air resistance would stop it in an instant. This do not break any of newtons laws since when it is absorbing air it will reflect less molecules and thus while it will be moving towards the hole the closed system would start moving in the other direction due to the air pressure getting slightly lower on one end. (Or the average of the gas will be moving in the direction the can is not moving) The easiest way to picture this is to imagine a square. Remove one of the walls and imagine that its interior gets filled by gas molecules, until the molecules who entered fills the whole square up the air pressure on the other side of it will create a net force and thus speed it up, and the moment it is full it will have a net force of 0 on both sides if we are neglecting the air resistance(And we can do that since air resistance will not slow it to a dead stop in such a limited time interval). The net force on the square gets exactly evened out by the net loss of reflected air from the removal of one of the walls. If you want an open system just imagine that we have an artificial air around it, all with a speed directed towards the can and only in a volume to have exactly enough air to hit the can untill it has reached the critical point were we stop observing it. In such a system we would get a gas sphere around the can except were the hole is, there we would also get an air hole in the sphere, and that is were the momentum gained by the can is taken from. In the end, the answer is B: It is moving to the right. I can not believe that so many missed this... Also yes even if the hole is extremely small, the fact that there is no air resistance unless the object is moving, and that until the last of the air have moved in there is always a net force working on the can from the higher pressure from the wall wo a hole it got to be moving at the time when equilibrium is reached. air resistance is not the only way the can can slow down. the air that enters the bottle will have a net velocity to the back of the can, which on average should slow it down (although the pressure would equalise before this happens, as it must pass through the equilibrium point before it reaches a higher pressure at the back wall.) thus, at equal pressure, the system is still moving, with the air which has entered the bottle possibly still moving with a net velocity in the opposite direction to the bottle wrt the bottle. the bottle moves because of the pressure on the side opposite to the hole, but the air entering the hole has it's own net velocity in the opposite direction, as it is forced in by the pressure outside the hole. + Show Spoiler + my previous answer confused the letter with the direction. I think the bottle will move towards the hole, will still be moving as the pressure equalises, but the air inside will be moving opposite to the can to slow it down. momentum has to be conserved eventually. this is why it needs to be decided what 'filled' means. | ||
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Klockan3
Sweden2866 Posts
On September 28 2008 12:39 betaben wrote: air resistance is not the only way the can can slow down. the air that enters the bottle will have a net velocity to the back of the can, which on average should slow it down The fact that the air moves faster in one direction relative to the bottle compared to the other is air resistance and there are no other slowing forces. | ||
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