Originally Posted By: paul
I think thats a load !!!


Then you think wrong. A space ship is a closed system...but we'll get back to that.

Originally Posted By: paul
if I have a nozzle at one end of a long pipe and the entire pipe is at 14.7 psi.

lets say the pipe is 60 inch diameter and 500 ft long.
and its inside your space ship.

and I have a compressor at the other end of the pipe that compresses air to 100 psi.

if I release the compressed air through the nozzle the reaction of the thrust will be felt by the pipe until the pressure in the pipe reaches 100 psi.

and if the compressor can maintain the 14.7 psi then
this can be continuous.


Absolutely. But you're missing the "closed system" bit.

Originally Posted By: paul
not only that but the friction between the pipe and the compressed air in the pipes that carries the compresed air back to the nozzle will also be a force in the direction of movement.


And here is where you are missing the closed system bit.

Based on your description you have what could be considered a linear air accelerator - air goes in the front, is compressed, and is then forced out the back. Thus giving you a vacuum at the front, providing a forward "pull", and a rear-facing nozzel, giving a "push". On earth this system would give you a great deal of thrust - in fact, add a bit of fire and you've got a jet engine.

Problem is, if you're spraying that air inside of a space ship that thrust disappears - for two reasons. Firstly, nature abhors a vacuum, and your system is generating one at the front of the pump. Hence, there will be an flow of air towards the front of your air pump equal to the air flow entering your pump. Since that air can only come from within your ship, that vacuum will be replaced by ship-board air, and the movement of that air will exert a force equal to the force produced by the vacuum. Newtons laws - opposite and equal reaction.

Secondly, the air coming out of your nozzle has a lot of momentum. That momentum is equal to, but in the opposite direction of, the thrust produced by the nozzle end of your engine. Since your ship is a closed system that air cannot leave the ship, and as such has no option but to transfer that momentum back to the ship. This transfer will occur in the form of impacts between the moving column of air and the walls, etc of your ship. This transfer of momentum back to the ship is in a direction opposite to the thrust of your engine. So just as with the "vacuum thrust", the amount of momentum transfered to the ship will be equal to the momentum of the moving column of air, which in turn is equal to the thrust of your engine. Net effect - zero nozzle thrust.

So:
Vacuum thrust = 0
Nozzle thrust = 0
------------------
Total thrust = 0

Originally Posted By: paul
according to your way of thinking if there was a hose attached to the place where the nozzle is attached instead of a nozzle and the hose was just laying there on the floor and someone turned on the air valve so that air could come out , the hose would just sit there , and it wouldnt move because you were taught that , and you could just walk up to it and pick it up.


Not even close to what I said. The hose will quite efficiently transfer the thrust of the air, resulting in movement of the hose. However, my workshop (equivalent in this case to your spaceship) will stay put as the thrust of that hose is countered because the energy of the air coming out of the hose is transfered to the air and walls of my workshop, producing a net thrust (in the context of my workshop) of zero.

So the hose will flow around and knock the unaware in the head. But people standing outside of my shop are 100% safe from being run over by a moving shop - momentum, in the context of the shop - is zero.

Originally Posted By: paul
me , I would want the air pressure to be turned off first. so the hose wouldnt slap me in the head as I approached it.

so if the hose will move then if I held the hose , the ship would move.


Only if the contents of that hose are free to leave the ship. If they are not, the momentum of the air will be transfered back to the ship, providing a net zero thrust.

Originally Posted By: paul
the problem here is that when you are driving down "think street" and you see a "do not enter" sign you do not enter , instead you turn onto "do not think street" , I dont see any "do not enter" signs.

so I stay on "think street"


LOL, big words for a guy obviously lacking an understanding of basic physics. You cannot generate momentum in a closed system that imparts a net momentum on the closed system itself. You ship is such a closed system.

Open the system though - eject that air out of the ship itself - and you'll move along quite nicely.

Here's the physics you seem to be missing. Read the pages, then come back and we'll talk:

http://en.wikipedia.org/wiki/Closed_system
http://en.wikipedia.org/wiki/Thermodynamic_system
http://en.wikipedia.org/wiki/Newtons_laws
http://en.wikipedia.org/wiki/Momentum

Bryan
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