Originally Posted By: paul

you state that the momentum moves the pipe , and you obviously are talking about the momentum of the air in the pipe , and then you state that at the same time the air
is stopping the pipe.

Yes, the air both starts it and stops it, not at the same time of course because each air molecule takes some time to slow down.


I cant understand your reasoning that the pipe will stop?

if you dont mind could you explain at which point the pipe
looses all momentum and stops?

I'll simplify it to be a gun instead of a rocket, and it's fixed to the front of the pipe, pointing backwards.

You fire the gun, and it gives momentum -p1 to the bullet. At the same time it gives momentum p1 to the gun and pipe. Exactly the same as recoil in a normal gun.

Clearly the pipe's now moving because it has momentum p1.

Eventually the back end of the pipe and the bullet collide, and the bullet becomes embedded in the back of the pipe. They were travelling in opposite directions, with zero total momentum (-p1 + p1 = 0). Now they're a single solid object with the same total momentum - 0.

The pipe has 0 momentum and has therefore stopped moving.

Please let me know if you think:

- Using a bullet analogy is a good enough idealization whether my explanation is correct or not.

- You agree with it but think it differs too much from the air jet case.