Originally Posted By: paul

those forces could come from within the pipe , such as the
momentum of the air.

If you consider the "thing" to be the pipe alone, then yes, that's fine, obviously anything inside the pipe is not part of the pipe, so it's classed as an external force. If you consider the "thing" to be the pipe and its contents, then it's the center of mass which maintains constant velocity, even for a normal rocket the center of mass doesn't accelerate - rocket goes one way, exhaust goes the other way, CoM stays at the same velocity as per the 1st law.


and you climb the ladder , you must place a force in a direction
in order to climb the ladder.

I'm glad you brought it back to a simple and easy to analyse system.

Yes, I agree with all your reasoning, but you omitted one important part. How do you stop at the top of the ladder? You have to apply a force to the ladder/pipe in the opposite direction. You might do it very slowly so you hardly feel it, but the total impulse (= force * time for constant force) has to be the negative of the impulse you used to accelerate. It decelerates both you and the pipe back to being stationary.

When it's all over, the pipe has moved a short distance then stopped.