If the pipe travels its length, then obviously none the air in it can remain where it was. Or it would become outside the pipe, left behind floating in space. So any air that was going in the opposite direction to the pipe has to change direction. That imparts a force to the pipe pushing it backwards.

There really is no way to make progress on this without either:
- Using the law of conservation of momentum or
- Working it out in more detail without giving up just before you get to the solution.

Claims of "there's nothing to stop it" have no value, because they're just claims without reasons.

Edited by kallog (03/10/11 02:25 PM)