Originally Posted By: paul
the force applied to the sides would not cause the pipe to move at all because all sides would feel the same force.

I didn't mean normal force against the wall, but longitudinal force in the direction of the air's initial motion. ie. parallel to the pipe walls. For a long pipe most of the air's momentum will be transferred to the pipe walls instead of the end cap. But in both cases it's in the same direction.

If you could somehow eliminate the drag on the pipe walls, then even for a long pipe the entire longitudinal momentum of the air when it was released from the tank will be transferred to the cap at the other end of the pipe.

Either way, no motion further than the length of the pipe.