Originally Posted By: paul
but note you need an external force. Whether you're talking about a tank of air, a gun, or guys running up & down ladders, all of those forces are internal

it doesnt matter where the force comes from , if it comes from inside the pipe or outside the pipe.
its pretty stupid of you to think that it does.

LOL, this from a guy whose continually shown a complete ignorer of newtons laws, and who thinks homeopathy is anything but a scam.

Internal forces are always balanced - equal and opposite as per newtons 3rd law. Ergo, external forces or the ability to impinge a force on the external environment are an absolute requirement for changes in momentum.

Originally Posted By: paul

if you were sitting inside a large trash can on earth
that is completely sealed.

then the trash can suddenly was in deep space.

and you stood up inside the can.

according to your understanding of physics the trash can would not move as you stand.

Nope, that is not what I said, not even close.

A trash can is open - I'd knock the lid off and fly out the top; the trash can would go the other way with equal momentum.

But if the trashcan was sealed, and large enough for me to stand, then when I stand the can would go one way, I the other. When I hit the top of the can the resulting force would cease all of the movement. End effect - can shifted one way, myself shifted the other, but the net center of mass exactly where it was when I started and the total momentum unchanged.

Originally Posted By: paul

I say the can moves away from you , because of the force that you applied to the can while standing.

But in the case you describe, the can is an open system - exactly what kellog and I have been telling you is needed since day 1.

If the lid were welded on - i.e. the trash-can equivalent of your pipe - the can wouldn't go anywhere. No changes in momentum are possible without external forces.