Originally Posted By: ImagingGeek

We all agree it is a body - 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

Careful there! Nature doesn't care if the ladder's bolted to the inside of the tube or to the outside.

It all depends what you define as the 'body'. If you say it's the pipe, then yes, Paul's right, it can be accelerated by things inside it, they're not the pipe, they're other things. You run into problems if you try to move the pipe further than you move the contents tho.

If you define the 'body' as the collection of the pipe and its contents, then conservation of momentum applies to the center of mass of the collection of things.

Well I'm sure you knew that, but getting careless is no way to sort out a disagreement!