water jet-pack
I said I would come back to this, and I am.  I am a man of my word.  Yesterday, I posted a link to a video of this really cool water rocket thingy.
How does this work?  What is the physics going on here? I think this can be best explained with the momentum principle.
Let me start by pretending like I have some object that shoots out a piece of water (or really it could be anything).  Also, let me pretend like this is in space ore something where there are no external forces.
Above is a before and after picture.  Initially, there is something (the box) with water inside.  Through some process…