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…