Firstly, this is not homework - it's a project I have started our of sheer curiosity. I have searched quite at length, but cannot really find anything that suits my problem. I want to model this on a computer and would like to implement it myself as I would like to understand the mathematics behind it rather than just using someone else's work. I want to model (only in 2D for now) an agent moving in a liquid. The agent may have flippers to propel it or move in a snake-like fashion (which I think, is basically flippers without a body). Is there any article that discusses the movement of the agent in such an environment. In a thought experiment I started with the following: 1. In a vacuum, the agent can flip all it wants, it will not move. 2. On top of solid (say a floor), the agent's movement pushing against the floor will push against the solid and all the force will result in acting against gravity and moving forward (depending on the angle). I guess I need something in between these two cases, so I suspect I need some way to model the viscosity of the liquid. How do I do that? The modelling does not have to be mathematically perfect - even a close approximation will be great. Any good articles? ...or a point in a direction?