Navier–Stokes equations
They are very useful because many important things flow. Scientists and engineers use the equations in mathematical models of weather, ocean currents, water flow in a pipe, how air flows around a wing, and how stars move inside a galaxy. The Navier–Stokes equations in their full and simplified forms help with the design of aircraft and cars, the study of blood flow, the design of power stations, the analysis of pollution, and many other things. Together with Maxwell's equations (the equations for electricity and magnetism) they can be used to model and study how things that can flow and conduct electricity can produce (and react to) magnetic fields.