Understanding the fundamentals of fluid mechanics is crucial for any analysis involving a system whereby a fluid serves as the working medium. Fluid mechanics is used to the design of practically all transportation modes. Knowledge of fluid mechanics is necessary for the design of subsonic and supersonic aircraft, ground effect machines, hovercraft, airplanes that take off and land vertically with a minimal surface ships, runway length, submarines, and vehicles. Even within fluid mechanics, there are subfields. Hydrodynamics is the field of study that focuses on the study of the movement of fluids that may be roughly considered incompressible. Hydraulics is a subfield of hydrodynamics that studies the movement of liquids across closed and open channels. The study of fluids whose density drastically varies during motion is known as gas dynamics; this includes the passage of gases via nozzles at high velocities. At either high or low speeds, aerodynamics examines how gases (particularly air) move over moving bodies like airplanes, rockets, and cars. Natural flows are the domain of many different subdisciplines, including meteorology, oceanography, and hydrology.