Aerodynamics, the branch of physics that deals with the motion of air and with the relative motion between air and objects in the air. The bodies in the air can be moving (for example, aircraft, missiles, birds, and automobiles), or they can be stationary structures (for example, bridges and buildings) acted upon by moving air. Aerodynamicists are concerned mainly with the design of airplanes and, to a lesser extent, automobiles. The flight of birds demonstrates many of the principles of aerodynamics.