Aerodynamic drag is the resistance (retarding effect) applied to a body as it passes through the air. It is made up of two components, caused by pressure and by friction. Resistance caused by pressure occurs because there is a difference in air pressure between the front and rear surfaces of the body. This has to be overcome to keep the body moving through the air, and is particularly significant in the case of relatively flat-surfaced, bulky objects such as motor vehicles. If the body moving through the air is slim and streamlined, however, like a modern aircraft, resistance due to friction represents a higher proportion of total aerodynamic drag. It is caused by the airflow rubbing against the surfaces as it passes over them.
Aerodynamic drag increases at the same rate as the square of the air speed. In other words if the speed doubles, the drag increases fourfold; if it quadruples, it increases sixteen-fold. See also: cd value.