A force acting on a moving body, opposite in direction to the movement of the body, caused by the interaction of the body and the medium it moves through. The strength of drag usually depends on the velocity of the body.♦ Drag caused by buildup of pressure in front of the moving body and a decrease in pressure behind the body is called pressure drag. It is an important factor in the design of aerodynamically efficient shapes for cars and airplanes.♦ Drag caused by the viscosity of the medium as the molecules along the body's surface move through it is called skin drag or skin friction. It is an important factor in the design of efficient surface materials for cars, airplanes, boat hulls, skis, and swimsuits. Compare lift. See Note at aerodynamics.

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