Drag is the resistance of a fluid to the motion of a body through it. In order for a body to move through a fluid, the particles of the fluid must move out of the way of the body. For this to happen the fluid particles must be subjected to a force. According to Newton's third Law, an equal and opposite force will be exerted on the object. The drag force is always in the opposite direction to the motion of the body. For small speeds the drag force will be proportional to the speed, but as the speed increases higher powers of the speed will become more important, as shown below.

In many circumstances, drag is a nuisance. To maintain a constant speedsome other external force must be applied. In a powered vehicle this force is supplied by the engine: If the drag force has magnitudeand no other forces act, the engine must work at a rateMany efforts are made to reduce drag, by streamlining for example.