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Net aerodynamic or hydrodynamic force: Drag acting opposite to the direction of movement of a solid object such as cars, aircraft, [3] and boat hulls. In the physics of sports, drag force is necessary to explain the motion of balls, javelins, arrows, and frisbees and the performance of runners and swimmers. [6]
Airships and some bodies of revolution use the volumetric drag coefficient, in which the reference area is the square of the cube root of the airship volume (volume to the two-thirds power). Submerged streamlined bodies use the wetted surface area.
The force on an object that resists its motion through a fluid is called drag. When the fluid is a gas like air, it is called aerodynamic drag or air resistance . When the fluid is a liquid like water it is called hydrodynamic drag, but never "water resistance".
Aerodynamic drag, sometimes called wind resistance, is an effect which a vehicle must overcome more and more as speed increases. Drag is a form of friction, though some of the air's effect on the vehicle is due to a low pressure zone behind the vehicle, and a high pressure zone in front.
At highway speeds, over 50% of the power of a car is used to overcome air drag. The most fuel-efficient cruising speed is about 70–80 km/h (about 45–50 mi/h). For this reason, during the 1970s oil crisis in the United States, maximum speeds on highways were set at about 90 km/h (55 mi/h).
Air resistance depends on the shape of the body (object) and the speed it’s travelling. Since drag force increases with speed, air resistance becomes important when objects move faster. A racing cyclist adopts a more streamline posture to reduce the effects of air resistance.
As the vehicle moves forwards through the air the air rushes past the vehicle. This causes friction, which we call drag, and this slows the vehicle down. Large amounts of money are spent by the designers of cars to try and make a car which has a little drag as possible.
