Il coefficiente di resistenza aerodinamica (anche noto con i simboli C_D o C_x) è un coefficiente adimensionale che misura la resistenza aerodinamica di un corpo in moto in un fluido. Dipende dalla forma del corpo ed è indipendente dalla sezione o dalla velocità cui viene effettuato il moto.

The power required to overcome the aerodynamic drag is given by:

Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

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