In physics, the drag equation gives the drag experienced by an object moving through a fluid. Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials have different properties, and so forth. ... For a solid object moving through a fluid or gas, drag is the sum of all the aerodynamic or hydrodynamic forces in the direction of the external fluid flow. ... A subset of the phases of matter, fluids include liquids, gases, plasmas and, to some extent, plastic solids. ...

where

D is the force of drag,

ρ is the density of the fluid*,

v is the velocity of the object relative to the fluid,

A is the reference area, and

C_d is the drag coefficient (a dimensionless constant, e.g. 0.25 to 0.45 for a car).

* Note that for the Earth's atmosphere, the density can be found using the barometric formula. In the case of air as a fluid at 15 °C and standard atmospheric pressure: In physics, a force is an external cause responsible for any change of a physical system. ... Jump to: navigation, search Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... // Basic explanation The velocity of an object is simply its speed and its direction. ... This article explains the meaning of area as a physical quantity. ... The drag coefficient (Cd or Cx) is a number that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation. ... In dimensional analysis, a dimensionless number (or more precisely, a number with the dimensions of 1) is a pure number without any physical units; it does not change if one alters ones system of units of measurement, for example from English units to metric units. ... In mathematics and the mathematical sciences, a constant is a fixed, but possibly unspecified, value. ... The Barometric Formula,sometimes called the exponential atmosphere, is a formula used to model how the pressure (or density) of the air changes with altitude. ... Saturns atmosphere is made up of hydorgen, helium and methane ...

• Density= 1.225 kilograms per cubic meter or 0.002378 slugs per cubic foot.

The reference area A is related to, but not exactly equal to, the area of the projection of the object on a plane perpendicular to the direction of motion (ie cross-sectional area). Sometimes different reference areas are given for the same object in which case a drag coefficient corresponding to each of these different areas must be given. The reference for a wing would be the plane area rather than the frontal area. Cross section may refer to the following In geometry, Cross section is the intersection of a 3-dimensional body with a plane. ...

The equation is based on an idealized situation where all of the fluid impinges on the reference area and comes to a complete stop, building up stagnation pressure over the whole area. No real object exactly corresponds to this behavior. C_d is the ratio of drag for any real object to that of the ideal object. In practice a rough unstreamlined body (a bluff body) will have a C_d around 1, more or less. Smoother objects can have much lower values of C_d. The equation is precise, it is the C_d (drag coefficient) that can vary and is found by experiment. Jump to: navigation, search Pressure (symbol: p) is defined as the magnitude of the normal force divided by the area over which the normal force acts. ... The drag coefficient (Cd or Cx) is a number that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation. ...

Of particular importance is the v² dependence on velocity, meaning that fluid drag increases with the square of velocity. Contrast this with other types of friction that generally decrease with velocity. Jump to: navigation, search In physics, friction is the non-conservative resistive force that occurs when two surfaces travel along each other when forced together. ...

See also

• angle of attack
• stall
• terminal velocity