# Drag equation ( for fluids)

## Description

Drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) refers to forces acting opposite to the relative motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers (or surfaces) or a fluid and a solid surface.

The drag equation is a formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid. The reference area is typically defined as the area of the orthographic projection of the object on a plane perpendicular to the direction of motion.

The drag equation with a constant drag coefficient gives the force experienced by an object moving through a fluid at relatively large velocity (i.e. high Reynolds number, Re > ~1000). This is also called quadratic drag.

## Variables

F_{D} | Drag force ( by definition the force component in the direction of the flow velocity) (N) |

ρ | Mass density of the fluid (kg/m^{3}) |

v | Velocity of the object relative to the fluid, (m/s) |

C_{D} | Drag coefficient (related to the object's geometry) (dimensionless) |

A | Area of the orthographic projection of the object on the plane perpendicular to the direction of motion. (m^{2}) |