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# 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.

Related formulas

## Variables

 FD Drag force ( by definition the force component in the direction of the flow velocity) (N) ρ Mass density of the fluid (kg/m3) v Velocity of the object relative to the fluid, (m/s) CD 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. (m2)