# Equation for drag force

1. Oct 3, 2009

### jason bourne

I know that standard formula is, FD = 1/2 CD *ApV^2

FD = Drag Force. SI: N
CD = Drag Coefficient. SI: Dimensionless (Typical Values)
A = Coss-sectional Area perpendicular to the flow. SI: m2
r = Density of the medium. SI: kg/m3
v = Velocity of the body relative to the medium. SI: m/s

But our prof also said theres another formula for drag force,

FD = 1/4 AV^2

(Its is not supposed to be equal but approximately)

So the question is when is the equation above false, whats the error in the equation that makes it approximate.