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.