Equation for drag force

  • #1
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.
 

Answers and Replies

  • #2
Baluncore
Science Advisor
9,165
3,702
Such a simple equation assumes that CD⋅ρ = 0.5
Where ∝ means proportional to; FD ∝ ¼⋅A⋅V2
can be further simplified to; FD ∝ A⋅V2
That can only be applied where changes to area or velocity occur, while maintaining a constant profile in a constant density fluid.
 

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