
Compute Drag Force, Velocity, or Area 

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Introduction
Drag force is caused by a fluid (such as water or air; or any liquid or gas) impinging
upon an object. The drag force is a function of the fluid velocity and density along with
the object's reference area and drag coefficient. The drag coefficient may further be a
function of the Reynolds number. Reynolds number depends on the fluid density, viscosity,
and velocity as well as the object's characteristic length.
Drag Force Equation (Blevins, 2003 and Munson et al.,
1998 and others)
F = 0.5 C ρA V^{2}
Notation
(ft=feet, g=gram, hr=hour, kg=kilogram, lb=Pound (force), m=meter, N=Newton, s=second)
Our calculation allows you to use a variety of units with all of the conversions completed
internally. The units below are consistent units for the drag force equation.
A = Reference area as (see figures above), m^{2}.
C = Drag coefficient (see figures above), unitless.
F = Drag force, N.
V = Velocity, m/s.
ρ = Density of fluid (liquid or gas), kg/m^{3}.
Drag coefficients for various objects (photographs taken by
Ken Edwards).
Tree (C values from Munson et al., 1998) A=Tree frontal area C=0.43 if V=10 m/s (36.0 km/h, 22.4 mph) C=0.26 if V=20 m/s (72.0 km/h, 44.7 mph) C=0.20 if V=30 m/s (108 km/h, 67.1 mph) 
Flag (C values from Munson et al., 1998) A=DL C=0.07 if L/D=1 C=0.12 if L/D=2 C=0.15 if L/D=3 
Thin Rectangular Plate (C from Blevins, 2003) Flow coming at the sign A=DL C=1.05 if L/D=1.0 C=1.10 if L/D=2.0 C=1.12 if L/D=4.0 C=1.20 if L/D=8.0 C=1.22 if L/D=10.0 C=1.22 if L/D=12.0 C=1.33 if L/D=17.8 C=1.90 if L/D=infinity 
Messages
"Need Density > 0", "Need C > 0", "Need
Area > 0", "Need Velocity > 0", "Need Force > 0".
References
Blevins, Robert D. 2003. Applied Fluid
Dynamics Handbook. Krieger Publishing Co.
Munson, Bruce R., Donald F. Young, and Theodore H. Okiishi. 1998. Fundamentals of Fluid
Mechanics. John Wiley and Sons, Inc. 3ed.
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