Drag Formula
Drag force is the resistance of a fluid, the force that it applies acting opposite to the motion of an object that is moving submerge in a certain fluid.
Drag = (density) * (square of the velocity) * (Drag coefficient) *(transversal area)
The equation is written
FD = ½ ρ * v2 * CD * A
We have:
FD: Drag force
ρ: fluid density
v: Relative velocity between the fluid and the object
CD: Drag coefficient
A: Transversal area or cross sectional area
Drag Formula Questions:
1) A car travels with the speed of 90 km/h have a drag coefficient of 0.32.
If the area of cross section is 5 m2, what is the drag force? The air density is 1.2 Kg/m3.
Answer: Using the formula for drag force, we can substitute directly,
FD = ½ ρ * v2 * CD * A
FD = ½ (1.2 Kg/m3)*(25 m/s)2 * 0.32 * 5 m2 = 600 Kg*m/s2 =600 N
2) what is the drag coefficient of an object moving in water and that feels a drag force of 500 N? The sectional area is 2 m2 and is moving at 30 m/s.
Answer: From the formula of drag we can find the drag coefficient,
CD = 2 * FD / (ρ * v2 * A)
Substituting the values in the formula above, with the density of the water 1 Kg/L,
CD = 2 * 500 N / (1 Kg/L * (30 m/s)2 * 2 m2)=
CD = 2 * 500 N / (1000 Kg/m3 * (30 m/s)2 * 2 m2)= 0.00055
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