Drag Formula

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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