# Drag Force Formula

## Drag Force Formula

Drag Force (D) is defined as the force that resists the motion of a body with fluid. If the motion of the body exists in the fluid like air it is known as aerodynamic drag. And, if the fluid is water it is known as hydrodynamic drag. The drag force always acts in the opposite direction to the flow of velocity.

## Drag Coefficient Formula

Following is the formula used to calculate the drag coefficient:

 $D=\frac{C_{d}*\rho *V^{2}A}{2}$

Where,

• Cd is the drag coefficient
• ρ is the density of the medium in kg.m-3
• V is the velocity of the body in m.km-1
• A is the cross-sectional area in m2

### Solved Examples

Question 1. A car travels with the speed of 80km.h-1 with a drag coefficient of 0.25. If the cross-sectional area is 6m2, calculate the drag force.

Solution:
Given:
Velocity, V=80km.h-1

Drag coefficient, Cd= 0.25

Cross-sectional area, A=6m2

Density of fluid, ρ=1.2kg.m-3

The drag force is given as:
$D=\frac{C_{d}*\rho *V^{2}A}{2}$

$D=\frac{0.25*1.2\frac{kg}{m^{2}}6400\frac{m}{km}6m^{2}}{2*3600}$

D=1.6N

Question 2. A plane moves with the velocity of 600km.h-1 with a drag coefficient of 0.25. If the cross-sectional area of the plane is 110m2, calculate the drag force.
Solution:
Given:
Velocity, V=600km.h-1

Drag coefficient, Cd= 0.25

Density of fluid, ρ=1.2kg.m-3

Cross-sectional area, A=110m2

The drag force is given as:
$D=\frac{C_{d}*\rho *V^{2}A}{2}$

$D=\frac{0.25*1.2\frac{kg}{m^{2}}360000\frac{m}{km}110m^{2}}{2*3600}$

D=1650N