Drag force is the resistance an object moving in a fluid, such as water or air, experiences. It reacts in the opposite direction of how an object is moving, hence slowing down the moving object. The strength of drag force depends on several factors that include an object's speed, the shape of the object, fluid density, and the area of the object facing the fluid.

Drag force is fundamental in many real-life applications: in aviations, it determines the automotive design, and sports play a great role in utilizing drag forces. For instance, at each step, airplanes or aerodynamic vehicles are made to be as drag-less as possible in order to ensure maximum fuel intake. Likewise, an athlete may use gear in a sport to lessen the drag force. This shows that with knowledge of the drag force, we can analyze and optimize the motion of an object within a fluid.

Formula

This is the formula that is used for determining the drag coefficient.

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 km.h-1

A is the cross-sectional area in m2

Solved Examples

Question 1: A car is moving with a speed of 80 km.h-1 whose drag coefficient is 0.25. Determine the drag force if the cross-sectional area is 6 m2.

Solution:

Given,

Velocity, V= 80 km.h-1

Drag coefficient, Cd= 0.25

Cross-sectional area, A= 6 m2

Density of fluid, ρ =1.2 kg.m-3

The drag force is given as

D=1.6 N

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

Solution:

Given,

Velocity, V=600 km.h-1

Drag coefficient, Cd= 0.25

The density of fluid, ρ=1.2 kg.m-3

Cross-sectional area, A=110 m2

Drag force is given by