Drag is a force of resistance when a body is

moving in water or air. In air, it is

aerodynamic drag. It opposes the thrust

force. It opposes forward motion.

Direction of drag is in opposite direction to

the moving object.  Firstly drag is directly

proportional to the velocity of the fluid in

a laminar flow. Secondly drag is

proportional to the square of the velocity

in a turbulent flow. At high speeds, almost

50 % power is spent in overcoming drag.

Aero-foil design has been found the best

to reduce drag. It is used in various vehicles.

Factors on Which Drag Force Depends

  1. Viscosity of the fluid (water or air)
  2. Upstream velocity
  3. Density of the fluid
  4. Shape, size and contour of the moving object in the fluid


Sr. No.

Name of drag

Cause of drag

How to reduce the drag?

Parasitic drag—It is of two types
Form drag or pressure drag
Drag depends on the shape of the body moving in a fluid.  For example, A body having larger cross section and blunt shape will have larger form drag and vice versa.
Make the shape like an aerofoil to reduce drag
 Skin friction drag
It is due to the surface roughness of the body moving n a fluid.
Make the surface more smooth
Profile drag
 Sum of skin friction drag and form drag is total drag.
Interference drag
Drag is due mixing different stream lines between aero-plane components such as for wings and fuselage at the wing.
Install the different parts to have minimum interference
Lift Induced drag
There is a lift force in a flying aircraft. It keeps the aircraft in air. During the stable flight lift is equal to the weight of the aircraft. Drag produced due to lift is called lift induced drag.
Wave drag
The drag produced at sonic and super- sonic speeds. These high speeds produce shock waves. These shock waves produce the wave drag.
Try to keep speed below sonic speed


Drag force is proportional to the square of the velocity and given by the equation
FD = CDρV2 A/2
Where FD is the drag force
CD is the drag coefficient
It depends on shape size and contour of the moving object. This also depends on Reynolds number. Drag coefficient is a dimensionless quantity
ρ is the density of the fluid
V is the relative velocity of the fluid
A is the projected surface area of the object perpendicular to the motion of the fluid
Drag force always opposes motion. It consumes power which is actually a waste. This power consumed is proportional to the cube of the relative velocity. Thus drag is very high at high speeds. It should be reduced to a minimum. It is achieved by adopting an aero foil design of objects moving in water or air.


The velocity at which drag force becomes equal to the weight is called a terminal velocity.
FD = mg
mg = CDρV2 A/2
m is the mass of the moving object
CD is the drag coefficient
ρ is the density of the fluid
V is the relative velocity
A is the surface area of the object normal to the fluid.

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