Drag Force: Types and Examples

Drag force is a type of resistive force that opposes the relative motion between an object and a fluid.

• This force acts parallel in the opposite direction of the relative motion.
• Total resistive force experienced by a body in a fluid has two components: Drag force and Lift force.
• The parallel component of the resistive force is known as the Drag force.
• The perpendicular component of the resistive force is known as the Lift force.
• Any substance that does not have a fixed shape is known as fluid. 
• Air and water are examples of fluids.
• Drag force acting on an object moving in a fluid such as air, is known as aerodynamic drag.
• While for an object moving in a fluid such as water, it is known as hydrodynamic drag.
• Drag force is a contact force since it is not produced by force fields such as electromagnetic force or gravitational force.

Some examples of drag force are

• An airplane moving in the air
• A submarine moving in water
• Flow of air over buildings

Key Terms: Relative motion, Drag force, Lift, Frictional force, Fluids, Motion, Aerodynamic drag, Hydrodynamic drag

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Concept of Drag Force

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The drag force is defined as

The resistive force that occurs whenever there is a relative motion between an object and a fluid.

• It is similar to the force of friction that occurs between two moving solid bodies.
• The friction between the molecules of the fluid and the surface of the object is the main cause of drag force.
• It is a vector quantity i.e. it has both magnitude as well as direction.
• The direction of drag force is opposite to the direction of relative motion.
• The SI unit of drag force is Newton (N).
• The dimensional formula of drag force is [M L T^-2].

Drag force occurs only when 

• There is physical contact between the object and the fluid, and
• There is a velocity difference between the object and the fluid.
• Drag force will not exist when an object rests in a stationary fluid.

Drag Force

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Types of Drag Force

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The following are the different types of drag force

Parasite Drag

This drag force occurs due to the motion of the object in a fluid.

• It consists of all types of drag force that oppose the motion of the object through a fluid.
• It includes form drag, skin friction drag, or interference drag.

Skin Friction Drag

This drag force occurs due to the friction between the rough outer surface of the body and the molecules of the fluid.

• The skin friction can be reduced by making the outer surface of the smooth.
• This is the reason that an Olympic swimmer wears tight and smooth suits.

Form Drag or Pressure Drag

This type of drag is caused due to the shape of the object moving through a fluid.

• It depends on the area of the cross-section of the object.
• The larger the cross-sectional area, the larger will be the form drag.

Interference Drag

Interference drag is caused due to the interaction of two or more airflows, having different speeds. 

• Multiple components of an aircraft cause this drag, such as airflow around a wing and airflow around a fuselage.
• The interference drag can be minimized by keeping the angle between these two components below 90 degrees.

Lift-Induced Drag

This type of drag is caused due to the result of the normal lift.

• It helps to stabilize the aircraft in the air by balancing its weight.
• The direction of lift-induced drag is perpendicular to the direction of the flow of air.
• The lift in an airplane is provided by its wings.

Wave Drag

This type of drag force is produced when an aircraft moves in the air with transonic speed i.e. speed almost equal to the speed of sound or supersonic speed i.e. speed almost equal to the speed of the sound.

• A shockwave is produced due to the high speed of the aircraft.
• This shockwave is the disturbance in the air particles that increases the drag force.

Drag Force Example

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Drag Force Formula

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The formula of drag force for a body moving in a fluid at uniform velocity is given by

\(F_D=C_DA \frac{\rho v^2}{2}\)

Where

• F_D is the drag force
• C_D is the coefficient of drag
• A is the projected area of the body perpendicular to the direction of flow
• ρ is the density of the fluid
• v is the uniform velocity of the object relative to the fluid

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Effects of Drag Force

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The following are the effects of drag force

• The helmets of cycles and bikes are specially designed to minimize the drag force.
• The orientation of the skydiver is very important while diving. 
• The drag force is directly proportional to the area of the body, therefore the terminal speed of the skydiver will be reduced if more area is exposed to air.
• The costumes for a swimmer are specially designed to reduce the skin friction drag. So that they can swim faster.
• The structure of the vehicles or artificially moving objects is designed in such a way that the drag force can be reduced.
• This will help in increasing the speed and fuel efficiency of the vehicle.
• The design of the structure of the submarines, sports cars, missiles, torpedoes, rockets, etc are the best examples of vehicles or objects having anti-drag properties.

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Examples of Drag Force

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Some real-life examples of drag force are

• Drag force acting on an airplane flying in the air
• Drag force on floating objects due to water
• Drag force on a submarine moving in oceans
• The flow of water over bridges causes drag force
• Flow of air over buildings
• Flying birds experience drag due to the air molecules
• A car moving on a highway experiences drag force due to air

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Things to Remember

• Drag force is the resistive force in the opposite direction of the motion of an object moving in a fluid.
• Drag force occurs when there is a relative motion between an object and a fluid.
• It is the parallel component of the total resistive force encountered by an object.
• Drag force depends on the relative velocity, area of the object, and density of the fluid.
• Drag force is a contact force.
• It is a vector quantity and its SI unit is Newton (N).
• The formula of drag force is given by \(F_D=C_DA \frac{\rho v^2}{2}\)

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