Drag Force

Drag force is present everywhere around us. We thrive in a ball of fluids (air and water). Drag forces appear whenever there is motion in air or water or in any other fluid.

When objects travel through fluids (a gas or a liquid), they will undoubtedly encounter resistive forces called drag forces.

The drag force always acts in the opposite direction to fluid flow. If the body’s motion exists in the fluid-like air, it is called aerodynamic drag. And, if the fluid is water, it is called hydrodynamic drag.

Drag Force

In order to minimise the influence of drag force, fast vehicles are created and designed, as streamlined as possible.

Examples of streamlined objects are everywhere in our nature – e.g. birds and dolphins have streamlined bodies to help them move quickly through the air and water, respectively.

What is a Drag Force?

Drag is an example of mechanical force.

When a solid body interacts with a fluid (liquid or gas), drag force is produced on the solid body.

Drag forces are not created by any force fields. In order to experience a drag force, an object has to come in physical contact with the fluid medium.

If the fluid doesn’t exist, there will be no resistance on the object’s surface; therefore, no drag will be created. Drag force is produced by the deviation or difference in velocity between the fluid and the object.

There should be movement between the fluid and the solid object. Without motion, drag is non-existent. In fact, there is no difference between whether the objects move in a static fluid or the fluid propagates past a stationary solid object.

As drag is a force, it’s a vector quantity with a magnitude as well as direction. Drag is produced in an order that is polar to the object’s motion.

Drag can also be explained as friction. Drag produced in the air is called ‘aerodynamic friction’. Friction between the molecules of air and the solid surface of the moving object is one of the common sources of drag force.

As friction is between a solid and gas, the skin friction value relies on the characteristics of both gas and solid. In the case of a solid, a waxed or smooth surface creates less friction than a rough surface.

When we consider the case of gas, value relies on air’s viscosity and the Reynolds number.

Video about Frictional Force

Drag Force Types

Parasite Drag

Parasite drag consists of every force that acts to slow down a vehicle’s movement.

There are a few variations of parasite drag: interference drag, skin drag force, form drag, etc.

Skin Drag Force

Skin friction drag is an air resistance generated on the aircraft due to air exposure to the craft’s outer surface.

Lift Induced Drag

Lift induced drag is the result of the normal lift mechanism. Lift Induced drag is generated by the aircraft’s wing (vortices on the tip).

Interference Drag

Interference drag is created due to the interference of multiple airflows, which have varying speeds. The interference of various aircraft components generates this drag force. It is precisely due to non-similar airflow around the fuselage and the wing.

Wave Drag

This unique drag force is limited to supersonic scenarios. It is a type of induced drag force produced from non-cancelling static pressure variables, to either side of a shock wave striking on the outer surface of the object, from which the wave is generated.

Drag Force Discovery

Sir George Cayley is known for discovering Drag Force and all other aerodynamic forces during flight – weight, thrust and lift. He also deduced the correlation between them.

Drag Force Equation

Drag Equation is used to find the force of drag on an object due to motion through an enclosed fluid system.

\(\begin{array}{l}F_{d} = \frac{1}{2}\rho \upsilon ^{2}c_{d}A\end{array} \)

\(\begin{array}{l}F_{d}\end{array} \)
is the drag force,

\(\begin{array}{l}\rho \end{array} \)
is the mass density of the fluid,

\(\begin{array}{l}\upsilon \end{array} \)
is the flow velocity relative to the object,

\(\begin{array}{l}A\end{array} \)
is the reference area,

\(\begin{array}{l}C_{d}\end{array} \)
is the drag coefficient

Drag Force

Relevance of Drag Force

  • Body orientation is a crucial part of skydiving because the object’s physical shape significantly affects the extent of air resistance experienced by the object. In fact, air resistance has a profound effect on terminal speed.
  • Professional bike and cycle helmets are specially designed to reduce drag force.
  • Swimming suits are also meticulously designed and fabricated to reduce drag force through water.
  • All efficient vehicles or artificial moving objects are designed around the drag force variable. Without implementing structural designs to counter drag force, no objects can travel smoothly through air or water. Speed and fuel efficiency will be dramatically reduced.
  • Examples of vehicles or objects with extreme anti-drag properties are submarines, missiles, rockets, torpedoes, sports cars and bikes, weather balloons, bullet trains, etc.
  • Fishes, birds and water mammals have various anti-drag mechanisms in their bodies. Specific examples of creatures with extreme anti-drag properties include lotus leaves, penguins, dolphins, sharks, Nepenthes pitcher plants, hawks, etc.
  • Whichever the case, drag reduction is very crucial in the survival of each of these creatures.
  • Humans have created efficient working models to counter drag forces in various scenarios by reverse-engineering these natural wonders.

Daily Life Examples of Drag Force

There are an infinite number of situations where drag force shows its undeniable presence.

  • Resistance on moving vehicles.
  • Floating objects.
  • Resistance during a storm or heavy wind.
  • Resistance on gliders and parachutes.


Frequently Asked Questions – FAQs

What is a drag force?

When a solid body interacts with a fluid (liquid or gas), drag force is produced on the solid body.
Drag forces are not created by any force fields. In order to experience a drag force, an object has to come in physical contact with the fluid medium.

What is aerodynamic drag?

If the body’s motion is through fluid-like air, it is called aerodynamic drag.

What is hydrodynamic drag?

If the body’s motion is through the fluid-like water, it is called hydrodynamic drag.

What does drag force depend on?

Drag relies on the square of the velocity, compressibility, air density, viscosity, size and structure of the body, etc.

Is drag a contact or non-contact force?

Drag forces are not created by any force fields. In order to experience a drag force, the object has to come in physical contact with the fluid medium.

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