Forces of Drag-What are they?

In this science blog we investigate the ‘forces of drag’ and develop an understanding of the three main types of drag which include aerodynamic, hydrodynamic and frictional drag.

You may also wish to refer to Understanding Gravity and Mass.


1) Force of Aerodynamic Drag- an introduction

An alternative way of saying ‘aerodynamic drag’ is ‘air resistance’. The two expressions are interchangeable.

  • Why is this 21st century racing cyclist wearing a strange looking helmet and cycling in a crouching position?
  • What do you notice that is different about these 19th century racing cyclists?

The 21st century cyclist racing at the 2012 London Olympic games is using the latest technologies to reduce the effects of ‘aerodynamic’ drag so he can cycle faster and hopefully win his race.

The features that are designed to reduce aerodynamic drag and help him cycle faster include:

the shape (or ‘form’) of his helmet; the shape (or ‘form’) of his bicycle and wheels; the crouching position he adopts as he cycles.

Everything about him and his bicycle is designed to increase his speed by reducing ‘air resistance’.

  • So what is ‘air resistance’? (or aerodynamic drag) How and why does the force of ‘air resistance’ slow the cyclist down?

Definition of Air Resistance

‘Air resistance’ is a force exerted on solid objects. This force resists (ie slows down) the forward motion of solid objects.

It means that when a solid object is moving forwards, the forces of air resistance act on that solid object in the opposite direction in which the solid object is traveling.

For ‘air resistance’ to occur the solid object(s) must be moving forwards through gases in the air.

The racing cyclist is moving forwards through gases in the Earth’s lower atmosphere, the main ones of which include nitrogen, oxygen, argon and carbon dioxide.

For racing cyclists, air resistance is an unwelcome and unwanted force.

Air resistance is also an unwanted force for airplanes in flight; it is a force that slows them down…

…However there are times when air resistance is a very welcome and necessary force. In the example below, after the light air craft’s engine has failed a parachute is deployed and the lives of both pilot and passengers are saved….

Air resistance explained

In the diagram below the smooth air flows, as represented by the parallel lines, collide with the ‘blunt’ red object. Colliding with a ‘blunt’ object means that many particles of air will be forced to change direction.

As a result of so many particles of air having to change direction, turbulent air flows are produced behind the blunt object. ‘Turbulent’ air flows slow down the speed of motion.

The more particles of air that have to change direction, the greater the turbulence, the greater the air resistance and the greater the reduction in velocity of the object in forward motion.

The ‘sofa car’ in the picture below was designed for fun and definitely not designed for reducing turbulent air flows. A large number of particles of air have to change direction when they collide with the sofa as the sofa is in forward motion, causing turbulent air flows to take effect behind the sofa.

The greater the speed of the sofa car, the greater the air resistance as more particles of air hit the ‘sofa car’ with greater force. Despite its ‘bad’ design, this sofa car has managed to reach a top speed of 87 mph. (140 kph)