Forces of Drag-What are they?

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

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?

 Alexander Vinokourov competing in the London 2012 Men's Olympic Time Trial.Image credit:DAVID ILIFF, Wikipedia,license CC-BY-SA 3.0

  • What do you notice that is different about these 19th century racing cyclists?

penny farthing race 19th century

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.

racing cyclist showing forces of thrust, friction, air resistance and gravity

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.

Earth's atmosphere taken from a low orbit showing curvature of the Earth Image credit:Wikipedia

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…

Aircraft flying showing forces of drag, thrust, gravity and liftImage credit:Wikipedia

…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….

Airplane with parachute slowing descent showing force of air resistance and gravityImage credit:NASA

two blunt pencils which need sharpening


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.

Effects of air resistance on blunt object in forward motion showing turbulenceImage credit:Wikipedia

This is what ‘turbulent’ air flows might ‘look’ like.

lines drawn in swirls and a chaotic fashion

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)

sofa car being driven on a road

  •  Can you see the turbulent air flows which act to slow down this car when it is in forward motion?

laminar and turbulent air flow in a wind tunnel

The fewer particles of air that have to change direction when they hit an object which is in forward motion, the less air resistance there will be. If a ‘blunt’ shape becomes a ‘streamlined’ shape then fewer particles of air will have to change direction; as a consequence there will be less ‘air resistance’.

‘Streamlining’ means fewer particles of air having to change direction following a collision with an object in forward motion.

In this diagram, very few particles of air have to change direction after colliding with this more ‘streamlined’ shape’. When they come into contact with the ‘streamlined’ object, the particles of air continue to flow in parallel lines either side of the streamlined object, producing little turbulence.

Diagram showing a streamlined shape

This motorbike has been designed with a more ‘streamlined’ shape to reduce the number of particles of air which have to change direction when the motorbike is in forward motion.

Motorbike being raced showing streamlined shape.Image credit:Wikipedia

In the picture of a racing cyclist, the ‘red’ regions indicate those regions of the body which receive the greatest number of ‘collisions’ with particles of air.

bike wind tunnel credit:

The ‘red’ regions of the body can therefore be said to be the regions which result in the greatest number of particles of air having to change direction following these collisions. This cyclist and his racing bicycle are both ‘streamlined’ with few turbulent air flows.

Formula 1 racing cars are also constructed in such a way as to minimize the negative effects of air resistance.

Reducing air resistance is achieved by fitting the following to Formula 1 cars:

#front spoilers to prevent turbulent air flowing under the racing car;

#front nose cones to provide more ‘streamlined’ air flows;

Racing car showing rear spoiler, front spoiler and nose cone Image credit:Wikipedia 

Not only are Formula 1 cars designed to decrease air resistance; strangely parts of them are designed to increase air resistance.

The rear spoilers harness (bring together and use) the force of air resistance to ensure that the rear tyres maintain a firm grip on the race track, as can be seen in the diagram below. When particles of air collide with the rear spoiler, the rear tyres are forced downwards.

F1 car showing streamlining and air resistance

traveling at very high speeds means that modern fighter jets need to combat the forces of aerodynamic drag in a very efficient way. Modern fighter jets have special triangular shaped ‘delta’ wings whose design considerably reduces aerodynamic drag.

Rafale F1 Fighter Aircraft showing delta wings Image credit:Wikipedia

Aircraft and bicycles aren’t the only solid objects which have to combat air resistance; we humans do as well!

Two people leaning into the wind on Macquarie IslandImage

2)  Fluid resistance (or hydrodynamic drag)wrist watch exposed to water showing that it is waterproof

The second type of drag we investigate is ‘fluid resistance’ or ‘hydrodynamic drag’. The forces of drag are greater in water than in air because water is denser than the gases in air.

We do not generally refer to ‘hydrodynamic drag’ as ‘water resistance’. If we did refer to ‘hydrodynamic drag’ as ‘water resistance’ we might become confused with ‘water resistance’ when it is used as an expression referring to watches.’Water resistance’ refers to watches that are ‘water proof’!

When a solid object such as a boat is moving forwards through water, the forces of fluid resistance act on that solid object in the opposite direction in which the solid object is traveling.

In this diagram the forces of hydrodynamic drag are acting on a ‘blunt’ solid object which is in forward motion in water. Because the object is ‘blunt’, many particles of water have to change direction, producing turbulent water currents.

fluid resistance with turbulence forming behind a 'blunt' object

  • Can you spot the turbulence in the water of this guitar boat in forward motion? Can you spot the particles of water changing direction?

Singer-songwriter Josh Pyke steering his guitar boat

When fluid resistance (or hydrodynamic drag) exerts a force on a solid object it slows that solid object down.

This submarine, when it is in forward motion on the surface of the sea, is not very streamlined.

  • Amazing! Look at all that turbulence!

Los Angeles class submarine produces turbulence as it is in forward motion Image credit:Wikipedia

fusiform shape which narrows at both ends

‘fusiform’ shape

In the natural world fish have sometimes evolved with ‘streamlined shapes’. This shark has evolved a ‘fusiform’ shape which is very streamlined. A ‘fusiform’ shape means that its body narrows at its front and rear ends, minimising hydrodynamic drag and enabling it to swim very fast to catch its prey.

Shark swimming in the sea surrounded by smaller fish              

In contrast these skunk anemones have not evolved with streamlined bodies. However their ‘thin’ bodies (‘laterally compressed’ bodies) allow them to hide more easily hide from predators.

skunk anemone near their anemone homeImage credit:Wikipedia

3) Frictional drag (or skin drag)

‘Frictional drag’ is the third type of drag that we discuss in this article.

Frictional drag is a force which resists the  forward movement (motion) of a fluid across the surface of an object (or ‘body’). As with hydrodynamic and aerodynamic drag, the force of frictional drag moves in the opposite direction to the direction of movement of the object.

frictional drag showing small friction on a smooth surface and large friction on a rough surfaceImage credit:Wikipedia

Dolphins need to swim very fast through water to catch their prey. Not only do they have ‘fusiform’ shapes to help reduce ‘hydrodynamic’ drag, but their skin has unique properties which helps them reduce ‘skin’ drag.

Bottlenose Dolphins perform a stunt by jumping into the air at Puerto de la Cruz, Tenerife, Spain. Image credit:Wikipedia

How do dolphins reduce frictional drag? Yes.. it’s true! Dolphins do this through shedding their skin- a case of bad dandruff!

Human scalp showing specks of white dandruffImage credit:Wikipedia

Dolphins shed their soft, flaky skin once every two hours when they are swimming; shedding skin helps dolphins reduce frictional drag.

As dolphins swim through the sea, thousands of tiny whirlpoopls of water called ‘vortices’ form close to the dolphins’ skin. These tiny vortices of water, if allowed to grow bigger, would normally slow a dolphin’s speed down.

several vortices all in a line which increase skin dragImage credit:Wikipedia

Dolphins’ flaking skin disrupts the formation of these tiny whirlpoopls of water. As a consequence less frictional drag is produced, allowing dolphins to swim faster.

Atlantic spotted dolphins swimming Image credit:NOAA

Human swimmers also have a way of reducing frictional drag through the use of swimsuits called  ‘bodyskins’. ‘Bodyskins’ are made of technologically advanced fabrics which help swimmers achieve faster times by decreasing frictional drag in the water.

 Two female and one male swimmers wearing competitive bodysuits Image credit:Wikipedia

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