# 4.3.5 Drag Forces

### Drag Forces

• Drag forces are forces that oppose the motion of an object moving through a fluid (gas or liquid)
• Examples of drag forces are friction and air resistance
• Drags forces:
• Are always in the opposite direction to the motion of the object
• Never speed an object up or start them moving
• Slow down an object or keeps them moving at a constant speed
• Convert kinetic energy into heat and sound
• Lift is an upwards force on an object moving through a fluid. It is perpendicular to the fluid flow
• For example, as an aeroplane moves through the air, it pushes down on the air to change its direction
• This causes an equal and opposite reaction upwards on the wings (lift) due to Newton’s third law

Drag forces are always in the opposite direction to the thrust (direction of motion). Lift is always in the opposite direction to the weight

• A key component of drag forces is it increases with the speed of the object
• This is shown in the diagram below:

Frictional forces on a car increase with speed

#### Worked Example

A car of mass 800 kg has a horizontal driving force of 3 kN acting on it.

Its acceleration is 2.0 m s-2.

What is the frictional force acting on the car?

### Air Resistance

• Air resistance is an example of a drag force that objects experience when moving through the air
• At a walking pace, a person rarely experiences the effects of air resistance
• However, a person swimming at the same pace uses up much more energy – this is because air is 800 times less dense than water
• Air resistance increases with the speed of an object, such as a vehicle
• However, there are other factors that also affect the maximum speed, such as:
• Cross-sectional area
• Shape
• Altitude
• Temperature
• Humidity

• Air resistance must be carefully considered in vehicle design, for example in racing cars, bicycles and aeroplanes:
• Racing cars have a streamlined design with a curve, angled front to experience less air resistance and travel faster
• Aeroplanes travel at high altitudes where there is less air resistance (since the air is less dense)
• However, they also travel through a variety of extreme temperatures and at very high speeds
• Therefore, aeroplane design is focused on producing the fastest, but also smoothest, journey possible
• A racing cyclist adopts a more streamlined posture to reduce the effects of air resistance
• Also, the bicycle, clothing and helmet are designed to allow them to go as fast as possible

Many factors such as posture, clothes and bicycle shape must be considered when trying to reduce air resistance

#### Air Resistance & Projectile Motion

• Air resistance decreases the horizontal component of the velocity of a projectile
• This means both its range and maximum height is decreased compared to no air resistance

A projectile with air resistance travels a smaller distance and has a lower maximum height than one without air resistance

• The angle and speed of release of a projectile is varied to produce either a longer flight path or cover a larger distance, depending on the situation
• For sports such as the long jump or javelin, an optimum angle against air resistance is used to produce the greatest distance
• For gymnastics or a ski jumper, the initial vertical velocity is made as large as possible to reach a greater height and longer flight path
• The perfect angle and speed for a projectile can be difficult to achieve
• For example, a footballer tries to kick a ball as high as possible but also with great speed to score a goal from a long distance

#### Exam Tip

If a question considers air resistance to be ‘negligible’ this means in that question, air resistance is taken to be so small it will not make a difference to the motion of the body. You can take this to mean there are no drag forces acting on the body.

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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