AQA A Level Physics

Revision Notes

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

Lift Force, downloadable AS & A Level Physics revision notes

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:

Drag force increases with speed, downloadable AS & A Level Physics revision notes

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?

WE - frictional force on car question image, downloadable AS & A Level Physics revision notes

WE - frictional force on car answer image, downloadable AS & A Level Physics revision notes

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

Racing cyclist, downloadable AS & A Level Physics revision notes

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

Projectile Motion Air Resistance, downloadable AS & A Level Physics revision notes

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

Football projectile, downloadable AS & A Level Physics revision notes

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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