Drag Forces (AQA A Level Physics): Revision Note

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

• Drag 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

• A key component of drag forces is it increases with the speed of the object

• This is shown in the diagram below:

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

Air resistance and 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

• 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