CIE AS Physics (9702) exams from 2022

Revision Notes

3.1.6 Drag Force & Air Resistance

Drag Forces

  • Drag forces are forces acting the opposite direction to an object moving through a fluid (either gas or liquid)
  • Examples of drag forces are friction and air resistance
  • 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 its speed

Worked Example

Worked Example - frictional force on car, downloadable AS & A Level Physics revision notes

Exam Tip

Remember to consider drag forces in your calculation for the resultant force. More details of this are in the notes “Force and acceleration”.

Air Resistance

  • Air resistance is an example of a drag force which 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 depends on the shape of the body (object) and the speed it’s travelling
  • Since drag force increases with speed, air resistance becomes important when objects move faster

Racing cyclist, downloadable AS & A Level Physics revision notes

A racing cyclist adopts a more streamline posture to reduce the effects of air resistance. The cycle, clothing and helmet are designed to allow them to go as fast as possible


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.

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