# 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:

Frictional forces on a car increase with its speed

#### 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

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.

### 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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