4.1.13

# Friction

Test yourself

## Friction, Drag and Air Resistance

When an object travels through a medium, it interacts with the particles in that medium. This can slow the object down or provide lift.

### Friction

• Friction is the resistance to motion due to contact forces.
• Usually, an object experiences friction due to collisions with particles in air or a rough surface of a solid.
• Friction always opposes motion, meaning that it slows down moving objects.
• Friction converts kinetic energy into other types of energy, mainly heat.
• This is why brakes on a car can overheat if used too much.

### Drag

• Drag is a frictional force.
• Objects that travel through a liquid or gas experience drag.
• Drag is due to collisions between the travelling object and the particles in the medium.
• The faster an object moves through a medium, the larger the drag.
• Drag is also affected by the shape of an object (how streamlined it is).

### Air resistance

• Air resistance is a type of drag.
• Air resistance is a frictional force which opposes the motion of an object travelling through air.

## Lift on a Wing

A moving wing can experience lift. Lift is an upward force due to the collisions with air particles on the underside of the wing.

### Equal and opposite force

• Lift is a result of Newton's Third Law of Motion:
• Every action has an equal and opposite reaction.
• The wing of a plane pushes air particles downward as it flies forward.
• As a result of this downward force on the air particles, there is an equal and opposite upward force on the wing.
• This force is called lift.
• This is the basic principle of how aeroplanes fly.

### Shape of the wing

• The wing is angled to maximise the amount of air it can push downwards, thereby generating the maximum force upwards.
• The wing is also smooth and slightly curved to reduce drag forces and turbulence.