# Friction

Friction is the force between two surfaces rubbing together.

When two smooth surfaces rub together there is very little friction.

When two rough surfaces rub together there is more friction.

There is less friction when there is a liquid (e.g. oil) between the two surfaces. This is because liquid particles move past each other much more easily.

There is more friction if the two surfaces are forced against each other.

### Friction makes things slow down

When we are on Earth - or any other planet for that matter- objects, once pushed, experience friction of some sort and therefore slow down.

This made Aristotle wrongly believe that movement was evidence of a force acting (as once you stopped pushing the thing slowed down!).

This is not the case!

It is the invisible force of friction that acts to oppose the motion and confuse us.

Acceleration (NOT movement) is evidence of a force acting.

### Friction can be a good thing:

Just think of walking on a wet marble floor or an icy pavement with new leather soled shoes: you might slip over! Worse still, if the tyres on your car are worn out and the road is wet and slippery, you would probably skid in the car and have an accident. Both your shoes and the car's tyres need good grip so their surfaces are rough. This increases the friction between them and the ground.

Friction is also very important for your car's brakes to work properly. When you put you foot on the brake pedal, some rough pads are squeezed tight against the brake discs. This friction slows the car down. If oil gets on the discs, the brakes will not work so well.

If the tyres are worn or the roads are wet braking can be a problem.

### Friction can be a bad thing:

Friction inside a car engine and inside the wheel axles will slow a car down and wear out the metal. To prevent this we put oil or even grease in them. This makes their surfaces more slippery and so reduces friction.

Friction in an engine causes a change of useful kinetic energy into heat energy. This is wasteful. A smooth running engine will be more efficient... because it will produce less unwanted heat energy and more of the type we want. Oiling a machine and keeping its it clean can make it more efficient.

NB Friction is a force NOT a type of energy!!