Friction

​​In a nutshell

Friction is a force that acts between two surfaces. The two surfaces can either be sliding against each other or trying to slide against each other. When the surfaces are sliding against each other, friction slows down the moving object. When the surfaces are trying to slide against each other, friction makes it harder for the object to start moving.

Direction

Friction works in the opposite direction to the one in which the object is moving or trying to move in. This is because the force is slowing down the object or keeping it from moving in the first place. 

Below is an image showing how friction acts on a block slipping down a slope, where friction slows the block down: 

​​1. Direction of friction as block moves down the slope.

Surfaces

The amount of friction depends on the type of material of the two surfaces. Rougher materials will cause more friction to be produced as it is harder for the surfaces to slide against each other. More friction being produced can be useful sometimes, but this can mean more wasted energy as friction produces heat as well. 

Example

If a marble is rolled on a wooden floor, it is going to roll further than it would on carpet. This is because carpet is rougher than a wooden floor, so there is more friction between the marble and the surface it is moving against. 

Heat production

Friction also produces heat. If two surfaces keep rubbing against each other, the surfaces will become hot because of the friction acting between them. Rougher surfaces sliding against each other will get hotter than smooth surfaces sliding against each other because there is more friction between the rougher surfaces.