Friction is the force resisting the motion of surfaces over each other. Friction arises when two solids slide (or attempt to slide) over one another.

Friction always opposes motion, and is often an unwanted force. It makes engines and many physical activities less efficient.

Friction is not always an unwanted force – it is the reason we can walk across surfaces, why cars can turn corners safely and why we can light a match.

Friction arises from surface roughness, surface deformation, and surface contamination.

The complexity and reliability of these interactions makes the calculation of friction difficult so experimental measurement is often used to determine its extent.

The effects of friction can be reduced by using lubricants such as oils or water. These allow the surfaces to move over one another more easily as in an engine or waterslide.

Friction problems are often presented using a co-efficient of friction \mu (a number between 0 and 1). The higher this number, the higher the effect of friction in the example. The co-efficient of friction is used to determine the opposing force acting in the problem.


A child attempts to pull a box attached to a piece of rope across a carpeted floor. The co-efficient of friction between the box and the floor is 0.2. The child pulls on the box with a force of 50N. Determine the force acting on the box after friction has been considered.

We can visualise the problem with a diagram:

Then we can calculate the opposing friction force using the co-efficient of friction \mu = 0.2

Opposing force = 50\times 0.2=10N

As we can see in the diagram above, we have a force acting to the right from the child and an opposing force acting to the left as a result of the friction.

Resultant force = 50-10=40N\quad right