The friction force is the force that opposes the motion of one body sliding over another. It is called friction when the objects are solid, viscosity in liquids but it always opposes the motion of an object.

The friction force depends on two factors:

a) The materials that are in contact. Rougher surfaces have a higher coefficient of frictions. This makes sense in terms of a model in which friction is described as arising from chemical bonds between the atoms of the two surfaces at their points of contact: very flat surfaces allow more atoms to come in contact.

b) The force pushing the two surfaces together. Pushing the surfaces together causes more of the asperities to come together and increases the surface area in contact with each other.

Its mathematical expression is:

Ffr = μ FN

where FN is the normal force and μ is the coefficient of friction.

Figure 1: A box on the grass experiences friction when pulled. The direction of the friction force is opposite to the direction of the pulling force applied by the man in the image. It is because of friction that we need to put an effort to move something.