Why does friction occur

Friction is understood to be the force that tries to oppose the easy motion of two surfaces touching against each other. This is, in a sense, the essence of friction put in extremely plain language. There are plenty of factors which support friction. However, the more prominent reasons behind the phenomenon of friction are molecular adhesive forces, roughness of surfaces and the digging (or deforming) effect.

The contribution of the adhesive molecular attractions was not taken into consideration until recent times. It was believed that friction was solely caused by surface roughness, but it has been established beyond doubt that the contribution of adhesive forces between two sliding surfaces is a major contributor towards the friction effect. The peripheral molecules of the two surfaces in contact are attracted towards each other, and form adhesive bonds. These bonds need to be snapped for the two surfaces to slide over each other. Although friction is generally visualized an effect relevant to solids, but the example of a liquid drop clinging on to a window pane glass is a pertinent indication of the adhesive forces leading to friction.

Surface roughness is another leading factor resulting in frictional forces that oppose easy motion. Most solid bodies do not have precisely uniform surfaces. Even visibly symmetrical geometries are not regular, and have protrusions as well as cavities. When two surfaces slide over each other, it is extremely likely that the respective protrusions and cavities would interlock amongst themselves, leading to a force that restricts the sliding motion. This was believed to be the sole cause behind friction for many years, but is now regarded as a minor effect as compared to the above mentioned adhesive bond effect.

Why does friction occur

The deformation effect is, at least to a small extent, mere logical extension of the roughness factor. Soft solid surfaces are prone to getting deformed and compressed upon impact from another relatively hard solid body. In such a case, the harder and more rigid body would sink a little in the depressions caused by the contact. This sinking inhibits the sliding motion of the solid bodies over each other, and has to be overcome for the bodies to continue moving.