Now that we understand the importance of friction, let us understand its origin. Take two pieces of sandpaper facing each other. Now bring them in contact with each other and try to rub them against each other. You will find it difficult to move. Why do you think so?
The answer is that the irregularities of both the surfaces get locked with each other and prevent slipping.
The same is the case between two sheets of paper, sheets of glass, even two different materials like a wooden plank on a concrete floor, being rubbed against each other. In the case of two well polished surfaces, interlocking occurs, but at a microscopic level.
Friction is the retarding force which comes into play when a body actually moves or tends to move over the surface of another body. To move a body we need force. When we move or try to move a body over another body by the aid of a force, the interlocking causes a retarding frictional force which acts in the opposite direction of the applied force.
The direction of the frictional force is always opposite to the direction of motion.
Modern theory of friction or surface adhesion theory of friction
Even though the laws of friction have been understood since a long time, some properties were understood properly, only after the discovery of atoms and molecules.
No matter how well polished two surfaces are, at the atomic level they are never plane. Sometimes, when these extremely well polished surfaces are brought in contact, a very small area out of the actual area of the total surface is in contact. This ratio could easily be 1: 104. Hence, the pressure on each of these atoms to the total pressure applied to the surfaces is of the order of 104: 1. For such a high pressure at the atoms, the atoms get deformed and bond with each other. In other words, the two surfaces get cold welded with each other.When one body is pulled across the other, these welds are constantly broken and constantly formed. This breaking of the welds causes a strong retarding force which is difficult to overcome.
Hence, rubbing two cold welded surfaces on each other is difficult.