ABSTRACT
It has been established since long that the surfaces of the bodies are never perfectly smooth. When even a very smooth surface is viewed under a microscope, it is found to have roughness and irregularities, which is not possible to get detected by normal touch. If a block of one substance is placed over the level surface of the same or of different material, a certain degree of interlocking of the minutely projecting particle takes place. This does not involve any force, so long as the block does not move or tends to move. But if one block moves or tends to move tangentially with respect to the surface on which it rests, the interlocking property of the projecting particles opposes the motion. This opposing force, which acts in the opposite direction of the movement of the upper block, is called the force of friction, or simply friction. It is thus evident that at every joint in a machine, force of friction arises due to the relative motion between the two parts and hence some energy is wasted in overcoming the friction. Although friction is considered to be undesirable, it plays important role both in nature and in engineering applications, for e.g. walking on road, playing a carom board, movement of shuttle in a loom.
