Give old and modern views of the cause of friction.

Explanation

Old view: A surface which looks smooth to naked eye has a number of irregularities, when observed through a powerful microscope. When a body is placed on another body, then these irregularities get interlocked. When a body starts moving over the other body, then these interlocks has to be broken. The force required to do so is called force of friction. It means, if irregularities are lesser in number, then force of friction is smaller. But it has been found that the force of friction increases, when bodies are made very smooth.
Modern view: According to modern theory, the actual area in contact is less than the area of contact as seen on the macroscopic scale. Thus there is larger pressure at the actual points of contact. These points appear to be welded. These welds are called cold welds. As a body moves over the other body, these cold welds has to be broken. The force required to break these welds is called force of friction. When bodies are made extra smooth, large area of the bodies come into contact. "This increases the number of cold welds, therefore friction increases.