1. If smooth surfaces are observed under powerful microscope, many irregularities and projections are observed.
2. Friction arises due to interlocking of these irregularities between two surfaces in contact.
3. The surfaces can be made extremely smooth by polishing to avoid irregularities but then case also, friction does not decrease but may increase.
4. Hence the interlocking of irregularities is not the real cause of friction.
5. According to modem theory, cause of friction is the force of attraction between molecules of two surfaces in actual contact in addition to the. force due to the interlocking between the two surfaces.
6. When one body is in contact with another body, the real microscopic area in contact is very small due to irregularities in contact.
7. Due to small area, pressures at points of contact is very high. Hence there is strong force of attraction between the surfaces in contact.
8. When the surfaces in contact become more and more smooth, the actual area of contact goes on increasing.
9. Due to this, the force of attraction between the molecules increases and hence the friction also increases.
