FRICTION

Modern theory: It has been found that if two surfaces in contact are highly polished, the force of friction increases. This implies that when the two surfaces in contact are highly polished, their molecules come in contact in the range of intermolecular distance. When the molecules are at distances of intermolecular range, the intermolecular forces of attraction between them become significant. When the intermolecular force of attraction between the molecules of the two surfaces in contact is significant, the sliding body has to overcome the intermolecular force of attraction. And, hence, the force of friction between the surfaces increases. When the smoothness of the surfaces is increased, more number of molecules come into the intermolecular range. As a result, the intermolecular force increases and the friction increases.