Friction can be ascribed to the interactions between bodies material zones that are in contact or moving relative to one another; it counteracts relative motion. External and internal types are differentiated. When friction is external, the different friction bodies material zones are in contact; when friction is internal, material zones that are in contact belong to one friction body or the interfacial medium.
It is a phenomenon that people have been interested in for over hundreds and even thousands of years and still today remains in the middle of the development of new products and technologies. It leads to energy dissipation and in micro-contacts, where extreme stress is present, to micro-fractures and surface wear. We often try to minimize friction during design in an attempt to save energy. There are, however, many situations in which friction is necessary. Without friction, we cannot enjoy violin music or even walk or drive. There are countless instances for example between tires and the road during braking.
Also, wear must not always be minimized. Fast and controllable abrasive techniques can actually form the basis for many technological processes, (e.g., grinding, polishing, and sandblasting).
The frictional force, F, exists because of the rough nature of the contact surface between the body and the ground. In some cases, where the contact is smooth or lubricated, the frictional force will be negligible and there will be a normal reaction force only. This is a special case only found under certain circumstances, e.g. contact surfaces in a lubricated bearing.
Friction and wear are very closely connected with the phenomenon of adhesion. For adhesion it is important to know if a close contact can be created between two bodies. While adhesion does not play a considerable role on a largescale in the contact between two “hard bodies” such as metal or wood, in instances in which one of the bodies in contact is soft, the role of adhesion becomes very noticeable and can be taken advantage of in many applications.
