Engineering Applications

6Friction

Friction, coming from the Latin word fricare, is the resistive force caused by the motion between two surfaces (objects) that are sliding or trying to slide across one another. This resistive force, i.e. friction force, which acts always opposite to the direction the object is moving or is trying to move is due to the irregularities and roughness of the surfaces in contact. Therefore, the magnitude of the resistive force depends on the materials from which the two surfaces are made.

The standard friction laws were discovered but not published by Leonardo da Vinci (1452–1519) a very famous Italian astronomer, engineer, mathematician, architect and sculptor. Considered one of the greatest painters of all time he also studied diverse engineering mechanisms such as screws, gears and bearings, and is considered a pioneer in tribology. The three laws of friction were reformulated by Guillaume Amontons in 1699 and verified and further developed by the French physicist Charles-Augustin de Coulomb in 1791. In an important work, namely “Théorie des machines simples”, considering the laws of friction, namely: (i) Amontons first law – the friction force is directly proportional to the normal force, (ii) Amontons second law – the friction force is independent of the apparent area of contact, and (iii) Coulomb’s Law – friction is independent of the relative velocity of sliding, Charles-Augustin de Coulomb investigated static and dynamic friction of sliding surfaces, rolling friction ...