Definition

Dating back to 1699, Amontons’ laws of friction assert that there exists a proportionality between friction force and the applied load, and that the friction force is independent of apparent contact area. While well-known in the field of tribology, these so-called “laws” are predated by the work of Leonardo da Vinci and are invalid in many practical situations.

Scientific Applications

Frenchman Guillaume Amontons (1663–1705) conducted research on friction during the late 1600 s and presented his work in a classic paper to the Royal Academy in December of 1699 (Amontons 1699). In the field of tribology, he is best known for two so-called Amontons’ laws of friction that derived from one of the conclusions presented in that paper; namely, “that the resistance caused by rubbing only increases or diminishes in proportion to greater or lesser pressure (load) and not according to the greater or lesser extent of the surfaces.” In other words:

  1. 1.

    The friction force is directly proportional to the applied load.

  2. 2.

    The friction force is independent of the apparent area of contact.

These relationships appeared to account for the results of early experiments on various pairs of materials and simple machines, leading to an assertion that the proportionality between the friction force and normal force tended toward 1/3. More comprehensive studies of friction using modern instruments have since shown that the frictional relationships put forth by Amontons, and inferred earlier by Leonardo da Vinci (1452–1519), are at best approximations. They do not in general reflect or predict the behavior of interfaces over a wider range of conditions than were originally treated by Amontons. The definition of the friction coefficient (see the article on Friction Coefficient) is largely based on the work of da Vinci, Amontons, and Coulomb (Dowson 1998); however, the proportionality between normal and friction forces in a tribosystem is a dimensionless quantity that depends on geometric, mechanical, environmental, thermal, and materials-related characteristics of that tribosystem. Broad generalizations such as Amontons’ laws are no longer widely accepted.

Key Applications

Amontons’ laws were derived based on macro-scale experiments and observations. While they played an historical role in understanding non-lubricated sliding behavior, they are not generally used in current mechanical design practices where precise measurements of friction forces and friction coefficients, specific to the application, are required. Experimental work or more tribosystem-specific models have largely taken their place.

Cross-References

Friction Coefficient

Friction, History of Research