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# What is the Coulomb friction law?

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As early as the 18th century, taking up the pioneering ideas developed by Leonardo da Vinci (1452 – 1519), C.A. Coulomb (1736 – 1806) postulated a simple law specifying the conditions that the contact force between two solids must verify for there to be a slip between them: the Coulomb friction law. This contact force comprises a normal component Fn, perpendicular to the contact plane between the two solids, and a tangential component Ft, belonging to the contact plane. C.A. Coulomb shows, from very precise experiments, that the slip between the two solids occurs if Fn and Ft verify the proportionality relationship: Ft = μ Fn, where μ is the coefficient of friction that characterizes the condition of the two surfaces in contact. As long as the force Ft is less than μ Fn (the contact force is said to remain within the Coulomb cone), sliding does not occur, and is referred to as adhesion between the two solids. Illustration of Coulomb’s law: by noting m the minimum mass that must be placed on the plate for the solid of mass M to slide on the support, the coefficient of friction between the solid and the plate is obtained by the relationship μ = m/M.

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