Coulomb Friction

The classic friction model where friction force never exceeds the friction coefficient times normal force, independent of sliding speed.

Coulomb friction is the empirical law describing contact between two dry solid surfaces: the tangential friction force f_t ≤ μ·f_n, where f_n is the normal (pressing) force and μ is the friction coefficient, commonly between 0.1 and 1. While not sliding, friction can take on any value up to that limit in whichever direction resists sliding; once sliding starts, friction equals μ·f_n exactly, points opposite the direction of sliding, and is independent of sliding speed or the nominal contact area. It's common to distinguish a static friction coefficient μs from a slightly smaller kinetic friction coefficient μk. The rule was first summarized by Guillaume Amontons in 1699 and studied in depth by Charles-Augustin de Coulomb in 1785. It's only an approximation, but it underlies both grasp analysis and physics engines: plotting f_t ≤ μ·f_n in 3D gives exactly the friction cone, and whether a contact will slip is just a matter of checking whether the contact force falls inside that cone.

ExampleA two-finger gripper presses a cup from both sides with 10 N each. If μ = 0.5, each contact point provides at most 5 N of friction, 10 N total — enough to lift a roughly 1 kg cup (about 9.8 N of weight); anything heavier slips out from between the fingers.

Also called

Coulomb Friction Model, Dry Friction

Related

Friction Coefficient (Coefficient of Friction) · Friction Cone · Static Friction (Stiction) and Stribeck Effect · Viscous Friction · Contact Model · Slip

Sources

Modern Robotics（Lynch & Park, 2017 预印本）12.2 节 Friction (Chinese)
Friction - Wikipedia