Friction Coefficient (Coefficient of Friction)

The ratio of maximum friction force to normal force, determining how easily a contact surface slips.

By Coulomb's law of friction, the friction force f between two contacting surfaces never exceeds the friction coefficient μ times the normal force N (the force pressing the surfaces together): f ≤ μN. μ is a dimensionless number that depends on the two materials and their surface condition, is largely independent of contact area, and can only be measured experimentally. There are two kinds: the static friction coefficient μ_s sets the largest tangential force an object can resist before it starts sliding, and the kinetic friction coefficient μ_k applies once it's already sliding; μ_s is usually larger than μ_k. Common dry materials fall mostly between 0.3 and 0.6; rubber on concrete is about 0.6–0.85; ice on ice is only 0.02–0.09. In robotics, μ determines how much grip force a gripper needs to avoid dropping something, and how much horizontal force a foot can push against the ground with before slipping; MuJoCo assigns each contact separate sliding, torsional, and rolling friction parameters, and μ is commonly randomized during training to narrow the sim-to-real gap.

ExampleA two-finger gripper holds a 0.5 kg cup, with normal force N per finger and μ = 0.5: total friction from both sides is 2×0.5×N, and supporting the roughly 4.9 N of weight needs N of at least about 4.9 N. Swap in a smooth cup surface with μ = 0.25, and the required grip force doubles to about 9.8 N.

Also called

μ, Coefficient of Friction (CoF)

Related

Coulomb Friction · Friction Cone · Normal Force and Tangential (Shear) Force · Slip · Static Friction (Stiction) and Stribeck Effect · Dynamics Randomization

Sources

Friction - Wikipedia
MuJoCo Documentation - Computation (contact, friction)