Coulomb friction

Coulomb friction is an approximate contact model in which tangential friction is limited by a coefficient of friction times compressive normal load. For a dry contact with normal force $N\ge 0$, the usual static and sliding versions arewhere $\mathbf{w}$ is contact slip. Static friction adjusts within its bound; it is not always equal to the limiting value. Kinetic friction opposes the relative sliding at the contact, which need not be opposite to the body's centre-of-mass velocity.^[ref]

For a horizontal contact with $N=100\mathrm{N}$ and stipulated ${\mu }_s=0.4$, an applied tangential force of $20\mathrm{N}$ can be balanced while sticking; the limiting value is $40\mathrm{N}$. In geological applications, a cohesive shear threshold is often added to the load-dependent term, with effective normal stress reduced by pore pressure. Real contact behavior can also depend on sliding history, speed and adhesion, so these constants describe a stated regime.