Coulomb friction
The dry-friction model game physics rests on: the friction force is at most μ times the normal force and opposes sliding along the surface.
The model says a contact can take a tangential force up to μ·N before it starts to slide. Below the limit, static friction holds the bodies still against each other; above it, kinetic friction takes over and brakes the slide. Charles-Augustin de Coulomb formulated the relation in 1785 from measurements, and it is approximate but surprisingly useful.
In a constraint solver the limit becomes a friction cone: the maximum friction impulse depends on the normal impulse in the same contact, so friction has to be solved together with, or after, the normal constraint. For speed the cone is often approximated by a pyramid — a box along two tangent axes — which makes crates on slopes slightly direction-dependent but cheap to compute.