friction

Friction is the umbrella name for a family of dissipative forces that arise when two surfaces are pressed together and one tries to slide against the other. At the macroscopic level it is captured by two numbers — the static coefficient μ_s, which sets the maximum force the surfaces can exert before they break loose, and the kinetic coefficient μ_k, which sets the force while they are sliding. Both scale with the normal force pressing the surfaces together, not with the contact area — a surprising empirical fact first nailed down by Guillaume Amontons in 1699.

At the microscopic level, friction is the net result of innumerable tiny interactions between the asperities (high points) of the two surfaces. Even a highly polished metal surface looks jagged on the nanometre scale, and when two such surfaces touch they make contact only at a sparse set of points. When they slide, those contact points deform, cold-weld briefly, and tear free again, each event converting a tiny amount of mechanical energy into thermal motion of the atoms. The macroscopic result is a drag force that feels proportional to the normal force and roughly independent of speed.

Friction is the mechanism by which almost all everyday mechanical systems lose energy. It is why hockey pucks slow down, why brakes work, and why the air warms slightly in a gust of wind. It is also the mechanism by which almost anything useful happens mechanically: walking, writing, climbing, gripping, tying knots, and driving cars all depend on the friction between surfaces behaving the way Amontons' laws say it should.