Static friction is the force that prevents a body from sliding relative to the surface it rests on. It acts parallel to the contact surface and is directed opposite to the incipient motion, that is, the motion the body would begin to have if friction were absent. It is the force that holds a book still on a slightly inclined surface, or that allows a weakly pushed crate to remain stationary: as long as the push is not too strong, static friction balances it exactly.
The crucial feature is that static friction does not have a fixed value: it is a constraint force, which takes on, case by case, the value “necessary” to prevent sliding, but only up to a maximum limit.
Principle — Static friction
Static friction takes on the value necessary to prevent motion, up to a maximum value: where is the coefficient of static friction and the normal constraint reaction. If the force required to keep the body still exceeds , the body slides and the friction becomes kinetic.
It is worth dwelling on the nature of this force. Like the normal reaction, static friction is not determined by a formula that fixes its value: it is determined by the equilibrium condition. If I push the crate with and it stays still, the static friction is exactly ; if I increase the push to and the crate is still motionless, the friction is . The inequality does not say what the friction equals, but up to what threshold it can go: once that threshold is exceeded, the constraint gives way and motion begins.
Tip
Static friction is a constraint force: like the normal reaction, it takes on the value necessary to satisfy the constraints of the problem. The difference is that it has a maximum value, beyond which it can no longer guarantee equilibrium.
Links
Topics: Attrito Concepts: Attrito statico · Forza normale
Related exercises: Problema — Coefficiente di attrito statico · Velocità massima in curva piana · La scatola di fiammiferi che accelera