Static Friction
Definition
The grip-like force that keeps an object from starting to slide — it matches your push exactly, up to a maximum limit.
Symbol: · Unit: N
Technical definition
The contact force opposing impending relative motion, with magnitude 0 ≤ f_s ≤ μ_s·N, where μ_s is the coefficient of static friction and N the normal force. It is self-adjusting: it equals the applied force until the applied force exceeds the maximum.
Example
A heavy box will not budge when you push lightly because static friction pushes back just as hard. Push harder than μ_s·N and it finally breaks free and starts sliding.
Static friction is not a fixed value — it is a range. Students often plug in f_s = μ_s·N automatically, but that is only the maximum. If you push with 10 N and the max is 50 N, static friction is 10 N, not 50 N.