static friction

Meaning: Static friction is the contact force, parallel to the surfaces, that keeps two touching surfaces from sliding past each other. It points opposite the direction the object would slide without it, and it adjusts: it grows with the applied force to hold the object at rest, up to a maximum. Past that, the surfaces slip and kinetic friction applies. It can point along the motion, as when tires push a car forward without skidding.

Equation: fs ≤ μs N, where fs is the static friction force (N), μs is the coefficient of static friction (unitless), and N is the normal force (N). Equality holds only at the threshold of slipping.

On the MCAT: Typical setups are a crate pushed on a floor, where you compare the applied force with μsN to see if it moves; a block on an incline, which just starts to slip when tanθ = μs; and a coin on a turntable, where static friction supplies the centripetal force. For the same surfaces, μs is usually greater than the kinetic coefficient μk, so a graph of friction against applied force rises, then drops to a flat kinetic value.

Common confusion: Students use fs = μsN every time. That holds only at the verge of slipping. Below it, find fs from a force balance (at rest on an incline, fs = mg sinθ).