Jerk
Definition
Jerk is the rate at which acceleration itself changes — the jolt you feel when a car starts braking harder or an elevator begins to move.
Symbol: · Unit: m/s³
Technical definition
The third time derivative of position, j = da/dt = d³x/dt³, with SI unit m/s³. Constant jerk means acceleration changes linearly with time.
Example
A sports car's acceleration ramping from 0 to 5 m/s² over 2 seconds has an average jerk of 2.5 m/s³; passengers feel this as the initial push growing stronger.
Common confusion
Jerk is not acceleration — acceleration is how fast velocity changes, while jerk is how fast acceleration changes. You can have zero acceleration but nonzero jerk, like the instant an elevator starts moving.