Acceleration
Definition
How quickly velocity changes. Speeding up, slowing down, and turning are all accelerations, because velocity includes direction as well as speed.
Symbol: · Unit: meters per second squared (m/s²)
Technical definition
The time rate of change of velocity, a = Δv/Δt, a vector quantity. Instantaneous acceleration is the derivative dv/dt. By Newton's second law it is proportional to net force: a = F/m.
Example
A sports car going from 0 to 27 m/s (about 60 mph) in 4.5 seconds has an average acceleration of 6 m/s² — every second, its speed grows by 6 m/s. A car turning a corner at steady speed is also accelerating, because its direction changes.
Zero velocity does not mean zero acceleration. A ball tossed straight up has zero velocity at the very top of its flight, yet gravity is still accelerating it downward at 9.8 m/s² the entire time — that is exactly why it starts falling back down. Velocity tells you where you are going; acceleration tells you how that is changing.