Acceleration is how quickly velocity changes. It is the change in velocity divided by the time taken: a = (v − u) ÷ t. Here a is the acceleration in metres per second squared (m/s2), v is the final velocity in m/s, u is the initial velocity in m/s and t is the time taken in seconds.
A car speeds up from 4 m/s to 20 m/s in 8 s. The change in velocity is 20 − 4 = 16 m/s, so a = 16 ÷ 8 = 2 m/s2. The velocity increases by 2 m/s every second. An object that starts from rest has an initial velocity of zero.
If an object slows down, its final velocity is less than its initial velocity, so the acceleration is negative. This is called deceleration.
The equation can be rearranged to give change in velocity = acceleration × time. A motorbike starting from rest and accelerating at 3 m/s2 for 5 s gains 3 × 5 = 15 m/s, so its final velocity is 15 m/s.