Average acceleration: The change in instantaneous velocity divided by the elapsed time.

Average acceleration is the change in instantaneous velocity over a period of elapsed time. Its definition is shown in Equation 1 to the right.

We will illustrate average acceleration with an example. Let's say you are initially driving a car at 12 meters per second and 8 seconds later you are moving at 16 m/s. The change in velocity is 4 m/s during that time; the elapsed time is eight seconds. Dividing the change in velocity by the elapsed time determines that the car accelerates at an average rate of 0.5 m/s².

Perhaps the car's acceleration was greater during the first four seconds and less during the last four seconds, or perhaps it was constant the entire eight seconds. Whatever the case, the average acceleration is the same, since it is defined using the initial and final instantaneous velocities.