acceleration

[noun]

The change in an object's velocity over time, measured in distance per unit time per unit time (for example meters per second per second or $\frac{m}{s^2}$). Compare to velocity. Acceleration (a) is calculated by dividing the change (symbolized by Δ, the Greek letter delta) in velocity (v) by the change in time (t):

This can also be written as:

$a=\frac{v_2-v_1}{t_2-t_1}$, where $v_1$ and $t_1$ denote the starting velocity and time and $v_2$ and $t_2$ denote the ending velocity and time.

To illustrate, imagine a car speeding up (accelerating) from a stand still (0 meters/second) to a speed of 15 meters/seconds over the course of 5 seconds. The car's total increase in velocity is 15 meters/second. During each of the 5 seconds that the car is accelerating, its velocity increases by 3 meters/second until it reaches its top speed. (After one second the car is traveling at a velocity of 3 meters/second; after 2 seconds, it's traveling at a velocity of 6 meters/second, and so on). Therefore, the car's rate of acceleration is 3 meters per second per second or 3 $\frac{m}{s^2}$. Using the equation above: