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 m/s2). 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):

a = Δv/ Δt.

This can also be written as:

a = v2-v1/ t2-t1, where v1 and t1 denote the starting velocity and time and v2 and t2 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 m/s2. Using the equation above:

a = v2-v1/ t2-t1
a = 15-0 / 5-0
a = 15/5
a = 3 m/s2

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