You can calculate the average acceleration of an object over a period of time based on its velocity (its speed traveling in a specific direction), before and after that time. To do this you need to know equation for acceleration: a = Δv / Δt where a is acceleration, Δv is the change in velocity, and Δt is the amount of time it took for that change to occur.  The unit for acceleration is meters per second per second or m/s2. Acceleration is a vector quantity, meaning it has both a magnitude and a direction. The magnitude is the total amount of acceleration whereas the direction is the way in which the object is moving. If it is slowing down the acceleration will be negative. You can further define Δv and Δt:  Δv = vf - vi and  Δt = tf - ti where vf is the final velocity, vi is the initial velocity, tf is the ending time, and ti is the starting time.  Because acceleration has a direction, it is important to always subtract the initial velocity from the final velocity. If you reverse them, the direction of your acceleration will be incorrect. Unless otherwise stated in the problem, the starting time is usually 0 seconds. First write down your equation and all of the given variables. The equation is a = Δv / Δt = (vf - vi)/(tf - ti). Subtract the initial velocity from the final velocity, then divide the result by the time interval. The final result is your average acceleration over that time.  If the final velocity is less than the initial velocity, acceleration will turn out to be a negative quantity or the rate at which an object slows down. Example 1: A race car accelerates uniformly from 18.5 m/s to 46.1 m/s in 2.47 seconds. What is its average acceleration?  Write the equation: a = Δv / Δt = (vf - vi)/(tf - ti)  Define the variables: vf = 46.1 m/s, vi = 18.5 m/s, tf = 2.47 s, ti = 0 s. Solve: a = (46.1 – 18.5)/2.47 = 11.17 meters/second2.   Example 2: A biker traveling at 22.4 m/s comes to halt in 2.55 s after applying brakes. Find his deceleration.  Write the equation: a = Δv / Δt = (vf - vi)/(tf - ti)  Define the variables: vf = 0 m/s, vi = 22.4 m/s, tf = 2.55 s, ti = 0 s. Solve: a = (0 – 22.4)/2.55 = -8.78 meters/second2.

Summary:
Define the equation for average acceleration. Understand the variables. Use the formula to find acceleration.