Quick Guide to Calculating Acceleration
Since acceleration is about how much an object changes velocity by changing direction,
speeding up, or slowing down in a period of time, we need to know two things:
1. The change in velocity (speed)
2. The amount of time it takes to change
Once we know these, we divide the change in speed by the amount of time it takes to change.
The end result will be a rate of change. Remember that:
 Increasing speed (speeding up) = positive acceleration = positive # in the answer
 Decreasing speed (slowing down) = negative acceleration = negative # in the answer
Final Speed
To figure this out, we use a simple formula
a = Change in Velocity
Time
a=
OR
Δv
Δt
OR
Starting speed
a = vf – vi
Δt
Here are some helpful hints:
Δ = change
t = time
V = velocity (we use the speed here)
Vf = final velocity (ending speed)
Vi = initial velocity (starting speed)
Amount of
time it takes
to change
speed
To calculate acceleration, you follow a few simple steps
1.
Write the formula
a=
Vf – Vi
t
2.
Place the numbers and units in the formula. For example, if you slow down from 5
m/s to 0 m/s and it takes you 2 seconds to change speed, you do this:
Remember to place the final speed FIRST!!!
0 m/s – 5 m/s
2s
Calculate.
1. Do subtraction of speeds first.
2. Divide that answer by the time.
a=
3.
a=
Subtract first; then divide!
-5 m/s
2s
4.
Write the answer and include units. Remember that it is a speed change divided by
time, so you need to include your speed units and your time units (such as m/s/s or mi/h/s)
In this case, the final answer is:
a=
-2.5 m/s/s
*Remember that in this case the answer is negative because your speed decreased
*This means that in every second you decreased speed by 2.5 m/s