- Spotting a police car ahead, the driver of the car in the previous
example slows from 32 m/s to 20 m/s in 2 sec. Find the car’s
average acceleration.
Here’s How to Crack It
Dividing the change in velocity (20 m/s − 32 m/s = −12 m/s) by the time interval
during which the change occurred (2 s) gives us ā = ∆v/∆t = (−12 m/s) / (2 s) = −6
m/s^2. The negative sign means that the direction of the acceleration is opposite the
direction of the velocity: The car is slowing down.
If an object has negative velocity, then a positive acceleration means it is slowing
down and a negative acceleration means it is speeding up. This can be confusing.
Just remember that if velocity and acceleration point in the same direction, the
object is speeding up and if they point in opposite directions, it is slowing down.
Note: If velocity and acceleration are perpendicular, the object is turning with
constant speed.