Understanding the Concepts

* Acceleration: The rate at which an object's velocity changes over time.

* Initial Velocity (vi): The starting speed of the car (20 m/s).

* Final Velocity (vf): The speed of the car when it comes to a stop (0 m/s).

* Time (t): The time it takes for the car to slow down (5 seconds).

Formula

The formula for acceleration is:

acceleration (a) = (final velocity (vf) - initial velocity (vi)) / time (t)

Calculation

1. Plug in the values:

a = (0 m/s - 20 m/s) / 5 s

2. Simplify:

a = -20 m/s / 5 s

3. Calculate:

a = -4 m/s²

Answer: The car's acceleration is -4 m/s².

Important Note: The negative sign indicates that the car is decelerating (slowing down).