Question

What is the acceleration of a car that slows down from 60 m/s to 40 m/s during a period of 10 seconds

Answer 1

Answer: $a=-2m/s^{2}$  

Acceleration $a$ is the variation of Velocity $\Delta V$ in time $\Delta t$ :

$a=\frac{\Delta V}{\Delta t}$   (1)

This can be written as:

$a=\frac{V_{f}-V_{o}}{t_{f}-t_{o}}$   (2)

Where:

$V_{f}=40m/s$  is the final velocity

$V_{o}=60m/s$  is the initial velocity

$t_{f}=10s$  is the final time

$V_{o}=0$  is the initial time

$a=\frac{40m/s-60m/s}{10s-0}$    

$a=\frac{-20m/s}{10s}$

Finally:

$a=-2m/s^{2}$  

The negative sign means the car is desaccelerating