Question

a car traveling at a speed of 30.0 m/s encounters an emergency and comes to a complete stop. how much time will it take for the car to stop if it decelerates at -4.0m/s^2

Answer 1

It will take 7.5 s for the car to stop.

One of the acceleration formulas is 

$v_{f} = v_{i}$ +at, where

$v_{f}$ is the final velocity,
$v_{i}$ is the initial velocity,
a is the acceleration, and
t is the time.

0 = 30.0 m·s⁻¹ + (-4.0 m·s⁻²)t

4.0 s⁻¹ t = 30.0

t = $\frac{30.0}{ 4.0 \text s^{-1}}$= 7.5 s