7. A car traveling at a speed of 30.0 m/s encounters an emergency and comes to a complete stop. How muchtime will it take for the car to stop if it slows down at -4.0 m/s2?

In this case we know that the acceleration is -4 m/s2 and that the initial velocity is 30 m/s and the final one is zero. Plugging this values into the equation above and solving for t we have:

$\begin{gathered} -4=(0-30)/(t) \\ t=(-30)/(-4) \\ t=7.5 \end{gathered}$

Therefore, it takes 7.5 seconds for the car to stop.