Question

A car moving along a straight road with speed of 144 km $h^{-1}$ is brought to a stop within a distance of 200 m. How long does it take for the car to stop?

• A. 5 s
• B. 10 s
• C. 15 s
• D. 20 s

Answer 1

The correct option is B 10 s

$u=144km/h=(144\times \frac{5}{18})m/s=40m/s$

$v=0s=200m$

By third equation of motion, $v^2=u^2+2as0=(40{)}^2+2a(200)-1600=400aa=-4m/s^2$

Now, by first equation of motion, $v=u+at0=40+(-4)\left(t\right)-40=-4tt=10sec$