A car moving with a speed of 40km/hr can be stopped by applying brakes after at least 2m. If the same car is moving at a speed of 80km/hr, what is the minimum stopping distance?

Using the third equation of motion,

v²-u² = 2as

As the car stops eventually, final velocity v=0

Therefore, Stopping distance, s=$\frac{u^2}{2a}$

$\frac{s_2}{s_1}=\frac{{\displaystyle \frac{{u_2}^2}{\cancel{2a}}}}{{\displaystyle \frac{{u_1}^2}{\cancel{2a}}}}$

$\frac{s_2}{s_1}=\frac{{u_2}^2}{{u_1}^2}$

$s_2={\left[\frac{u_2}{u_1}\right]}^2{s}_1$

 

$s_2={\left[\frac{80}{40}\right]}^{2}2$

s_{2 = 8m}