An auto mobile travelling with a speed of 60 km/h can be stopped by brakes after  covering distance of 20 m. If the same car is going at speed of 120 km/hr, the minimum stopping distance will be

$\begin{array}{l}\\ given final velocity is zero i.e. V=0; dis\tan ce s_1=20m;\\ initial velocity=u_1=60km/h; initial velocity=u_2=120km/h\\ V^2-{\mathrm{u}}^2=2\mathrm{as}\\ 0^2-{\mathrm{u}}^2=2\mathrm{as}\\ \Rightarrow -{\mathrm{u}}_1^2=2{\mathrm{as}}_1---\left(1\right)\\ \Rightarrow -{\mathrm{u}}_2^2=2{\mathrm{as}}_2---\left(2\right)\\ eqn\left(2\right) divided by eqn\left(1\right)\\ \Rightarrow \frac{{\mathrm{s}}_2}{{\mathrm{s}}_1}=\frac{{\mathrm{u}}_2^2}{{\mathrm{u}}_1^2}\Rightarrow \frac{{\mathrm{s}}_2}{{\mathrm{s}}_1}=\frac{{120}^2}{{60}^2}\\ {\mathrm{s}}_2=4{\mathrm{s}}_1 \mathrm{substitute} {\mathrm{s}}_1 \mathrm{value}\\ \Rightarrow {\mathrm{s}}_2=4\times 20\mathrm{m}\\ \Rightarrow {\mathrm{s}}_2=80\mathrm{m}\end{array}$