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  4. An automobile, travelling at 40 km/h, can be stopped at a distance of 40 m by applying brakes. If the same automobile is travelling at 80 km/h, the minimum stopping distance, in metres, is (assume no skidding) :

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Q. An automobile, travelling at $40\, km/h$, can be stopped at a distance of $40\, m$ by applying brakes. If the same automobile is travelling at $80\, km/h$, the minimum stopping distance, in metres, is (assume no skidding) :

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Solution:

Given, automobile travelling at $40\, km / h$ can be stopped at a distance of $40\, m$, then stopping distance
$=\frac{v^{2}}{2 a} \Rightarrow 2 a=\frac{v^{2}}{\text { stopping distance }}$
$=\frac{(40)^{2}}{40 m }=40$
Therefore, stopping distance of automobile travelling at $80\, km / h$ is
Stopping distance $=\frac{v^{2}}{2 a}=\frac{(80)^{2}}{2 a}=\frac{(80)^{2}}{40}=160\, m$
Thus, minimum stopping distance of automobile travelling at $80\, km / h$ in metres is $160\, m$.