Question

A Driver is driving a car at speed of 72 km/hr on road. He sees a boy on road at a distance of 48 m from him. If his reaction time is 0.4 sec, find minimum retardation required to save the boy.

• A. $4m/s^2$
• B. $4.4m/s^2$
• C. $3m/s^2$
• D. $5m/s^2$

Answer 1

The correct option is D $5m/s^2$


By the time he reacts, he travels a distance of 8m. So, he is 40 m away from the car.
$v_0^2-0^2=2as\Rightarrow {20}^2=2\times a\times 40$
$\Rightarrow a=\frac{v_o^2}{2s}=\frac{20\times 20}{2\times 40}=5m/s^2$