Question

A person driving a car with a speed 72 km/h, suddenly sees a boy crossing by car before the person applies brakes is 5 m, the reaction time of the person will be

• A. 0.5 s
• B. 0.10 s
• C. 10 s
• D. 0.25 s

Answer 1

The correct option is A 0.5 s

$72\times \frac{5}{18}=20m/sec$
Using third equation of motion,
$2as=v^2-u^2$
Here, a is negative.
$-2as=0-(20{)}^2$
$+2\times a\times s=+400$
a = 40 m/$s^2$
Using first equation of motion,
v = u + at
4 = u - at
0 = +20 - 40 t
20 = 40 t
t = 0.5 sec