Question

35
14) 1.5 m/s
A person driving a car with a speed 72 km/h, suddenly sees a boy crossing the road. If the distance moved
by car, before the person applies brakes is 5 m, the reaction time of the person is
(1) 0.5 s
(2) 0.10 S
(3) 10 s
247 0.25s​

Answer 1

u=72km/h=20m/s

v=0

s=5m

2as=v²-u²

$2 \times a \times 5 = {0}^{2} - {20}^{2} \\ = > 10a = - 400 \\ = > a = - 40ms ^{ - 2}$

Therefore reaction time,

v=u+at

$0 = 20 + ( - 40) \times t \\ = > 20 = 40t \\ = > t = 0.5s$

Answer 2

0.5sec

Explanation:

as u=20m/s

v=0

and s=5m

v²-u²=2as

a=-40m/s

v=u +at

t=u/a

t=0.5sec