Question

A car is moving on a straight road with uniform
velocity 72 kmh 'Suddenly the driver saw a child
in the middle of road and applies brakes. The just
able to save the child, then reaction time of the
driver is (Initial distance between car and child is
5 m and due to application of brakes uniform
retardation of 50 media developed)
(1) 0.04s
(2) 0.4 s
(3) 0.05 s
(4) 0.45 s​

Answer 1

v=72km/hr

=20m/s

u=0m/s

a=50m/s^2

v=u+at

t=v-u/a

t=20-0/50

t=20/50

t=0.4s

Thus, (2) is the correct option.

Answer 2

Explanation:

v = 0 m /s

u = 20m /s

a = -50 m/s^2

s = v^2-u^2/2a = 4 m

S ' = 5m -4m= 1m

S' = u t'

→ 1m = 20m/s • t'

→ t' = 0.05 s