Question

A car is moving in a straight line with speed 18 km/hr. It is stopped in 5s by applying the brakes. Find (i) the retardation and (ii) speed of the car after 2s of applying the brakes.

Answer 1

u=18 kmph = 18*5/18 m/s = 5m/s

t= 5s

v= 0

v=u-at

= v-u = -at

= -a = (v-u)/t

= -a = 0-5/5 = -5/5 = -1

= retardation = 1m/ss

case 2 :

u=5m/s

t=2s; a=-1

v=u-at

= v=5-1*2 = 5-2 = 3

speed=3m/s = 3*18/5 = 10.8 km/h