Question

A car is traveling at 72 km/hr along a road.A child runs out into the road 50m ahead and the car driver steps on the brake pedal.What must be car's accelerationbe if the car is to stop before it reaches the child?

Answer 1

speed of car =72km/ hr
child speed =50m
acceleration of car = 72/60×50=60

Answer 2

u = 72 km/hr = 20 m/s
s = 50 m
v = 0 m/s
So,v2 - u2 =2as
=> a = v2 -u2 /2s = (0-20^2)/2x50 = -400/100 = -4 m/s2Hence,acceleration = -4 m/s2 and retardation  = 4 m/s2