A car is travelling at 20m/s along a road. A child runs out into the road 50m ahead and the car driver steps on the brake pedal. What must the car's declerations be if the car is to slop just before it reaches the child .

Answer 1

50 = Vi t + .5 a t^2

v = Vi + a t = 0 at end
so
a = -Vi/t = -20/t

50 = 20 t - .5 (20/t)t^2

50 = 20 t - 10 t = 10 t

t = 5 seconds
a =-20/5 = -4 meters/second^2

Answer 2

-4m/s^2