Robbers in a car travelling at 20 m/s pass a policeman on a motor cycle at rest.The policeman immediately starts chasing the robbers.The policeman accelerates at 3 m/s2 for 12 s and thereafter travels at a constant velocity .Calculate the distance covered by the policeman bfore he overtakes the car ?
Suppose after time, t policeman overtakes the car.
Also, distance covered by both car and motor cycle is same, x(say).
For robber,
v =x/t
=> 20 = x/t
=> x = 20t .......................(1)
For policeman,
for first 12s, u =0
v =0+ 3*12 = 36m/s.
s = 0 + 0.5*3*12*12
=> s = 216m
for time, t
x = s + 36*t' = s +36(t - 12)
=> 20t = 216 +36t - 432 (using eqn. 1)
=> 16t = 216
=> t = 13.5s
So, x = 20*t = 20* 13.5 = 270m.