Question

A speeder in an automobile passes a stationary policeman who is hiding behind a bill board with a motorcycle. After a 2.0 sec delay (reaction time) the policeman accelerates to his maximum speed of 150 km/hr in 12 sec and catches the speeder 1.5 km beyond the billboard. Find the speed of speeder in km/hr.

Answer 1

maximum velocity of policeman = 150KM/hr = 125/3m/s

for policeman , applying V = U+at  upto that instant when it has reached its maximum velocity...

  125/3 = 0+a(12)

a(accleration)= 125/36 m/s2  

distance covered by policeman upto this instant = 1/2 at2 = 250m = 0.25km

total distance moved by policeman upto the instant when he catches the speeder is 1.5Km (given )

  distance covered with constant velocty = 1.5-0.25 = 1.25KM = 1250m

V = D/t

t = 1250/125/3 = 30sec

it means , total time by policeman to catch speeder = 30+12=42sec

now , policeman starts his journey 2 sec later so , total time for speeder = 42+2 = 44sec

now let speeder moves with U , then distance covered in 44sec = 44U = 1.5km

  44u = 1500

     u = 34.09m/s

in km/h=122.7 km/hr

Answer 2

Hope it helpful

Total time is 14 sec because

2 sec is reaction time so

12+2=14sec