A driver takes 0.20 s to apply the brakes after he sees need for it. If he is driving a car at speed of 54 km/h and the brakes cause deacc. of 6 m/s^2,find the distance travelled by car after he sees need to put brakes. (ANSWER GIVEN: 22 m) JUSTIFY your answer

The car is moving at speed of 54 km/hr i.e., 15 m/sec

that is initial speed after applying brakes is (u) = 15 m/sec

say  at point A,  he sees the need to put brakes

        at point B,  he puts brakes and then car starts deceleratin

         at point C,  car finally comes to rest

from A to B it moves with speed of 15m/sec in 0.2 sec

so distance covered is S₁ = v.t = 15*0.2 = 3 mts

from B to C it moves with constant deceleration of -6 m/sec2 and initial speed of 15m/sec

Using relation V² - u² = 2*(-a)*S

S₂ = V²/2*a = 15*15 /(2*6) =18.75 mts

the distance travelled by car after he sees need to put brakes is S₁+S₂

 = 21.75 mts  ( nearly 22 mts)