Mechanics> motion with constant acceleration...
a driver takes .20s to apply the brakes after he sees a need for it.if he is driving at a rate of 54 km/hr and the brakes are causing a deceleration of 6m/s2,find the distance travelled by the car after he sees the need to put the brakes on ? 
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2 Answers
u = 54 km/hr = 15 m/s
the car will cover a distance of 300 m in the first 20 sec at const. speed of 15 m/s
after applying brakes
u = 15 m/s
a = -6 m/s2
v = 0 m/s
s = (v^2 - u^2)/2a
= 225/12
=18.75 m
hence total distance = 300 + 18.75 = 318.75 m
velocity of car=54km/h=15m/s
ditance covered before applying breaks=0.2*15=3m
deceleration due to breaks= 6m/s2
v2=u2+2as
0=225-2*6*s
s=18.6
total distance=18.6+3=21.6
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