Question

A driver takes 0.2 seconds to apply the brakes after he sees a need for it this is called a reaction time of the driver in driving a car at a speed of 54 km per hour and the because of declaration of 6 metre per second square find the distance travelled by the car after this is the need to put the brakesbrakes

Answer 1

Solution:

Initial velocity=u=54km/h=54x5/18=15m/s

Time=t=0.2sec

Final velcoity=v=0m/s

Deceleration =a= 6m/s^2

Now,

Distance travelled by car during reaction time : 

Distance=Speed x time
Distance=15x0.2
=3m

From third equation of motion:
v²-u²=2as
0²-15x15=2x-6xs
s=-15x15/-12
S=225/12a
s=18.75m 

So, the total distance travelled by the driver after  he seeds the need to put the brakes ON= 18.75+3=21.75m