Question

A driver takes 0.20 s to apply the brakes after
he sees a need for it. This is called the reaction time of the
driver. If he is driving car at a speed of 54 kmh-1 and the
brakes cause a deceleration of 6.0 ms-2, find the distance
travelled by the car after he sees the need to put the brakes.​

Answer 1

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

time=t=0.2sec

Final velcoity=v=0m/s

deceleration =a= 6m/s2

Now, Distance travelled by car during reaction time :

Distance=Speedx 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

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Answer 2

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

time=t=0.2sec

Final velcoity=v=0m/s

deceleration =a= 6m/s2

Now, Distance travelled by car during reaction time :

Distance=Speedx 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