Question

A car traveling at 20 m/s when the driver sees a child standing in the road. He takes 0.80 s to react, then steps on the brakes and slows at 7.0 m/s2. How far does the car go before it stops?

Answer 1

When driver see the child standing on road his speed is 20 m/s

So here at that instant his reaction time is 0.80 s

He will cover a total distance given by product of speed and time

`d_1 = v* t`

`d_1 = 20 * 0.8`

`d_1 = 16 m`

now after this he will apply brakes with acceleration a = 7 m/s^2

so the distance covered before it stop is given by

`v_f^2 - v_i^2 = 2 a d`

`0 - 20^2 = 2*(-7)*d_2`

`d_2 = 28.6 m`

so the total distance covered by it

`d = d_1 + d_2`

`d = 16 + 28.6 = 44.6 m`

so it will cover a total distance of 44.6 m