Question

A car is traveling 20m/s when the driver sees a child standing in the road. He takes 0.8s to react, then steps on the brakes and slows at 7.0m/s^2. How far does the car go before it stops?

i don't know where to start.

Answer 1

First figure out how far the car went in those first .8 seconds at 20 m/s

Then do the deacceleration problem

a = -7 m/s^2
vo = 20

v = v0 + a t = 20 - 7 t
so use that to find t when stopped, v = 0
0 = 20 - 7 t
solve for t, that is how long it deaccelerated to stop. Then use that t in
x = x0 + 20 t -(1/2)(7) t^2
that xo, the initial position, is the distance you got from the .8 second reaction time.

Answer 2

13.76m