Question

A car is traveling at 20.0 m/s, and the driver sees a traffic light. turn red. After 0.530 s (the reaction time), the driver applies the brakes,. and the car decelerates at 7.00 m/s2. What is the stopping distance of the. car, as measured from the point where the driver first sees the red light?

Answer 1

Before the driver applies the brakes ( with the reaction time ):
d 1 = v0 · t = 20 m/s · 0.53 s = 10.6 m
After that: 
v = v0 - a · t1  
0 = 20 m/s - 7 · t1
7 · t1 = 20
t1 = 2.86 s
d 2 = v 0 · t1 - a · t1² / 2
d 2 = 20 m/s · 2.86 s - 7 m/s² · (2.86 s)²/2 = 57.2 m - 28.6 m = 28.6 m
d = d 1 + d 2 = 10.6 m + 28.6 m = 39.2 m
Answer: the stopping distance of a car is 39.2 m.