Question

A car is approaching a traffic light at 18.0 m/s when the light turns red. The driver takes 0.80 s to react and puts his foot on the brake pedal after which the car decelerates at 2.0 m/s^2 . What is the car's stopping distance?

Answer 1

Use formula s= ut+(12)at^2
u=18 m/s
t =0.8s
a= 2 m/s^2

Source: http://www.mathskey.com/question2answer/

Answer 2

Total stopping distance may be represented by equation s = s1 + s2
where s1 represents the distance traveled by the car before the he reacts.
This is also represented by equation s1 = vt = 18 (0.8)

Also, s2 represents the distance traveled by the car when he reacted.
You may use the kinematic equation Vf = Vo + 2as,
And hence, s2 = (Vf - Vo)/2a
where, Vf = 0 (because the car stops), Vo = 18m/s and a = - 2m/s^2 (negative since it is decelerating)

Substituting the values, you'll have:
s2 = (0-18)/2(-2)

Just add s1 and s2 to solve for the total stopping distance s.