100 m from a traffic light, a motorist sees it to red. If he needs to accelerate at -1 m.s ^2 to stop at the traffic light, what is the initial speed, and how long did he take to stop?

Answer 1

Let initial speed = Vo
Average speed while decelerating = Vo/2
X = 100 m
Time to decelerate to zero =
T = 2 X/Vo
also,
T = Vo/a.
Therefore
2X/Vo = Vo/a
Vo = sqrt(2 a X) = 14.1 m/s
T = 14.1/1 = 14.1 s