Question

You're driving at 39km/h , when the traffic light 25m away turns yellow.

I need to find the constant acceleration required to stop at the light, the stopping time, and weather the acceleration is reasonable.

I know how to find the average and instantaneous acceleration, but how to obtain the constant, I have no clue.

Answer 1

39,000 m/3600 s = 10.83 m/s

average speed during braking = 10.83/2 = 5.42

25 = 5.42 t
t = 25/5.42 = 4.62 seconds to stop

v = Vi + a t
0 = 10.83 + a (4.62)
a = -2.34 m/s^2

1/4 g is not an unreasonable deacceleration

Answer 2

39 km/hr = 10.83 m/s

s = Vi*t + 1/2 at^2
25 = 10.83t + .5 at^2

v = Vi + at
10.83 + at = 0
so,

t = 2.35s
a = -4.46 m/s^2

Answer 3

Go with Damon - I switched the a and the t.