Motion in one dimension physics problem

[vv_ramirez259]

Homework Statement

Determine the stopping distances for an automobile with an initial speed of 90 and human reaction time of 1.0 : (a) for an acceleration = -5.0 , (b) for = -7.5 .

Homework Equations

vf = vo + at
avg velocity = (vf + vo) /2
d = vo)t + (1/2) at2
vf2 = vo2 + 2ad

The Attempt at a Solution

For part A)I tried using the formula s=ut+1/2at^2, I had that
u=25m/s, t=1.0s, a=-5.0,and then solved for s, which was =23m

For part B) I used the same formula and everything else was
the same, except for this part I let a=-7.5, which was = 21m

 

[Doc Al]

The 1.0 seconds is the time before the driver steps on the brakes and starts accelerating. During that time he travels at constant speed. How far? Add that to the distance that he travels once he starts braking.