# Kinematic Equations (Constant Acceleration)

1. Oct 16, 2012

### Physics 134

1. The problem statement, all variables and given/known data
The speed limit in a school zone is 40 km/h (about 25 mi/h). A driver traveling at this speed sees a child run onto the road 13m ahead of his car. He applies the brakes, and the car decelerates at a uniform rate of 8.0 m/s^2. If the driver's reaction time is 0.25 seconds, will the car stop before hitting the child?
vo=25mi/h or 11.1 m/s
a=-8.0 m/s^2
xo=0
x=13

2. Relevant equations
x = xo + 1/2(v + vo)t
x = xo + vot + 1/2at^2
v^2=vo^2 + 2a(x-xo)

3. The attempt at a solution
I changed all the values to mi/h to m/s. I tried to plug in the variables to one of the equations but I am just getting crazy answers that don't make any sense. I have no clue what time is and where the reaction time fits in

2. Oct 16, 2012

### mishek

The drivers reaction time would be the time between spotting the child and applying the brakes. During that period, the driver travels some path with full speed.

Hint for time:

v=v0+a*t