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Kinematic Equations (Constant Acceleration)

  1. Oct 16, 2012 #1
    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. jcsd
  3. Oct 16, 2012 #2
    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
     
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