Kinematic Equations (Constant Acceleration)

  • #1

Homework Statement


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

Homework Equations


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


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
 

Answers and Replies

  • #2
93
3
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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