How to find distance with the given information?

[jle1092]

The speed limit in a school zone is 40 km/hr. A driver traveling at this speed sees a child run into the road 13 m ahead of the car. He put on his breaks and decelerates at a rate of 8.0 m/s^2. If the drivers reaction time is .25 s, will the car stop before hitting the child?



a= - 8.0 m/s^2
t= .25s
d= ?

I am just confused about what equation to use.

 

[verty]

There isn't an equation per se because many things happen. The equations can only be used when acceleration is constant. You will use one equation for each time period in which the acceleration is constant.

The way to do it is to write down the important points in time (so that the periods between those points have constant acceleration), then calculate displacement and time for each point. By the end, you will have the final displacement and you can check whether it is <= 13m.