1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Kinematics, driving question

  1. Oct 1, 2008 #1
    1. The problem statement, all variables and given/known data
    you're driving down the highway late one night at 20ms when a deep steps onto the road 35m infront of you. your reaction time before steeping on the brakes is 0.5s and the max deceleration of ur car is 10m/s^2

    what is the max speed you could have and still not hit the deer?

    deltaX=35m V=?
    a=-10m/s^2 reaction time = 0.5s

    2. Relevant equations

    Xf= Xi+ViT+1/2aT^2

    3. The attempt at a solution
    so this is what i tried

    i pluged the numbers into the first equation to get


    and i have 2 missing variables so i tried to get it from the other equation

    0-Vi=(-10)T then that didnt work so i tried the other equation

    Vi= 13.23m/s

    and i know that Vi can't be right because i did the question before it and it had Vi of 20m/s and it was 5m from hitting the deer...i dont know what i did wrong can someone help please?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 1, 2008 #2


    User Avatar
    Science Advisor

    In order to JUST miss hitting the deer, it must take all 35 meters to slow to 0 m/s. So you have TWO of the equations you just gave: 35=0+ViT+1/2(-10)T^2 for the 35 m and 0-Vi=(-10)T for the time to slow to 0. Now you have two equations in the two "unknown" numbers Vi and T. 0-Vi= -10T tells you that Vi= 10T. Replace Vi by that in the first equation to get a simple equation for T alone. Once you have found T, put that into Vi= 10T to find Vi.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Kinematics, driving question
  1. Kinematics Questions (Replies: 4)

  2. Kinematics Question (Replies: 6)

  3. Kinematic Question (Replies: 2)