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Homework Help: Speeding Car on Intersection

  1. Sep 10, 2008 #1
    1. The problem statement, all variables and given/known data
    You are driving a car at the 25-mi/h speed limit when you observe the light at the intersection 70 m in front of you turn yellow. You know that at that particular intersection the light remains yellow for exactly 4.0 s before turning red. After you think for 1.0 s, you then accelerate the car at a constant rate. You somehow manage to pass your 4.3 m car completely through the 15.0 m wide intersection just as the light turns red, thus narrowly avoiding a ticket for running a red light. Immediately after passing through the intersection you take your foot off the accelerator, relieved. However, down the road you are pulled over for speeding. You assume you were ticketed for the speed of your car as it exited the intersection. Determine this speed and decide whether you should fight this ticket in court.

    How fast were you going as you exited the intersection?

    2. Relevant equations
    Basic Kinematics Equations

    3. The attempt at a solution
    Initial Velocity in m/s = 11.18

    The distance to travel is 80.12, since the 4.3m car has to get through all the way, and you already covered 11.18 meters in the second you were thinking.
    So the equation becomes
    a=(80.12m/s-11.18m/s^2*3s)/(.5*9s)=10.3511 m/s^2
    And then I used v^2=Vo^2 +2a(x-xo) and plugged in the number like so:
    V = [tex]\sqrt{11.18^2 + 2*10.3511*80.12}[/tex] and got the answer 42.233, which is incorrect. Can anyone help me please? This is due kind of soon. =/

    I realized my problem: I did not revert it back to mi/hr, so the correct answer is 91.414 mi/hr.
    Last edited: Sep 10, 2008
  2. jcsd
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