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Homework Help: Can someone please help me do this problem

  1. Sep 20, 2009 #1
    1. The problem statement, all variables and given/known data

    A person driving her car at 50 km/h approaches an intersection just as the traffic light turns yellow. She knows that the yellow light lasts only 2.0 s before turning to red, and she is 30 m away from the near side of the intersection (Fig. 2-29). Should she try to stop, or should she make a run for it? The intersection is 15 m wide. Her car's maximum deceleration is -5.8 m/s2, whereas it can accelerate from 50 km/h to 70 km/h in 6.0 s. Ignore the length of her car and her reaction time.
    If she hits the brakes, how far will she travel before stopping?
    If she hits the gas instead, how far will she travel before the light turns red?

    2. Relevant equations

    i tried vf^2=Vi^2+2ad

    3. The attempt at a solution
    i didnt get the right answer. so can some please help??!!
    here's the pic http://i38.tinypic.com/2h4j86d.gif
  2. jcsd
  3. Sep 20, 2009 #2
    First, Did you get your units done properly... everything in meters and seconds perhaps?
  4. Sep 21, 2009 #3
    actually yea my units were wrong. i found the answer. its 16.632 and 29.63. thnx for ur help pgardn
    Last edited: Sep 21, 2009
  5. Sep 21, 2009 #4
    Emmm ...

    Question # 1

    Should she try to stop, or should she make a run for it ?

    We have Her driving speed = 50*(1000/3600) m/s
    Distance From car to traffic Light = 30 m
    The time that yellow light can stand = 2.0 s

    Speed = Distance / Time So Time = Distance / Speed

    t=d/v , t= [30] / [50*(1000/3600)]

    Now If t > 2.0 then she Should she try to stop
    And If t < 2.0 then she should make a run for it

    I Think it could be theright answer for Q1 So try it and let us know ^^

    I'll come back for next 2 Questions.
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