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Homework Help: Chosing to stop or accelerate through a traffic light

  1. Aug 30, 2007 #1
    1. A person driving her car at 47 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 -5.6 m/s2, whereas it can accelerate from 47 km/h to 70 km/h in 6.9 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. V^2=Vi^2+2ax

    3. 47 km/h=13.05 m/s 0=(13.05)^2+29-5.6)=159.1 (That just doesn't seem right what am I doing wrong)
  2. jcsd
  3. Aug 30, 2007 #2


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    Please rewrite the equation. " (13.05)^2+29-5.6) " is not correct.

    Can the driver stop in 30 m or less?

    V^2=Vi^2+2ax is correct. Solve for x knowing that v = 0, vi = 13.05 m/s and a = -5.6 m/s.

    I get an answer an order of magnitude less than 159.1.
  4. Aug 30, 2007 #3
    I do that and it doesn't work its the wrong answer everytime...
  5. Aug 30, 2007 #4


    and what do u get for x now?
  6. Aug 30, 2007 #5
    thanks...I got the answer I needed.
  7. Aug 30, 2007 #6
    what about the second part of the problem?
  8. Aug 30, 2007 #7
    I guess you haven't showed any work for it =XP
  9. Aug 30, 2007 #8
    ok what I did was:

    Vi=13.05 Vf=26.38 t=6.9 a=?

    26.38=13.05+(a)(6.9) = 1.93

    then I used.


    and that's not the right answer...
  10. Aug 30, 2007 #9
    70 km/h = 19.4444444 meters / second
  11. Aug 30, 2007 #10
  12. Aug 30, 2007 #11
  13. Aug 30, 2007 #12
    A rock is dropped from a sea cliff and the sound of it striking the ocean is heard 4.2 s later. If the speed of sound is 340 m/s, how high is the cliff?

    I don't even know where to start....can you help?
  14. Aug 30, 2007 #13
    the stone striked the ground after 't' seconds,
    and, then the sound starts travelling at v of 340 m/s (I guess without the influence of gravity), and reaches to the top in " t' " seconds

    and, you know t+t'=4.2 s,
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