1. Not finding help here? Sign up for a free 30min 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!

How do I finish this?

  1. Aug 28, 2005 #1
    A person driving her car at 57 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 .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 -6.6 m/s2, whereas it can accelerate from 57 km/h to 70 km/h in 3.9 s. Ignore the length of her car and her reaction time.

    If she hits the brakes, how far will she travel before stopping?
    19 m (I know how to do this one )

    If she hits the gas instead, how far will she travel before the light turns red?
    ? m (How would I do this one?)
     
  2. jcsd
  3. Aug 28, 2005 #2

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    "whereas it can accelerate from 57 km/h to 70 km/h in 3.9 s."
    How can you find her possible acceleration from this piece of info?
     
  4. Aug 28, 2005 #3

    Fermat

    User Avatar
    Homework Helper

    her speed changes from 57km/h to 70 km/h, which is a change of 13 km/h, in 3.9s.
    13 km/h = 13/3.6 = 3.6 m/s
    So, accln is 3.6/3.9 = .925 m/s²
     
  5. Aug 28, 2005 #4

    Fermat

    User Avatar
    Homework Helper

    You know the accln.
    You know her initial speed.
    You know the time limit during which she can travel.

    Use the suvat eqn for distance travelled under constant accln.
     
  6. Aug 28, 2005 #5
    a=.93
    Initial speed =15.83
    time = 2secs
    is that right? I just cant seem to get the right answer.
     
  7. Aug 29, 2005 #6

    Fermat

    User Avatar
    Homework Helper

    That's right - you should have used the eqn: s = ut + ½at².

    If you used that, what did you get.
     
  8. Aug 29, 2005 #7
    Is the correct answer 33.52 ?
     
  9. Aug 29, 2005 #8

    Fermat

    User Avatar
    Homework Helper

    yep - close enough, depending on rounding errors.

    a = 0.923 m/s²
    u = 15.83 m/s
    t = 2 s

    s = ut + ½at²
    s = 15.83*2 + ½*0.923*4
    s = 31.66 + 1.846
    s = 33.506 m
    ========
     
  10. Aug 29, 2005 #9
    Thanks alot for everything!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?