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Velocity/Acceleration word problem. I'm so confused

  1. Sep 29, 2009 #1
    1. The problem statement, all variables and given/known data
    You are traveling at 20 m/s and the stop light turns yellow. You have to make an insantaneous decision: Do i brake before the intersection or do I hit the gas pedal and beat the light? Keep in mind that this particular intersection is photo-enforced, and an $85 traffic ticket will be issued if you run the stop light.

    total time must not exceed 5 seconds
    You have a 1 second reaction time.
    The intersection is 15m wide.
    When you first saw the light turn yellow, you were 50m in front of the intersection.
    Acceleration rate is 2m/s^2
    Deceleration rate is 5m/s^2



    2. Relevant equations
    a= vf-vi/t


    3. The attempt at a solution

    If you stop at the light:

    Time Action Velocity
    0 Saw Light Turn 20m/s
    1 Reaction 15 m/s
    2 10m/s
    3 5 m/s
    4 0 m/s
    5 -5 m/s

    If you try to beat the light:

    Time Action Velocity
    0 Saw Light Turn 20 m/s
    1 Reaction 22m/s
    2 Hit the Gas 26m/s
    3 Speed Up 32 m/s
    4 Speed Up 40 m/s
    5 Speed Up 50 m/s

    There are more questions but this is the main part i am unsure about. :/
     
  2. jcsd
  3. Sep 29, 2009 #2
    [tex]x=v_ot+\frac{1}{2}at^2[/tex]
    You can use that. If he decides to stop he needs to travel less than 50m in the time.
    If he decides to speed up he needs to travel more than 50m during that time.

    In the second part, your acceleration is constant at 2. You went all over the place with your acceleration.
     
  4. Sep 29, 2009 #3
    What is vo?

    Sorry, she doesn't teach much. she jsut hands out the paper and tells us to complete it.

    also, what are you finding? (what is x standing for? Velocity? Acceleration?)
     
  5. Sep 29, 2009 #4
    Vo is your initial velocity and x is the distance traveled.
     
  6. Sep 29, 2009 #5
    Alright, I'll work on this in a few minutes and let you know what I get.

    I'm trying to find the velocity though.

    With a beginning velocity of 20 m/s and in increase of 2m/s^2

    I get the answers:

    Time Velocity
    0 20 m/s
    1 22m/s
    2 26m/s
    3 32 m/s
    4 40 m/s
    5 50 m/s

    But you said it was constant so it should be:

    20
    22
    24
    26
    28
    ?
     
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