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AP Phyz: Constant Acceleration

  1. Aug 29, 2005 #1
    Hi, our teacher requires us to do problem solving questions, which means a full one page report about the situation. I got stuck on this part and would like some advise.

    P: A person driving her car at 50km/h approaches an intersection just as the traffic light turns yellow. She knows that the yellow light lasts only 2.0s before turning to red, and she is 30m away from the near side of the intersection. Should she try to stop or should she make a run for ti? The intersection is 15m wide. Her car's max deceleration is -6.0m/s^2, whereas it can accelerate from 50 to 70 km/h in 6.0s. Ignore the length of her car and her reaction speed.

    Must find out how long it takes for the car to reach the end of the intersection. First, find the acceleration from 50-70km/h.
    After conversions...
    (19.4m/s-13.9m/s)/6.0s= 0.917m/s^2

    Now must find time it took for her to reach the end.
    d= 45m

    How do I find t and how do I find out if she can make the braking distance?
  2. jcsd
  3. Aug 29, 2005 #2


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    I'm not sure what you want for t. I would just use the 2 seconds the traffic light will take to change and use the equations of motion to see if she is through by then (if x(t=2 sec) > 45 m).
  4. Aug 29, 2005 #3
    First make sure to check the significant figures, as in you're rounding too early, you can round but be consistant and don't make your answer more accurate then the place you rounded to. Have you learned the kinematics equations yet, or what eqs have you learned recently?
  5. Aug 29, 2005 #4
    We've only covered constant acceleration formulas. They are as follow:

    a= (V-Vo)/t
    Average Velocity= Vo+V/2

    Keep in mind I just started AP Phyz, skipped Chem and Phyz.
  6. Aug 29, 2005 #5


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    So you can use that third equation to see how far she gets in two seconds of accelerating from 50 km/hr towards 70 km/hr, and whether she clears the intersection in that time.
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