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Homework Help: Issues finding time in acceleration problem

  1. Sep 10, 2010 #1
    1. The problem statement, all variables and given/known data

    You are driving to the grocery store at 16 m/s. You are 130 m from an intersection when the traffic light turns red. Assume that your reaction time is 0.50 s and that your car brakes with constant acceleration.

    x1 = 0 m
    v1 = 16 m/s
    t1 = 0 s
    x2 = ?
    v2 = 16 m/s
    t2 = 0.50s
    x3 = 130 m
    v3 = 0 m/s
    t3 = ?

    2. Relevant equations

    (a) How far are you from the intersection when you begin to apply the brakes?
    (b) What acceleration will bring you to rest right at the intersection?
    (c) How long does it take you to stop?

    3. The attempt at a solution

    for (a)
    (x2-x1) = ((v1+v2)/2)*(t2-t1)
    x2 = 8m

    for (c)
    (x3-x2) = ((v2+v3)/2)*delta t
    122m=(16m/s/2)*delta t
    16s=delta t

    for (b)
    a=delta v/delta t
    a=-1 m/s^2

    I know (a) is correct, where I am having issues is computing the final time so I can then obtain the acceleration.


  2. jcsd
  3. Sep 10, 2010 #2


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    Gold Member

    Hello Bracing,

    Welcome to Physics Forums!
    These are really part of the problem statement, not the relevant equations. But now I'm just being nitpicky...
    You have calculated that the car travels 8 m in 0.5 seconds, which is fine. But the problem statement is asking you how far away the car is from the intersection. (But I see from below that you have already figured out it is 122 m. If you wanted to get that value from your equation directly, you could do it by putting in the 130 m into your (x2 -x1) term somewhere.)
    Be careful of your math. 122/8 isn't exactly 16.
    Your approach is sound. But you'll need to correct that previous mistake first (the time isn't exactly 16 seconds)
  4. Sep 10, 2010 #3
    16s is rounded to significant figures as my instructor instructed the class on how he wanted it. I personally think I'm losing a bit by doing it but he continually stresses that we do.

    I am dumb-founded over this as every example given in class had a stated acceleration, then my homework questions posted only do not.
  5. Sep 10, 2010 #4


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    Well okay, if that's what your instructor wants. But I'm just saying that 122/8 is actually a little closer to 15 than it is 16.

    But apart from the rounding stuff your solution looks okay to me.
    Oh, but that's part of the joy of physics. By learning some basic principles, you can figure problems out from scratch, even without specific examples. That's one reason why physics is phun! :smile:
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