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Homework Help: Am I missing something? Basic acceleration

  1. Sep 7, 2008 #1
    Hey everyone, quick question from the new guy. I don't know maybe I'm just making it harder than it is.

    Car A is traveling east at 15.7 m/s. As car A crosses the intersection, car B starts from rest 42 m north of the intersection and moves south with a constant acceleration of 1.2 m/s2.
    What is the distance between A and B 5 s after A crosses the intersection?

    So car A is obviously 78.5 meters east of the intersection. It seems like a simple a^2 +b^2=c^2 problem but for some reason I cant convert car B's acceleration of 1.2 m/s^2 to a correct displacement. It seems to me car B should be 36m from the intersection but that cant be correct.
    So can someone please explain to me how to convert acceleration to displacement.

    P.S. I tried using delta x=vit+1/2axt2 with no luck either.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 7, 2008 #2


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    That's strange. I would have tried that. Could you show us the details of how you tried to use it?
  4. Sep 7, 2008 #3
    I must be using it incorrectly. I used,
    (1.2)*(5)+1/2(1.2)(5)^2= 21

    then ((21)^2 +(78.5)^2)^1/2 = 81.3

    Is my initial velocity wrong and what causing the problem?
  5. Sep 7, 2008 #4


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    What is the (1.2)*(5) ?
  6. Sep 7, 2008 #5
    acceleration multiplied by time
  7. Sep 7, 2008 #6


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    Yes. I know what it is. Just wondering why you put that term in there?

    Wasn't car B at rest?
  8. Sep 7, 2008 #7
    I see what your saying. The initial velocity isn't 1.2 its 0. That makes sense and gives a correct answer.

    thanks for the help. This is the first time I have used this site and you guys where great.:!!) Thanks again.
  9. Sep 7, 2008 #8
    one more quick question though

    the next question is asking what is the speed and magnitude of acceleration of car B relative to A

    What exactly do they mean by relative?
  10. Sep 7, 2008 #9
    In relation to each other, I'm guessing.
  11. Sep 7, 2008 #10


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    Yes that is correct.
  12. Sep 7, 2008 #11
    can you be a little more specific? I cant seem to find anything in the text about how to go about seting these up.
  13. Sep 7, 2008 #12
    am I wrong assuming the final velociy of car B is 6 m/s at T=5?
  14. Sep 7, 2008 #13


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    Start out by thinking about what is the equation for the distance car B travels relative to the origin (intersection) in terms of velocity and acceleration.

    Likewise for the distance Car A travels from the origin (intersection).

    Perhaps you can exploit the fact at any point in time the distance traveled by both is along the sides of a right triangle?
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