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Equations of the three parabolas

  1. May 11, 2007 #1
    1. The problem statement, all variables and given/known data

    Computing the equations of the three parabolas involved

    2. Relevant equations

    y=A1x^2 + B1

    y= A2x^2+B2


    3. The attempt at a solution

    Tried to get the constants A and B but unsuccessful!!

    See the attached file for the three parabolas.The dotted points indiacate the change in curvature.
    How to get 2 constants in each of the three parabolas?Please help!!

    Attached Files:

  2. jcsd
  3. May 11, 2007 #2


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    Staff: Mentor

    Pretty hard to solve for the numbers when there are no number labels on the graphs..... Am I missing something?
  4. May 11, 2007 #3


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    You will need to specify the spline points, that is, where your parabolas meet.

    You should also match the slopes at those points.

    Next trouble is that you have:

    [tex] A_n x^2 + B_n [/tex] where n = 1,2,3
    But you need:
    [tex] A_n (x - {k_n})^2 + B_n [/tex] where n = 1,2,3
    Where the [itex] k_n [/itex] specify the x coordinate of the apex of the parabola.

    Matching the slopes seems to requires:
    [tex] A_n (x_n- k_n) = A_{n+1}(x_n-k_{n+1})[/tex] for n = 1 2 where [itex] x_n [/itex] is your 2 spline points.

    By matching the slopes you will avoid having a corner at the matching point.
    Last edited: May 11, 2007
  5. May 12, 2007 #4
    The total length is 7 metres.
    The first parabola meets the second at 0.7 metres.
    The second parabola meets the third at 6.3 metres.

    As you see,at the points where the parabolas meet, there is a change in curvature
  6. May 14, 2007 #5


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    Staff: Mentor

    Not enough data. What are the x,y points of join?
  7. May 14, 2007 #6


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    The more information you can provide the easier it will be to help. Currently we are having to guess what it is you are trying to do. Could you please make an effort to actually express what it is you are trying to do. Use more then 3 words in a sentence.

    Now, that said, go back and read my last post. Until you get a correct experssion for each of the parabolas there is no way you can arrive at a correct solution. The equations in your original post cannot work. They represent 3 parabolas centered at zero, if I guess correctly NONE of your parabolas have a center at zero.

    If you do not want to match slopes at the splines that is your call, just means you do not have those as constraints.
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