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Find Equation From a Graph

  1. Mar 4, 2007 #1
    Finding an Equation from a Graph

    Hi,

    I need to find an equation from a graph.

    The graph itself is not very complex. Just need some help.

    I cant really show you the graph but I'll give you the points so you can plot it to paste them in to Excel or something.

    x co-ords y co-ords
    0 0
    1 1.5
    2 3
    3 3
    4 3
    5 1.5
    6 0
    7 0
    8 0
    9 -1
    10 -2
    11 -2
    12 -2
    13 -1.5
    14 -1
    15 -0.5
    16 0

    Please help . . . your help is much appreciated

    Thanks
     
  2. jcsd
  3. Mar 4, 2007 #2
    Hi,

    Well this isnt really homework - Its just buggin me how I would do this.

    I want to find an equation from a graph.

    The graph itself is not very complex. Just need some help.

    I cant really show you the graph but I'll give you the points so you can plot it to paste them in to Excel or something.

    x co-ords y co-ords
    0 0
    1 1.5
    2 3
    3 3
    4 3
    5 1.5
    6 0
    7 0
    8 0
    9 -1
    10 -2
    11 -2
    12 -2
    13 -1.5
    14 -1
    15 -0.5
    16 0

    Please help . . . your help is much appreciated

    Thanks
     
  4. Mar 4, 2007 #3
    Finding Equation from Graph

    Hi,

    Yeahhh I just posted this in the wrong forum.
    Anyway this isnt homework - Its just buggin me how I would do this.

    I want to find an equation from a graph.

    The graph itself is not very complex. Just need some help.

    I cant really show you the graph but I'll give you the points so you can plot it to paste them in to Excel or something.

    x co-ords
    0
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16

    corresponding y co-ords
    0
    1.5
    3
    3
    3
    1.5
    0
    0
    0
    -1
    -2
    -2
    -2
    -1.5
    -1
    -0.5
    0
    Please help . . . your help is much appreciated

    Thanks
     
  5. Mar 4, 2007 #4
    what's the problem? You just plot them into a CAS-tool and use regression of some kind? and then you'll get x as a function of y..
     
  6. Mar 4, 2007 #5

    StatusX

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    Homework Helper

    There are many, many equations whose graphs would pass through those points, and even more which would come close to them. Exactly what type of equation are you looking for (polynomial, exponential, etc)?
     
  7. Mar 4, 2007 #6
    Hey thanks for your reply. Whats a CAS tool? Like what?
    Im not sure I have the right software for it. Is there any manual way to do this?


    Thanks
     
  8. Mar 4, 2007 #7
    I just want to relate x and y over those values via function
    How would i Go about doing this?
     
  9. Mar 4, 2007 #8
    it's like an advanced calculator like a Texas Instruments Ti-89 Titanium like mine, or you could use Excel like you say..



    I'm from Denmark and use the Danish excel so I'm not sure what the different menus and functions is called in English..

    [​IMG]

    not exactly a "pocket" calculator, but really good..
     
    Last edited by a moderator: Mar 4, 2007
  10. Mar 4, 2007 #9

    StatusX

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    Homework Helper

    Think of it like this. Say you find a function f that passes through all of those points. Now think of another function g that is zero at x=0,1,...,16. You should be able to find a lot of these (eg, g=0, or g=sin(pi*x), or g=x(x-1)(x-2)...(x-16), or g=x(x-1)(x-2)...(x-16)(x-123), etc). Then adding f to g gives another function that passes through all the points. So, like I said, there are a lot of them. What is this for?
     
  11. Mar 4, 2007 #10

    arildno

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    Gold Member
    Dearly Missed

    There are an infinite number of continuous functions whose graphs go through all of those points.

    that is, your question is ill-posed.
     
  12. Mar 4, 2007 #11

    arildno

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    Dearly Missed

    Do not double post your questions.
     
  13. Mar 4, 2007 #12

    ssd

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    What do you want: a curve passing through all the points or a curve to substitute the function from which the points were derived with some possible errors in obsrevation.
     
  14. Mar 4, 2007 #13

    ZapperZ

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    Education Advisor

    2 threads have been merged because the OP made multiple posts. So if the flow makes no sense, don't blame me.

    Zz.
     
  15. Mar 4, 2007 #14
    basically its some values I determined experimentally in the labs. And over 16secs the graph is drawn. Now, I'm using a program to simulate my findings but I need an equation to relate this graph. Can anyone give me an equation ofr tell me how I can work one out?
     
  16. Mar 4, 2007 #15
    Hey thanks for replies

    basically its some values I determined experimentally in the labs. And over 16secs the graph is drawn. Now, I'm using a program to simulate my findings but I need an equation to relate this graph. Can anyone give me an equation ofr tell me how I can work one out?
     
  17. Mar 4, 2007 #16
    I would suggest that you plot them in to a coordinate-system using Excel.. Then looking at them and choosing the right kind of regression (power, quad, lin, exp and so on)...

    Make an experiment and see which regression brings the R^2 closest to 1...
     
    Last edited by a moderator: Mar 4, 2007
  18. Mar 4, 2007 #17
    Thanks for your replies!!

    But you aint really helped. Is there anyone who can give me a simple explanation how to do this or who can actually do it and walk the walk.

    No offence I'm thankful for your efforts but I need telling simply because I am rather simple myself.


    thanks
     
  19. Mar 4, 2007 #18

    StatusX

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    Homework Helper

    Do you understand what we mean when we say there are many equations whose graphs touch those points? There's no single "equation for this graph". So unless you're more specific about what you want, we can't help you.
     
  20. Mar 4, 2007 #19
    As i said make an experiment and see which regression brings the R^2 closest to 1...
     
  21. Mar 4, 2007 #20

    Integral

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    Gold Member

    The only way we could help you arrive at a meaningful function discription is if you tell us what the physcal system these measurments come from. That way we couldmodle the system and not the data.
     
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