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Why do i get a stright line in both double and single logarithm paper ?

  1. Oct 15, 2011 #1
    1. The problem statement, all variables and given/known data
    i have some data and i have to find out if they can be put in the function f(x)=a^x or f(x)=x^a
    it is about how much time there goes before the population soemwhere grows or such.

    2. Relevant equations
    data: it is about the growth or a population in a villlage (way hard to explain)
    time data: 15,59, 146,362, 571, 900
    population; 72,69,67,65,64,63
    addition: see the attachments. the red apaper is the doublelogarithm paper where i have put my data, and the grey is the single logarithm where i have put my data....but as you see the lines are both straight which is the problem


    3. The attempt at a solution
    then i have been thinking that u can actually just draw it and see if it makes a stright line in a double or single logarithm paper. and where it makes the stright line must be that funktion, but when i did draw it turned out to be stright in both the single logarithm paper and the double logarithm paper ....that is the reason why i need help...because i know it should only be stright in one of them yet it turned out to be in both

    i hope that someone will help me
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

    Last edited: Oct 15, 2011
  2. jcsd
  3. Oct 15, 2011 #2
    why isnt anyone answering ? is my question to hard or am i explaining it wrong?
    please give me an answer anyone
     
  4. Oct 15, 2011 #3

    Mark44

    Staff: Mentor

    It would be helpful if you included the data you are working with.
     
  5. Oct 15, 2011 #4
    hello mark
    i just added my data and two attachments on how the results look when i put them in both of the logaritm papers...only one line should be straight yet they both are and i cant figure out why....
     
  6. Oct 15, 2011 #5

    Ray Vickson

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    1) Your plots seem to show only 3 points, but you have 6 data points.
    2) Change the scale on the second plot's x-axis, to spread it out better.
    3) When I plot your data in Maple I get a straight line only for a log-log plot. The semi-log and direct plots are decidedly nonlinear.

    RGV
     
  7. Oct 15, 2011 #6
     
  8. Oct 15, 2011 #7

    Ray Vickson

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  9. Oct 16, 2011 #8
    scan0008.jpg
     
    Last edited: Oct 16, 2011
  10. Oct 16, 2011 #9

    Mark44

    Staff: Mentor

    Log-log paper has logarithmic scales on both axes. Semilog paper has a log scale only on the vertical axis.

    This is the data that you eventually added to your first post.
    I don't see how you got the plot that you show in post #8 from this data. The graph you show is a straight line (on log-log paper) with a positive slope, but your data shows that as time increases, the population is decreasing.
     
  11. Oct 16, 2011 #10

    Ray Vickson

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  12. Oct 16, 2011 #11
    [/COLOR]
    And the thing about that attachment is that everyone here says that my data can only make a straight line on a log-log paper, but how come it also turns straight on a semilog when i put them.......see the last attachment again
    Besides i think i didn't really say it correctly what i am trying to do...but the teacher told that we can put the data on a normal paper, on a semi log and on a log-log and see which of the papers craetes a straight line...he said that we shouldn't take the log or anything of the numbers...just put them...and the time should be on the y-axis
     
    Last edited: Oct 16, 2011
  13. Oct 16, 2011 #12

    SammyS

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    Anytime you make a plot with scaling set so that either the horizontal component of the data are very compressed or the vertical component of the data are very compressed, the result will be either a nearly horizontal line, or a nearly vertical line.
     
  14. Oct 16, 2011 #13

    Mark44

    Staff: Mentor

    The image in post #8 is not good enough to be able to read the coordinates of the points. In any case, as I said before, you should not be getting a line with a positive slope.
    The 15 in your time data should be paired with the 72 in your population data, and so on. Are you getting the order reversed?
     
  15. Oct 16, 2011 #14

    Ray Vickson

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  16. Oct 17, 2011 #15
    what do you mean with getting it reversed? i did exactly as you are saying, but but it is going to kill me since iv'e used like three day and when i put my data in a exponential function they also are right and everything....the only problem is that my line in the log-log paper also is a straight line and i may not have two straight lines
     
  17. Oct 17, 2011 #16

    Mark44

    Staff: Mentor

    Well, that explains why the slope appears to be positive. The image quality is so poor and the axes are not labelled, so I was not able to see this.
     
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