Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Double log scale

  1. Oct 28, 2007 #1
    double log scale - please help

    I am trying to find/create the equation for a straight line on a double logarithmic scale graph - what is the format of the equation or how do I do this using data points from the graph.
    Last edited: Oct 28, 2007
  2. jcsd
  3. Oct 28, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    If you mean a log-log graph, a straight line would be [itex]\log y=m\cdot\log x + b[/itex]. If you mean a double-log graph, a straight line would be [itex]y = m\cdot\log\log x + b[/itex]. Simplify these as you see fit.
  4. Nov 1, 2007 #3
    Yes - it is for the equation log y= m*logx+b - how do I generate this equation from a straight line on a log-log graph - particularly not sure about b.
  5. Nov 1, 2007 #4


    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    If the paper is log log, then what you really want is the "y" values for the vertical axis, and the "x" values for the horizontal axis. Each of your coordinates should range over at least one magnitude; otherwise, you probably will not obtain a line. If the result is not a line, then the use of log log paper was probably not the correct choice (but then this is what you are trying to find out). Note carefully, you are plotting the x and y values here on log log paper - NOT their logarithms.
  6. Nov 2, 2007 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Well, you plot logx against logy, in the same way that you would plot x against y. If you have an equation for a straight line y=mx+b, what would b be?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook