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Matlalb problem with two first order ODEs

  1. Mar 21, 2008 #1
    1. The problem statement, all variables and given/known data

    I have two questions:
    1) If i have two first order ODE y(1) and y(2) (in terms of time), i know how to plot y(1) versus time and y(2) versus time but i dont know how to plot y(2) versus y(1)

    2)I have two second order ODES X''=... and Z''=... to solve this, we make the substitutions y(1)=X, y(2)=X', y(3)=Z, and y(4)=Z'
    then we have two first order ODEs: y(2)'=... and y(4)'=...
    I typed this into Matlab (dydt=[(the equation for y(2)'; the equation for y(4)']) but an error came up since i didnt define y(4). how do i do this?
    2. Relevant equations

    3. The attempt at a solution
    1) i tried putting:
    plot(y(1),y(:,2)), and i also tried plot(y(1),y(2)) but these didnt work.

    Any help would be very much appreciated.
    Than you.
  2. jcsd
  3. Mar 22, 2008 #2
    1) plot(y(:,1),y(:,2)) ought to work, assuming you just left out the iterate through all rows part in the first column.

    2) Can you post your exact system of equations function?
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