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Homework Help: Mathematica Differential Equation, help me see whats wrong

  1. Apr 16, 2009 #1
    Basically I am trying to recreate this http://mathforum.org/advanced/robertd/magneticpendulum.html

    now my differential equation is
    NDSolve[{x''[t] + Rx'[t] - Sum from i=1to3 of (x-x[t])/(rootof(x-x[t])^2 +[y-y[t])^2 +d^2)^3 + Qx[t]==0,
    {y''[t] + Ry'[t] - Sum from i=1to3 of (y-y[t])/(rootof(x-x[t])^2 +[y-y[t])^2 +d^2)^3 + Qy[t]==0, x[0]==-.95, y[0]==0.09, x'[0]==0, y'[0]==0}, {x,y}, {t,0,100}, MaxSteps -> 1000]
    I have assigned values to R Q and d, and please note than the x's and y's which are not of t, should be subscript i.

    I get the error,
    "The first argument must have both an equation and an initial condition."

    I keep compare my equation to other example equations but I can't see whats wrong.

    Any anyone shed any light?

    Very much appreciated.
  2. jcsd
  3. Apr 16, 2009 #2


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    Gold Member

    As far as I can tell, you started off the set of differential equations with an opening { but then re-opened with another {. The form should be NDSolve[{f1(vars),f2(fars)},{vars}.....] and so on and so forth.
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