Mathematica NDSolve conditions help

[Hertz]

Hi, I'm having a problem with NDSolve in mathematica where it is interpreting my initial conditions as "True" or "False".

Here is the code:

soln = NDSolve[{eqn1[[1]], eqn1[[2]], x[0] == y[0] == 0, x'[0] == 1, 
    y'[0] == 0}, {x, y}, t];

where eqn1 is determined by the following code:

Clear[soln, eqn1, xeqn, yeqn, eqn, sink, source, field,  k, m, a,  f, F, U]
a = 1;
sink = {0, 
    a} - (1/(x^2 + (y - a)^2))*(x^2 + (y - a)^2)^(-1/2)*{x, y - a};
source = {0, -a} + (1/(x^2 + (y + a)^2))*(x^2 + (y + a)^2)^(-1/2)*{x, 
     y + a};
field = sink + source;
F = m*k*field;
U[x_, y_] = Integrate[F[[1]], x] + Integrate[F[[2]], y];
T = (m/2) (x'[t]^2 + y'[t]^2);
L = T - U[x[t], y[t]];
xeqn = D[L, x[t]] == D[D[L, x'[t]], t];
yeqn = D[L, y[t]] == D[D[L, y'[t]], t];
eqn = {xeqn, yeqn};
eqn1 = eqn /. {k -> 1, m -> 1}

Here is the error:

NDSolve::deqn: Equation or list of equations expected instead of False in the first argument
{eqn1[[1]], eqn1[[2]], x[0] == y[0] == 0, False, False}

It looks to me like I must be typing my velocity initial conditions in wrong or something, but every example I can find online does it this same way. I've also tried typing it in other ways like using the D function etc, but I keep having the same issue. :\

 

[Dale]

Interesting. I don't get an error using Mathematica 9.0. Try quitting your kernel and re-running it.

 

[Hertz]

Wow, I was almost certain that wasn't going to work. I've been dealing with the problem for a long time now, just never bothered to ask. Turns out, restarting mathematica fixed it though. I just did a restart and copy/pasted what was here.

Is there anything to take from this? Any ideas why that might have happened? I wonder if saving my work and restarting mathematica would have also worked. Guess I'll have to experiment more later on.

 

[Dale]

It looks like somehow y'[0] and x'[0] were previously set to some specific value, and it was causing problems.

 

[alphy]

Hi there,

It seems like the issue might be with your initial conditions. The error message is indicating that the initial conditions for x'[0] and y'[0] are being interpreted as False, which is not a valid input for NDSolve.

I would recommend checking your initial conditions and making sure they are formatted correctly. It's possible that there is a typo or some other mistake that is causing NDSolve to interpret them as False. Also, double check that your equations are correct and that you are passing the correct variables to NDSolve.

If you continue to have trouble, you may want to try reaching out to the Mathematica community for further assistance. They may be able to provide more specific guidance on how to resolve this issue. Good luck!