# Mathematica NDSolve conditions help

• Mathematica
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:
Code:
soln = NDSolve[{eqn1[], eqn1[], x == y == 0, x' == 1,
y' == 0}, {x, y}, t];
where eqn1 is determined by the following code:
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[], x] + Integrate[F[], 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:
Code:
NDSolve::deqn: Equation or list of equations expected instead of False in the first argument
{eqn1[], eqn1[], x == y == 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. :\

Related MATLAB, Maple, Mathematica, LaTeX News on Phys.org
Dale
Mentor
Interesting. I don't get an error using Mathematica 9.0. Try quitting your kernel and re-running it.

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