problems with NDSolve

*To*: mathgroup at smc.vnet.net*Subject*: [mg131110] problems with NDSolve*From*: Axel Sielaff <axel.sielaff at googlemail.com>*Date*: Wed, 12 Jun 2013 05:37:32 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-outx@smc.vnet.net*Delivered-to*: mathgroup-newsendx@smc.vnet.net

Hi, I'm new in Mathematica and have a problem you can maybe help me with. I try to solve the differential equation given by T. G. Myers and J. P. F. Charpin (http://dx.doi.org/10.1063/1.3155185) as follows: rho = 1700 g = 9.81 sigma = 0.00001 c = rho g / sigma z0 = 0.1 sol = NDSolve[{x'[t] == c (z0 - z[t]) - tan[x[t]]/(r Sqrt[1 + tan[x[t]]^2]) , y'[t] == cos[x[t]], z'[t] == sin[x[t]], x[0] == 0.0001, y[0] == 0, z[0] == 0}, {x[t], y[t], z[t]} , {t, 0, 1} ] I get this result and don't know what to do. NDSolve::ndnum: Encountered non-numerical value for a derivative at t == 0.`. >> NDSolve[{Derivative[1][x][ t] == -(tan[x[t]]/(r Sqrt[1 + tan[x[t]]^2])) + 1.6677*10^9 (0.1 - z[t]), Derivative[1][y][t] == cos[x[t]], Derivative[1][z][t] == sin[x[t]], x[0] == 0.0001, y[0] == 0, z[0] == 0}, {x[t], y[t], z[t]}, {t, 0, 1}] Do you have any ideas? Thanks