Re: problems with NDSolve

*To*: mathgroup at smc.vnet.net*Subject*: [mg131121] Re: problems with NDSolve*From*: "Kevin J. McCann" <kjm at KevinMcCann.com>*Date*: Thu, 13 Jun 2013 02:36:15 -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*References*: <kp9epj$2sr$1@smc.vnet.net>

You will probably get a lot of replies to this: 1) Sin, Cos, Tan, Sqrt all start with a capital letter. 2) you have an undefined variable r. In a numerical calculation, everything except the things you are solving for must be numerical. Hope this helps. Kevin On 6/12/2013 5:25 AM, Axel Sielaff wrote: > 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 >