       Re: problems with NDSolve

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• Subject: [mg131121] Re: problems with NDSolve
• From: "Kevin J. McCann" <kjm at KevinMcCann.com>
• Date: Thu, 13 Jun 2013 02:36:15 -0400 (EDT)
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```You will probably get a lot of replies to this:

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 == 0.0001, y == 0,
>     z == 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[x][
>      t] == -(tan[x[t]]/(r Sqrt[1 + tan[x[t]]^2])) +
>      1.6677*10^9 (0.1 - z[t]), Derivative[y][t] == cos[x[t]],
>    Derivative[z][t] == sin[x[t]], x == 0.0001, y == 0,
>    z == 0}, {x[t], y[t], z[t]}, {t, 0, 1}]
>
> Do you have any ideas?
> Thanks
>

```

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