Re: Can someone tell me why NDsolve isn't working here?

• To: mathgroup at smc.vnet.net
• Subject: [mg44055] Re: Can someone tell me why NDsolve isn't working here?
• From: Hugh Walker <hwalker at gvtc.com>
• Date: Sun, 19 Oct 2003 01:11:03 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```On Saturday, October 18, 2003, at 02:12 AM, Mizrandir wrote:

> Hello, Im trying to solve a system of 6 differential equations with
> NDSolve but I can't get Mathematica to do it. I wrote:
>
> R = 1;
> M = 1;
> L = 1;
> NDSolve[{Rx[t] + Fx[t] == 2M x''[t], Ry[t] + Fy[t] == 0,
>     Fx[t] R == -M R x''[t], -Rx[t] ==
>       M x''[t] + (M L \[Theta]''[t] Cos[-\[Theta][t]])/
>           2 + (M L \[Theta]'[t]^2 Sin[-\[Theta][t]])/2, -Ry[
>           t] == (M L \[Theta]''[t] Sin[-\[Theta][t]])/
>           2 - (M L \[Theta]'[t]^2 Cos[-\[Theta][t]])/
>           2, -(L Cos[\[Theta][t]] Rx[t])/2 + (L Sin[\[Theta][t]])/
>           2 == (M L^2 \[Theta]''[t])/12, x[0] == 0, \[Theta][0] ==
> Pi/2,
>     Ry[0] == -M/2, Rx[0] == 0, Fy[0] == 5M/2, Fx[0] == 0}, {x,
> \[Theta], Ry,
>     Rx, Fy, Fx}, {t, 0, 1}]
>
> And Mathematica gave me the following error:
>
> NDSolve::"ndode": "Input is not an ordinary differential equation."
>
> Does anybody know what am I doing wrong. Can I use Mathematica to
> solve this?
>

I see two things that will give trouble, although the sort indicated by
the error message. a) One of the six equations does not contain a
derivative. b) Two of the six equations contain 2nd derivatives
implying that the set of initial conditions are insufficient for
numerical integration.

========
Hugh Walker
Gnarly Oaks

```

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