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? > > Thanks in advance, miz. 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