Re: System of differential equations

*To*: mathgroup at smc.vnet.net*Subject*: [mg128988] Re: System of differential equations*From*: "Nasser M. Abbasi" <nma at 12000.org>*Date*: Fri, 7 Dec 2012 01:38:13 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net*References*: <k9pq4e$hnp$1@smc.vnet.net>*Reply-to*: nma at 12000.org

On 12/6/2012 3:56 AM, hazem.abdelhafiz at gmail.com wrote: > Dear all, > I want to solve this system of equations but I always get an error > > NDSolve::ndnum:Encountered non-numerical value for a derivative at t==0 > > can any one help me on this > > s=NDSolve[{ > ph1''[t]+ph1'[t]+sin[ph1[t]]-mu[t]-mu'[t]==0.005, > ph2''[t]+ph2'[t]+sin[ph2[t]]+mu[t]+mu'[t]==0.005, > 0.2mu'[t]+mu[t]==ph1'[t]-ph2'[t], > ph2[0]==0, > ph1[0]==0, > ph2'[0]==0, > ph1'[0]==0, > mu[0]==0 > }, > {ph1[t],ph2[t],mu[t]}, > {t,0,5000} > ] > It is Sin[] not sin[]. Mathematica is Case Sensitive. It is better to write things in more lines than one so it is easier to read and modify. -------------------------- eq1 = ph1''[t] + ph1'[t] + Sin[ph1[t]] - mu[t] - mu'[t] == 0.005; eq2 = ph2''[t] + ph2'[t] + Sin[ph2[t]] + mu[t] + mu'[t] == 0.005; eq3 = 0.2 mu'[t] + mu[t] == ph1'[t] - ph2'[t]; ic = {ph2[0] == 0, ph1[0] == 0, ph2'[0] == 0, ph1'[0] == 0,mu[0] == 0}; s = NDSolve[Flatten@{eq1, eq2, eq3, ic}, {ph1[t], ph2[t], mu[t]}, {t, 0.01,5000}] -------------------------------- {{ph1[t] -> InterpolatingFunction[][t], ph2[t] -> InterpolatingFunction[][t], mu[t] -> InterpolatingFunction[][t]}} From the warning messages I see, this looks like a stiff system, you might want to look into using options to handle this. see NDSolve documentation for more information on this. --Nasser