Re: A simple ordinary differential equation

*To*: mathgroup at smc.vnet.net*Subject*: [mg98240] Re: A simple ordinary differential equation*From*: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>*Date*: Fri, 3 Apr 2009 04:37:39 -0500 (EST)*References*: <gr21kh$lpg$1@smc.vnet.net>

In the definition of f you specify a derivative of f, but Mathematica cannot know that it is supposed to be a dervative with respect to t. And, BTW, Sqr should be Sqrt. A few quick fixes: Clear[sol] f = 1 - 1111*x'[t]^2; sol = NDSolve[{x''[t] + (16/10^4) x'[t]^2* Sqrt[f]/x[t]^2 - (1458/10^9)*f/x[t]^2 == 0, x[0] == 1, x'[0] == 0}, x[t], {t, 0, 100}, AccuracyGoal -> 8, PrecisionGoal -> 8, WorkingPrecision -> 30, MaxSteps -> Infinity] Plot[D[x[t] /. sol, t] // Evaluate, {t, 0, 100}, PlotRange -> All, Frame -> True, Axes -> None Cheers -- Sjoerd On Apr 2, 11:48 am, "I. Shechtman" <shech... at netvision.net.il> wrote: > What is wrong with this equation? > > Clear[sol] > f[x_?NumericQ] = 1 - 1111*x'^2; > sol = NDSolve[{x''[t] + (16/10^4)x'[t]^2*Sqr[ > f[x[t]]]/x[ > t]^2 - (1458/10^9)*f[x[t]]/x[t]^2 == > 0, x[0] == 1, x'[0] == 0}, x[t], { > t, 0, 100}, AccuracyGoal -> 8, > PrecisionGoal -> 8, WorkingPre= cision -> > 30, MaxSteps -> Infinity]; > Plot[Evaluate[x'[t] /. sol], {t, 0, 100}]