Re: A simple ordinary differential equation
- To: mathgroup at smc.vnet.net
- Subject: [mg98213] Re: A simple ordinary differential equation
- From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
- Date: Fri, 3 Apr 2009 04:32:36 -0500 (EST)
- Organization: Uni Leipzig
- References: <gr21kh$lpg$1@smc.vnet.net>
- Reply-to: kuska at informatik.uni-leipzig.de
Hi,
what is wrong with
f[x_?NumericQ] = 1 - 1111*x'^2;
?
A derivative x' from a numeric value f[x_?NumericQ]
what is wrong with Sqr[] is it a miss-typed Sqrt[] or is it
a square Sqr[x_]:=x^2
f[x_[t_]] := 1 - 1111*x'[t]^2;
sol = NDSolve[{x''[t] + (16/10^4) x'[t]^2*
Sqrt[f[x[t]]]/x[t]^2 - (1458/10^9)*f[x[t]]/x[t]^2 == 0,
x[0] == 1, x'[0] == 0}, {x[t], x'[t]}, {t, 0, 100},
AccuracyGoal -> 8, PrecisionGoal -> 8, WorkingPrecision -> 30,
MaxSteps -> Infinity];
and
Plot[Evaluate[x'[t] /. sol], {t, 0, 100}]
work fine.
Regards
Jens
I. Shechtman 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, WorkingPrecision ->
> 30, MaxSteps -> Infinity];
> Plot[Evaluate[x'[t] /. sol], {t, 0, 100}]
>