MathGroup Archive 2009

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: A simple ordinary differential equation


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}]
> 


  • Prev by Date: release memory
  • Next by Date: Re: Interpolation: Method->"Spline"
  • Previous by thread: Re: A simple ordinary differential equation
  • Next by thread: Re: A simple ordinary differential equation