Re: Pendulum Help

[Jordi Gutiérrez Hermoso]

On 9 October 2011 20:32, Nelson Chung <address@hidden> wrote:
> Dear all:
>
> Can someone tell me how to compute the motion and the period for this
> pendulum equation?
>
> x" + sinx = 0.

I suppose you mean how to solve the nonlinear IVP.

Rewrite it as

    u' = v
    v' = -sin(u)

So that for vector y, you have the autonomous system

    y' = f(y)

where

    f(y) = [y(2), -sin(y(1))].

Subject to f(0) = [a, b].

Thus define in Octave

    f = @(y) [y(2), -sin(y(1))];

and pick some initial conditions,

    a = [1, 0]

and use lsode to solve it, for the period t = linspace(0,10,200);

    y = lsode(f, a, t);

and plot it:

    plot(t, y(:,1));

HTH,
- Jordi G. H.