- #1
alexfloo
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Homework Statement
We're considering the following special case of the Duffing oscillator, in the limit of small nonlinearity:
[itex]\ddot x + x + \epsilon x^3 = 0[/itex].
The problem asks us to find a conserved quantity, and use it to write the period as an integral. Then, to write it as a power series in [itex]\epsilon[/itex] and evaluate it, then check the first two terms agains a power series that was given in our text. Mine didn't match up. I'm pretty sure I got the integrand right, but I really just don't know how select the bounds.
Homework Equations
The Attempt at a Solution
First of all, this system resembles a force on a unit mass which doesn't depend on velocity. Therefore, there is a well-defined potential energy as a function of only x, which in this case is
[itex]\frac{1}{2}x^2+\frac{\epsilon}{4}x^4[/itex]
Therefore,
[itex]C=\frac{1}{2}x^2+\frac{\epsilon}{4}x^4+\frac{1}{2}\dot{x}^2[/itex]
is conserved. (We can check that will time derivatives but I won't.)
Therefore, along a particular trajectory, there is some constant C such that
[itex]\frac {dx}{dt}=\dot x=\sqrt{C-x^2-\frac{\epsilon}{2}x^4}[/itex].
Therefore
[itex]dt=\frac{dx}{\sqrt{C-x^2-\frac{\epsilon}{2}x^4}}[/itex].
So the period will be
[itex]T=\int \frac{dx}{\sqrt{C-x^2-\frac{\epsilon}{2}x^4}}[/itex].
The problem is I have no idea what to set the bounds of the integral as. The x-value at the start and end of the period are the same, so the integral will equal zero.
I recognize also that it'd make more sense to cast it as a closed line integral on the set
[itex]y=\sqrt{C-x^2-\frac{\epsilon}{2}x^4}[/itex],
but I'm not sure how to effectively parametrize that set, or else I should be writing it in a very different form.