# Perturbation Techniques and Theory for Nonlinear Systems

## Homework Statement

Given the equation
$$\ddot{\theta}=\Omega^2\sin{\theta}\cos{\theta}-\frac{g}{R}\sin{\theta}$$
Determine a first-order uniform expansion for small but finite theta.

## Homework Equations

Other than the equation above, none so far as I am aware.

## The Attempt at a Solution

The only thing I could think to do was try to solve this differential equation via the method of undetermined coefficients, which I do not think is right at all. I then planned to expand my solution in a Taylor series about 0. This is from Ali Hasan Nayfeh's Introduction to Perturbation Techniques. My professor gave us a packet of the fourth chapter of the aforementioned text as a basis to solve this and other problems. Nowhere in the text does it give a clear example of what exactly a "first order uniform expansion" is, nor do I even know where to begin. My professor's research interests lie in nonlinear dynamics and chaos, and I fear he is going a little too in depth for my second year physics course. Thank you for any input.

## Answers and Replies

vela
Staff Emeritus
Science Advisor
Homework Helper
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I'm only making an educated guess here, but I think what you want to do is expand the trig functions using the Taylor series and retain only the lowest-order non-vanishing term. This will leave you with a linear second-order differential equation. Then you want to convert this second-order equation into a system of two first-order equations.

fluidistic
Gold Member
I think that finding the solution to the original ODE and then expand it using Taylor series is equivalent to solve the "simplified" ODE that vela suggests. Vela's way is much easier for sure.

That's something along the lines of what I thought of doing. I read up on the subject, and "uniform expansion" only means "without secular terms", so the approximation of my system won't blow up as t→∞. I'm just going to do the Taylor series DE thing. Thanks for your input, guys. It's greatly appreciated.