# 20. Parametric Resonance

*Michael Fowler*

## Introduction

(*Following Landau para 27*)

A one-dimensional simple harmonic oscillator, a mass on a spring,

has two parameters, and For some systems, the parameters can be changed externally (an example being the length of a pendulum if at the top end the string goes over a pulley).

We are interested here in the system's response to some
externally imposed periodic variation of its parameters, and in particular
we'll be looking at *resonant* response, meaning large response to a small
imposed variation.

Note first that imposed variation in the mass term is easily dealt with, by simply redefining the time variable to , meaning

Then

,

and the equation of motion becomes

This means we can always transform the equation so all the parametric variation is in the spring constant, so we'll just analyze the equation

Furthermore, since we're looking for *resonance*
phenomena, we will only consider a small parametric variation at a single frequency,
that is, we'll take

where , and is positive (a trivial requirement -- just setting the time origin).

(*Note*: We prefer where
Landau useswhich is often used for a resonance *width*
these days.)

We have now a driven oscillator:

How does this differ from our previous analysis of a driven oscillator? In a very important way!

*The amplitude is a factor in the
driving force*.

For one thing, this means that if the oscillator is
initially at rest, it stays that way, in contrast to an ordinary externally
driven oscillator. But if the amplitude increases, so does the driving force.
This can lead to an *exponential *increase in amplitude, unlike the linear
increase we found earlier with an external driver. (Of course, in a real
system, friction and nonlinear potential terms will limit the growth.)

What frequencies will prove important in driving the
oscillator to large amplitude? It responds best, of course, to its natural
frequency . But if it is in fact already oscillating
at that frequency, then the driving force, *including the factor of* , is proportional to

,

with no component at the natural frequencyfor a general .

The simplest way to get resonance is to take Can we understand this physically? Yes.
Imagine a mass oscillating backwards and forwards on a spring, and the spring
force increases just after those points where the mass is furthest away from
equilibrium, so it gets an extra tug inwards twice a cycle. This will feed in
energy. (You can drive a swing this way.) In contrast, if you drive at the
natural frequency, giving little push inwards just after it begins to swing
inwards from one side, then you'll be giving it a little push *outwards*
just after it begins to swing back from the other side. Of course, if you push
only from one side, like swinging a swing, this works -- but it isn't a single
frequency force, the next harmonic is doing most of the work.

## Resonance near Double the Natural Frequency

From the above argument, the place to look for resonance is close to Landau takes

and, bearing in mind that we're looking for oscillations close to the natural frequency, puts in

with slowly varying.

It's important to realize that this is an *approximate*
approach. It neglects nonresonant frequencies which must be present in small
amounts, for example

and the term is thrown away.

And, since the assumption is that are slowly varying, their second derivatives are dropped too, leaving just

This must equal

Keeping only the resonant terms, we take and so this expression becomes

The equation becomes:

The zeroth-order terms cancel between the two sides, leaving

Collecting the terms in :

The sine and cosine can't cancel each other, so the two coefficients must both be identically zero. This gives two first order differential equations for the functions , and we look for exponentially increasing functions, proportional to , which will be solutions provided

The amplitude growth rate is therefore

Parametric resonance will take place ifis real, that is, if

a band of widthabout .

## Example: Pendulum Driven at near Double the Natural Frequency

A simple pendulum of length, mass is attached to a point which oscillates vertically. Measuring downwards, the pendulum position is

The Lagrangian

The purely time-dependent term will not affect the equations of motion, so we drop it, and since the equations are not affected by adding a total derivative to the Lagrangian, we can integrate the second term by parts (meaning we're dropping a term ) to get

(We've also dropped the term from the potential energy term -- it has no ordependence, so will not affect the equations of motion.)

The equation for small oscillations is

Comparing this with

we see that , so the parametric resonance range around is of width