## Introduction

A **harmonic oscillator** is any physical system that varies above and below its mean value with a characteristic frequency, *f*. Common examples of harmonic oscillators include pendulums, masses on springs, and RLC circuits.

The following article discusses the harmonic oscillator in terms of classical mechanics. See the article quantum harmonic oscillator for a discussion of the harmonic oscillator in quantum mechanics.

## Full Mathematical Definition

Most harmonic oscillators, at least approximately, solve the differential equation:

where*t*is time,

*b*is the damping constant, ω

_{o}is the characteristic angular frequency, and

*A*

_{o}cos(ω

*t*) represents something driving the system with amplitude

*A*

_{o}and angular frequency ω.

*x*is the measurement that is oscillating; it can be position, current, or nearly anything else. The angular frequency is related to the frequency,

*f*, by:

### Simple Harmonic Oscillator

A simple harmonic oscillator is simply an oscillator that is neither damped nor driven. So the equation to describe one is:

In the case of a mass hanging on a spring, Newton's Laws, combined with Hooke's law for the behavior of a spring, states that:

*k*is the spring constant,

*m*is the mass,

*y*is the position of the mass, and

*a*is its acceleration. Rewriting the equation, we obtain:

*d*

^{2}

*z*/

*dt*

^{2}∝ -

*z*,

*z*is some form of sine. So we try the solution:

*A*is the amplitude, δ is the phase shift, and ω is the angular frequency. Substituting, we have:

*A*cos(ω

*t*+ δ)):

*A*and δ). That means that what was labelled ω is in fact ω

_{o}. This will become important later.

### Driven Harmonic Oscillator

Good example:AC LC circuit.

a few notes about what the response of the circuit to different AC frequencies.

### Damped Harmonic Oscillator

good example:weighted spring underwater

Note well: underdamped, critically damped

### Damped, Driven Harmonic Oscillator

example:Notes for above apply, transient vs steady state response, and quality factor.

## A Final Note on Mathematics

For a more complete description of how to solve the above equation, see the article on Differential equations.