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| {{about|the harmonic oscillator in classical mechanics|its uses in [[quantum mechanics]]|quantum harmonic oscillator}}
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| {{Classical mechanics|cTopic=Basic motions}}
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| In [[classical mechanics]], a '''harmonic oscillator''' is a system that, when displaced from its equilibrium position, experiences a [[restoring force]], ''F'', [[Proportionality (mathematics)|proportional]] to the displacement, ''x'':
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| :<math> \vec F = -k \vec x \, </math>
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| where ''k'' is a positive [[coefficient|constant]].
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| If ''F'' is the only force acting on the system, the system is called a '''simple harmonic oscillator''', and it undergoes [[simple harmonic motion]]: [[sinusoidal]] [[oscillation]]s about the equilibrium point, with a constant [[amplitude]] and a constant [[frequency]] (which does not depend on the amplitude).
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| If a frictional force ([[damping]]) proportional to the [[velocity]] is also present, the harmonic oscillator is described as a '''damped oscillator'''. Depending on the friction coefficient, the system can:
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| * Oscillate with a frequency smaller than in the non-damped case, and an [[amplitude]] decreasing with time (''underdamped'' oscillator).
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| * Decay to the equilibrium position, without oscillations (''overdamped'' oscillator).
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| The boundary solution between an underdamped oscillator and an overdamped oscillator occurs at a particular value of the friction coefficient and is called "critically damped."
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| If an external time dependent force is present, the harmonic oscillator is described as a '''driven oscillator'''.
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| Mechanical examples include [[pendulum|pendula]] (with [[Small-angle approximation#Motion of a pendulum|small angles of displacement]]), masses connected to [[spring (device)|spring]]s, and [[acoustics|acoustical system]]s. Other [[#Equivalent systems|analogous systems]] include electrical harmonic oscillators such as [[RLC circuit]]s. The harmonic oscillator model is very important in physics, because any mass subject to a force in stable equilibrium acts as a harmonic oscillator for small vibrations. Harmonic oscillators occur widely in nature and are exploited in many manmade devices, such as [[clock]]s and radio circuits. They are the source of virtually all sinusoidal vibrations and waves.
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| == Simple harmonic oscillator ==
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| {{main|Simple harmonic motion}}
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| [[Image:Simple harmonic motion animation.gif|thumb|right|Simple harmonic motion]]
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| A simple harmonic oscillator is an oscillator that is neither driven nor [[Damping|damped]]. It consists of a mass ''m'', which experiences a single force, ''F'', which pulls the mass in the direction of the point ''x''=0 and depends only on the mass's position ''x'' and a constant ''k''. Balance of forces ([[Newton's second law]]) for the system is
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| :<math>F = m a = m \frac{\mathrm{d}^2x}{\mathrm{d}t^2} = -k x. </math>
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| Solving this [[differential equation]], we find that the motion is described by the function
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| :<math> x(t) = A\cos\left( \omega t+\phi\right), </math>
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| where
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| :<math>\omega = \sqrt{\frac{k}{m}} = \frac{2\pi}{T}.</math>
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| The motion is [[Periodic function|periodic]], repeating itself in a [[sine wave|sinusoidal]] fashion with constant amplitude, ''A''. In addition to its amplitude, the motion of a simple harmonic oscillator is characterized by its [[Frequency|period]] ''T'', the time for a single oscillation or its frequency ''f'' = {{frac|''T''}}, the number of cycles per unit time. The position at a given time ''t'' also depends on the [[phase (waves)|phase]], ''φ'', which determines the starting point on the sine wave. The period and frequency are determined by the size of the mass ''m'' and the force constant ''k'', while the amplitude and phase are determined by the starting position and [[velocity]].
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| The velocity and [[acceleration]] of a simple harmonic oscillator oscillate with the same frequency as the position but with shifted phases. The velocity is maximum for zero displacement, while the acceleration is in the opposite direction as the displacement.
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| The potential energy stored in a simple harmonic oscillator at position ''x'' is
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| :<math>U = \frac{1}{2}kx^2.</math>
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| This equals the total oscillation that was given to you before.
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| == Damped harmonic oscillator ==
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| {{main|Damping}}
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| [[File:Damping 1.svg|thumb|200px|Dependence of the system behavior on the value of the damping ratio ''ζ'']]
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| [[File:Oscillatory motion acceleration.ogv|thumb|A damped harmonic oscillator, which slows down due to friction]]
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| In real oscillators, friction, or damping, slows the motion of the system. Due to frictional force, the velocity decreases in proportion to the acting frictional force. While simple harmonic motion oscillates with only the restoring force acting on the system, damped harmonic motion experiences friction. In many vibrating systems the frictional force ''F''<sub>f</sub> can be modeled as being proportional to the velocity ''v'' of the object: {{nowrap|1=''F''<sub>f</sub> = −''cv''}}, where ''c'' is called the ''viscous damping coefficient''.
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| Balance of forces ([[Newton's second law]]) for damped harmonic oscillators is then
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| :<math> F = -kx - c\frac{\mathrm{d}x}{\mathrm{d}t} = m \frac{\mathrm{d}^2x}{\mathrm{d}t^2}.</math>
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| This is rewritten into the form
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| :<math> \frac{\mathrm{d}^2x}{\mathrm{d}t^2} + 2\zeta\omega_0\frac{\mathrm{d}x}{\mathrm{d}t} + \omega_0^{\,2} x = 0, </math>
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| where
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| :<math>\omega_0 = \sqrt{\frac{k}{m}}</math> is called the 'undamped [[angular frequency]] of the oscillator' and
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| :<math>\zeta = \frac{c}{2 \sqrt{mk}}</math> is called the 'damping ratio'.
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| [[Image:Step response for two-pole feedback amplifier.PNG|thumbnail|200px|Step-response of a damped harmonic oscillator; curves are plotted for three values of {{nowrap|1=''μ'' = ''ω''<sub>1</sub> = ''ω''<sub>0</sub>{{radic|1−''ζ''<sup>2</sup>}}}}. Time is in units of the decay time {{nowrap|1=''τ'' = 1/(''ζω''<sub>0</sub>)}}.]]
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| The value of the damping ratio ''ζ'' critically determines the behavior of the system. A damped harmonic oscillator can be:
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| *''Overdamped'' (''ζ'' > 1): The system returns ([[exponential decay|exponentially decays]]) to steady state without oscillating. Larger values of the damping ratio ''ζ'' return to equilibrium slower.
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| *''Critically damped'' (''ζ'' = 1): The system returns to steady state as quickly as possible without oscillating. This is often desired for the damping of systems such as doors.
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| *''Underdamped'' (''ζ'' < 1): The system oscillates (with a slightly different frequency than the undamped case) with the amplitude gradually decreasing to zero. The [[angular frequency]] of the underdamped harmonic oscillator is given by
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| :<math>\omega_1 = \omega_0\sqrt{1 - \zeta^2}.</math>
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| The [[Q factor]] of a damped oscillator is defined as
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| :<math>Q = 2\pi \times \frac{\text{Energy stored}}{\text{Energy lost per cycle}}.</math>
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| ''Q'' is related to the damping ratio by the equation <math>Q = \frac{1}{2\zeta}.</math>
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| == Driven harmonic oscillators == <!-- Driving DAB entry and [[Stiff equation]] link here, pls do not change -->
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| Driven harmonic oscillators are damped oscillators further affected by an externally applied force ''F''(''t'').
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| [[Newton's second law]] takes the form
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| :<math>F(t)-kx-c\frac{\mathrm{d}x}{\mathrm{d}t}=m\frac{\mathrm{d}^2x}{\mathrm{d}t^2}. </math>
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| It is usually rewritten into the form
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| :<math> \frac{\mathrm{d}^2x}{\mathrm{d}t^2} + 2\zeta\omega_0\frac{\mathrm{d}x}{\mathrm{d}t} + \omega_0^2 x = \frac{F(t)}{m}. </math>
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| This equation can be solved exactly for any driving force, using the solutions ''z''(''t'') which satisfy the unforced equation:
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| :<math> \frac{\mathrm{d}^2z}{\mathrm{d}t^2} + 2\zeta\omega_0\frac{\mathrm{d}z}{\mathrm{d}t} + \omega_0^2 z = 0,</math>
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| and which can be expressed as damped sinusoidal oscillations,
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| :<math>z(t) = A \mathrm{e}^{-\zeta \omega_0 t} \ \sin \left( \sqrt{1-\zeta^2} \ \omega_0 t + \varphi \right), </math>
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| in the case where ''ζ'' ≤ 1. The amplitude ''A'' and phase ''φ'' determine the behavior needed to match the initial conditions.
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| ===Step input===
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| {{See also|Step response}}
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| In the case ''ζ'' < 1 and a unit step input with ''x''(0) = 0:
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| :<math> {F(t) \over m} = \begin{cases} \omega _0^2 & t \geq 0 \\ 0 & t < 0 \end{cases}</math>
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| the solution is:
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| :<math> x(t) = 1 - \mathrm{e}^{-\zeta \omega_0 t} \frac{\sin \left( \sqrt{1-\zeta^2} \ \omega_0 t + \varphi \right)}{\sin(\varphi)},</math>
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| with phase ''φ'' given by
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| :<math>\cos \varphi = \zeta. \, </math>
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| The time an oscillator needs to adapt to changed external conditions is of the order ''τ'' = 1/(''ζω''<sub>0</sub>). In physics, the adaptation is called [[relaxation (physics)|relaxation]], and τ is called the relaxation time.
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| In electrical engineering, a multiple of τ is called the ''settling time'', i.e. the time necessary to ensure the signal is within a fixed departure from final value, typically within 10%. The term ''overshoot'' refers to the extent the maximum response exceeds final value, and ''undershoot'' refers to the extent the response falls below final value for times following the maximum response.
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| ===Sinusoidal driving force===
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| [[File:Resonance.PNG|thumb|300px|Steady state variation of amplitude with frequency and [[damping]] of a driven [[simple harmonic oscillator]].<ref>{{cite book|last=Ogata|first=Katsuhiko|title=System dynamics|year=2004|publisher=Pearson Education|location=Upper Saddle River, NJ|isbn=9780131247147|edition=4th}}</ref><ref>
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| {{cite book
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| | title = Optics, 3E
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| | author = [[Ajoy Ghatak]]
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| | edition = 3rd
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| | publisher = Tata McGraw-Hill
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| | year = 2005
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| | isbn = 978-0-07-058583-6
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| | page = 6.10
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| | url = http://books.google.com/books?id=jStDc2LmU5IC&pg=PT97&dq=damping-decreases+resonance+amplitude#v=onepage&q=damping-decreases%20resonance%20amplitude&f=false
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| }}</ref>]]
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| In the case of a sinusoidal driving force:
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| :<math> \frac{\mathrm{d}^2x}{\mathrm{d}t^2} + 2\zeta\omega_0\frac{\mathrm{d}x}{\mathrm{d}t} + \omega_0^2 x = \frac{1}{m} F_0 \sin(\omega t),</math>
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| where <math>\,\!F_0</math> is the driving amplitude and <math>\,\!\omega</math> is the driving [[frequency]] for a sinusoidal driving mechanism. This type of system appears in [[alternating current|AC]] driven [[RLC circuit]]s ([[Electrical resistance|resistor]]-[[inductor]]-[[capacitor]]) and driven spring systems having internal mechanical resistance or external [[air resistance]].
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| The general solution is a sum of a [[Transient (oscillation)|transient]] solution that depends on initial conditions, and a [[steady state]] that is independent of initial conditions and depends only on the driving amplitude <math>\,\!F_0</math>, driving frequency, <math>\,\!\omega</math>, undamped angular frequency <math>\,\!\omega_0</math>, and the damping ratio <math>\,\!\zeta</math>.
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| The steady-state solution is proportional to the driving force with an induced phase change of <math>\,\!\phi</math>: | |
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| :<math> x(t) = \frac{F_0}{m Z_m \omega} \sin(\omega t + \phi)</math>
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| where
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| :<math> Z_m = \sqrt{\left(2\omega_0\zeta\right)^2 + \frac{1}{\omega^2}\left(\omega_0^2 - \omega^2\right)^2}</math>
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| is the absolute value of the [[Mechanical impedance|impedance]] or [[linear response function]] and
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| :<math> \phi = \arctan\left(\frac{2\omega \omega_0\zeta}{\omega_0^2-\omega^2}\right)</math>
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| is the [[phase (waves)|phase]] of the oscillation relative to the driving force, if the arctan value is taken to be between -180 degrees and 0 (that is, it represents a phase lag, for both positive and negative values of the arctan's argument).
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| For a particular driving frequency called the [[resonance]], or resonant frequency <math>\,\!\omega_r = \omega_0\sqrt{1-2\zeta^2}</math>, the amplitude (for a given <math>\,\!F_0</math>) is maximum. This resonance effect only occurs when <math>\,\zeta < 1 / \sqrt{2}</math>, i.e. for significantly underdamped systems. For strongly underdamped systems the value of the amplitude can become quite large near the resonance frequency.
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| The transient solutions are the same as the unforced (<math>\,\!F_0 = 0</math>) [[damping|damped harmonic oscillator]] and represent the systems response to other events that occurred previously. The transient solutions typically die out rapidly enough that they can be ignored.
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| == Parametric oscillators ==
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| {{main|Parametric oscillator}}
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| A [[parametric oscillator]] is a driven harmonic oscillator in which the drive energy is provided by varying the parameters of the oscillator, such as the damping or restoring force. | |
| A familiar example of parametric oscillation is "pumping" on a playground [[swing (seat)|swing]].<ref name=Case>{{cite web |title=Two ways of driving a child's swing |url=http://www.grinnell.edu/academic/physics/faculty/case/swing/ |first=William |last=Case |accessdate=27 November 2011}}</ref><ref name=Case96>{{cite doi|10.1119/1.18209}}</ref><ref name=Roura>{{cite journal |last1=Roura |first1=P. |last2=Gonzalez |first2=J.A. |year=2010 |title=Towards a more realistic description of swing pumping due to the exchange of angular momentum |journal=European Journal of Physics |volume=31 |issue=5 |pages=1195–1207 |doi=10.1088/0143-0807/31/5/020 }}</ref>
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| A person on a moving swing can increase the amplitude of the swing's oscillations without any external drive force (pushes) being applied, by changing the moment of inertia of the swing by rocking back and forth ("pumping") or alternately standing and squatting, in rhythm with the swing's oscillations. The varying of the parameters drives the system. Examples of parameters that may be varied are its resonance frequency <math>\omega</math> and damping <math>\beta</math>.
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| Parametric oscillators are used in many applications. The classical [[varactor]] parametric oscillator oscillates when the diode's capacitance is varied periodically. The circuit that varies the diode's capacitance is called the "pump" or "driver". In microwave electronics, [[waveguide (electromagnetism)|waveguide]]/[[Yttrium aluminum garnet|YAG]] based parametric oscillators operate in the same fashion. The designer varies a parameter periodically to induce oscillations.
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| Parametric oscillators have been developed as low-noise amplifiers, especially in the radio and microwave frequency range. Thermal noise is minimal, since a reactance (not a resistance) is varied. Another common use is frequency conversion, e.g., conversion from audio to radio frequencies. For example, the [[Optical parametric oscillator]] converts an input [[laser]] wave into two output waves of lower frequency (<math>\omega_s, \omega_i</math>).
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| Parametric resonance occurs in a mechanical system when a system is parametrically excited and oscillates at one of its resonant frequencies. Parametric excitation differs from forcing, since the action appears as a time varying modification on a system parameter. This effect is different from regular resonance because it exhibits the [[instability]] phenomenon.
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| ==Universal oscillator equation==
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| The equation
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| :<math>\frac{\mathrm{d}^2q}{\mathrm{d} \tau^2} + 2 \zeta \frac{\mathrm{d}q}{\mathrm{d}\tau} + q = 0</math>
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| is known as the '''universal oscillator equation''' since all second order linear oscillatory systems can be reduced to this form. This is done through [[nondimensionalization]].
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| If the forcing function is ''f''(''t'') = cos(''ωt'') = cos(''ωt<sub>c</sub>τ'') = cos(''ωτ''), where ''ω'' = ''ωt''<sub>''c''</sub>, the equation becomes
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| :<math>\frac{\mathrm{d}^2q}{\mathrm{d} \tau^2} + 2 \zeta \frac{\mathrm{d}q}{\mathrm{d}\tau} + q = \cos(\omega \tau).</math>
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| The solution to this differential equation contains two parts, the "transient" and the "steady state".
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| === Transient solution ===
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| The solution based on solving the [[ordinary differential equation]] is for arbitrary constants ''c''<sub>1</sub> and ''c''<sub>2</sub>
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| <math>q_t (\tau) = \begin{cases} \mathrm{e}^{-\zeta\tau} \left( c_1 \mathrm{e}^{\tau \sqrt{\zeta^2 - 1}} + c_2 \mathrm{e}^{- \tau \sqrt{\zeta^2 - 1}} \right) & \zeta > 1 \text{ (overdamping)} \\ \mathrm{e}^{-\zeta\tau} (c_1+c_2 \tau) = \mathrm{e}^{-\tau}(c_1+c_2 \tau) & \zeta = 1 \text{ (critical damping)} \\ \mathrm{e}^{-\zeta \tau} \left[ c_1 \cos \left(\sqrt{1-\zeta^2} \tau\right) +c_2 \sin\left(\sqrt{1-\zeta^2} \tau\right) \right] & \zeta < 1 \text{(underdamping)} \end{cases}</math>
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| The transient solution is independent of the forcing function.
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| === Steady-state solution ===
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| Apply the "[[complex analysis|complex variables]] method" by solving the [[auxiliary equation]] below and then finding the real part of its solution:
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| :<math>\frac{\mathrm{d}^2 q}{\mathrm{d}\tau^2} + 2 \zeta \frac{\mathrm{d}q}{\mathrm{d}\tau} + q = \cos(\omega \tau) + \mathrm{i}\sin(\omega \tau) = \mathrm{e}^{ \mathrm{i} \omega \tau} .</math>
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| Supposing the solution is of the form
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| <!-- The \,\! is to keep the formula rendered as PNG instead of HTML. Please don't remove it.-->
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| :<math>\,\! q_s(\tau) = A \mathrm{e}^{\mathrm{i} ( \omega \tau + \phi ) } . </math>
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| Its derivatives from zero to 2nd order are
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| :<math>q_s = A \mathrm{e}^{\mathrm{i} ( \omega \tau + \phi ) }, \ \frac{\mathrm{d}q_s}{\mathrm{d} \tau} = \mathrm{i} \omega A \mathrm{e}^{\mathrm{i} ( \omega \tau + \phi ) }, \ \frac{\mathrm{d}^2 q_s}{\mathrm{d} \tau^2} = - \omega^2 A \mathrm{e}^{\mathrm{i} ( \omega \tau + \phi ) } .</math>
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| Substituting these quantities into the differential equation gives
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| <!-- The \,\! is to keep the formula rendered as PNG instead of HTML. Please don't remove it.-->
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| :<math>\,\! -\omega^2 A \mathrm{e}^{\mathrm{i} (\omega \tau + \phi)} + 2 \zeta \mathrm{i} \omega A \mathrm{e}^{\mathrm{i}(\omega \tau + \phi)} + A \mathrm{e}^{\mathrm{i}(\omega \tau + \phi)} = (-\omega^2 A \, + \, 2 \zeta \mathrm{i} \omega A \, + \, A) \mathrm{e}^{\mathrm{i} (\omega \tau + \phi)} = \mathrm{e}^{\mathrm{i} \omega \tau} .</math>
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| Dividing by the exponential term on the left results in
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| <!-- The \,\! is to keep the formula rendered as PNG instead of HTML. Please don't remove it.-->
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| :<math>\,\! -\omega^2 A + 2 \zeta \mathrm{i} \omega A + A = \mathrm{e}^{-\mathrm{i} \phi} = \cos\phi - \mathrm{i} \sin\phi . </math>
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| Equating the real and imaginary parts results in two independent equations
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| :<math>A (1-\omega^2)=\cos\phi \qquad 2 \zeta \omega A = - \sin\phi.</math>
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| ==== Amplitude part ====
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| [[Image:Harmonic oscillator gain.svg|thumb|Bode plot of the frequency response of an ideal harmonic oscillator.]]
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| Squaring both equations and adding them together gives
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| :<math>\left . \begin{array}{rcl} A^2 (1-\omega^2)^2 & = & \cos^2\phi \\[6pt] (2 \zeta \omega A)^2 & = & \sin^2\phi \end{array} \right \} \Rightarrow A^2[(1-\omega^2)^2 + (2 \zeta \omega)^2] = 1. </math>
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| Therefore,
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| :<math>A = A( \zeta, \omega) = \text{sign} \left( \frac{-\sin\phi}{2 \zeta \omega} \right) \frac{1}{\sqrt{(1-\omega^2)^2 + (2 \zeta \omega)^2}}.</math>
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| Compare this result with the theory section on [[resonance]], as well as the "magnitude part" of the [[RLC circuit]]. This amplitude function is particularly important in the analysis and understanding of the [[frequency response]] of second-order systems.
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| ==== Phase part ====
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| To solve for φ, divide both equations to get
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| :<math>\tan\phi = - \frac{2 \zeta \omega}{ 1 - \omega^2} = \frac{2 \zeta \omega}{\omega^2 - 1} \Rightarrow \phi \equiv \phi(\zeta, \omega) = \arctan \left( \frac{2 \zeta \omega}{\omega^2 - 1} \right ). </math>
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| This phase function is particularly important in the analysis and understanding of the [[frequency response]] of second-order systems.
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| === Full solution ===
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| Combining the amplitude and phase portions results in the steady-state solution
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| :<math>\,\! q_s (\tau) = A(\zeta,\omega) \cos(\omega \tau + \phi(\zeta,\omega)) = A\cos(\omega \tau + \phi).</math>
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| The solution of original universal oscillator equation is a [[Superposition principle|superposition]] (sum) of the transient and steady-state solutions
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| <!-- The \,\! is to keep the formula rendered as PNG instead of HTML. Please don't remove it.-->
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| :<math>\,\! q(\tau) = q_t (\tau) + q_s (\tau).</math>
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| For a more complete description of how to solve the above equation, see [[Ordinary differential equation#Linear ODEs with constant coefficients|linear ODEs with constant coefficients]].
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| ==Equivalent systems==
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| Harmonic oscillators occurring in a number of areas of engineering are equivalent in the sense that their mathematical models are identical (see [[#Universal oscillator equation|universal oscillator equation]] above). Below is a table showing analogous quantities in four harmonic oscillator systems in mechanics and electronics. If analogous parameters on the same line in the table are given numerically equal values, the behavior of the oscillators—their output waveform, resonant frequency, damping factor, etc.—are the same.
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| {|class="wikitable" cellpadding="4" style="background:#F8F8F8;"
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| !width="225" align="left"|Translational Mechanical
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| !width="225" align="left"|Torsional Mechanical
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| !width="225" align="left"|[[RLC Circuit#Series_RLC_circuit|Series RLC Circuit]]
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| !width="225" align="left"|[[RLC Circuit#Parallel_RLC_circuit|Parallel RLC Circuit]]
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| |-
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| |Position <math>x\,</math>||Angle <math> \theta\,\! </math>||[[charge (physics)|Charge]] <math>q\,</math>||[[Flux linkage]] <math>\phi\,</math>
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| |[[Velocity]] <math>\frac{\mathrm{d}x}{\mathrm{d}t}\,</math>||[[Angular velocity]] <math>\frac{\mathrm{d}\theta}{\mathrm{d}t}\,</math>||[[Electric current|Current]] <math>\frac{\mathrm{d}q}{\mathrm{d}t}\,</math>||[[Voltage]] <math>\frac{\mathrm{d}\phi}{\mathrm{d}t}\,</math>
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| |[[Mass]] <math>M\,</math>||[[Moment of inertia]] <math>I\,</math>||[[Inductance]] <math>L\,</math> ||[[Capacitance]] <math>C\,</math>
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| |[[Hooke's law|Spring constant]] <math>K\,</math>|| [[Torsion spring#Torsion coefficient|Torsion constant]] <math>\mu\,</math>||[[Capacitance#Elastance|Elastance]] <math>1/C\,</math>||[[Susceptance]] <math>1/L\,</math>
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| |[[Damping]] <math>\gamma\,</math>||[[Torsion spring#Motion of torsion balances and pendulums|Rotational friction]] <math>\Gamma\,</math>||[[Electrical resistance|Resistance]] <math>R\,</math>||[[Electrical conductance|Conductance]] <math>G=1/R\,</math>
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| |Drive [[force]] <math>F(t)\,</math>||Drive [[torque]] <math>\tau(t)\,</math>||[[Voltage]] <math>e\,</math>||[[Electric current|Current]] <math>i\,</math>
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| |colspan="4" align="center"|Undamped [[Resonance|resonant frequency]] <math>f_n\,</math>:
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| |<math>\frac{1}{2\pi}\sqrt{\frac{K}{M}}\,</math>||<math>\frac{1}{2\pi}\sqrt{\frac{\mu}{I}}\,</math>||<math>\frac{1}{2\pi}\sqrt{\frac{1}{LC}}\,</math>||<math>\frac{1}{2\pi}\sqrt{\frac{1}{LC}}\,</math>
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| |colspan="4" align="center"|Differential equation:
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| |<math>M\ddot x +
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| \gamma\dot x + Kx = F\,</math>||<math>I\ddot \theta + \Gamma\dot \theta + \mu \theta = \tau\,</math>||<math>L\ddot q + R\dot q + q/C = e\,</math>||<math>C\ddot \phi + G\dot \phi + \phi/L = i\,</math>
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| |}
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| == Application to a conservative force ==
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| The problem of the simple harmonic oscillator occurs frequently in physics, because a mass at equilibrium under the influence of any [[conservative force]], in the limit of small motions, behaves as a simple harmonic oscillator.
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| A conservative force is one that has a [[potential energy]] function. The potential energy function of a harmonic oscillator is:
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| :<math>V(x) = \frac{1}{2} k x^2</math>
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| Given an arbitrary potential energy function <math>V(x)</math>, one can do a [[Taylor series|Taylor expansion]] in terms of <math>x</math> around an energy minimum (<math>x = x_0</math>) to model the behavior of small perturbations from equilibrium.
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| :<math>V(x) = V(x_0) + (x-x_0) V'(x_0) + \frac{1}{2} (x-x_0)^2 V^{(2)}(x_0) + O(x-x_0)^3</math>
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| Because <math>V(x_0)</math> is a minimum, the first derivative evaluated at <math>x_0</math> must be zero, so the linear term drops out:
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| :<math>V(x) = V(x_0) + \frac{1}{2} (x-x_0)^2 V^{(2)}(x_0) + O(x-x_0)^3</math>
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| The [[constant term]] ''V''(''x''<sub>0</sub>) is arbitrary and thus may be dropped, and a coordinate transformation allows the form of the simple harmonic oscillator to be retrieved:
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| :<math>V(x) \approx \frac{1}{2} x^2 V^{(2)}(0) = \frac{1}{2} k x^2</math>
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| Thus, given an arbitrary potential energy function <math>V(x)</math> with a non-vanishing second derivative, one can use the solution to the simple harmonic oscillator to provide an approximate solution for small perturbations around the equilibrium point.
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| ==Examples==
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| ===Simple pendulum===
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| [[Image:Simple pendulum height.svg|thumb|right|250px|A [[simple pendulum]] exhibits simple harmonic motion under the conditions of no damping and small amplitude.]]
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| Assuming no damping and small amplitudes, the differential equation governing a simple pendulum is
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| :<math>{\mathrm{d}^2\theta\over \mathrm{d}t^2}+{g\over \ell}\theta=0.</math>
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| The solution to this equation is given by:
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| :<math>\theta(t) = \theta_0\cos\left(\sqrt{g\over \ell}t\right) \quad\quad\quad\quad |\theta_0| \ll 1</math>
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| where <math>\theta_0</math> is the largest angle attained by the pendulum. The [[Sine|period]], the time for one complete oscillation, is given by <math>2\pi</math> divided by whatever is multiplying the time in the argument of the cosine (<math>\sqrt{g\over \ell}</math> here).
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| :<math>T_0 = 2\pi\sqrt{\ell\over g}\quad\quad\quad\quad |\theta_0| \ll 1.</math>
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| ===Pendulum swinging over turntable===
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| Simple harmonic motion can in some cases be considered to be the one-dimensional [[Projection (mathematics)|projection]] of two-dimensional [[circular motion]]. Consider a long [[pendulum]] swinging over the turntable of a [[record player]]. On the edge of the turntable there is an object. If the object is viewed from the same level as the turntable, a projection of the motion of the object seems to be moving backwards and forwards on a straight line orthogonal to the view direction, sinusoidally like the pendulum.
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| ===Spring/mass system===
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| [[Image:Harmonic oscillator.svg|thumb|right|250px|Spring–mass system in equilibrium (A), compressed (B) and stretched (C) states.]]
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| When a spring is stretched or compressed by a mass, the spring develops a restoring force. [[Hooke's law]] gives the relationship of the force exerted by the spring when the spring is compressed or stretched a certain length:
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| :<math>F \left( t \right) =-kx \left( t \right) </math>
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| where ''F'' is the force, ''k'' is the spring constant, and ''x'' is the displacement of the mass with respect to the equilibrium position. The minus sign in the equation indicates that the force exerted by the spring always acts in a direction that is opposite to the displacement (i.e. the force always acts towards the zero position), and so prevents the mass from flying off to infinity.
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| By using either force balance or an energy method, it can be readily shown that the motion of this system is given by the following differential equation:
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| :<math> F(t) = -kx(t) = m \frac {\mathrm{d}^{2}}{\mathrm{d}{t}^{2}} x \left( t \right) = ma. </math>
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| ...the latter being [[Newton's laws of motion#Newton's second law|Newton's second law of motion]].
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| If the initial displacement is A, and there is no initial velocity, the solution of this equation is given by:
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| :<math> x \left( t \right) =A \cos \left( \sqrt{k \over m}t \right).</math>
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| Given an ideal massless spring, <math>m</math> is the mass on the end of the spring. If the spring itself has mass, its [[Effective mass (spring-mass system)|effective mass]] must be included in <math>m</math>.
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| ====Energy variation in the spring–damping system====
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| In terms of energy, all systems have two types of energy, [[potential energy]] and [[kinetic energy]]. When a spring is stretched or compressed, it stores elastic potential energy, which then is transferred into kinetic energy. The potential energy within a spring is determined by the equation <math> U = k{x}^{2}/ 2 . </math>
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| When the spring is stretched or compressed, kinetic energy of the mass gets converted into potential energy of the spring. By conservation of energy, assuming the datum is defined at the equilibrium position, when the spring reaches its maximum potential energy, the kinetic energy of the mass is zero. When the spring is released, it tries to return to equilibrium, and all its potential energy converts to kinetic energy of the mass.
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| ==See also==
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| *[[Anharmonic oscillator]]
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| *[[Critical speed]]
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| *[[Effective mass (spring-mass system)]]
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| *[[Normal mode]]
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| *[[Parametric oscillator]]
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| *[[Phasor]]
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| *[[Q factor]]
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| *[[Quantum harmonic oscillator]]
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| *[[Bertrand's_theorem#Radial_harmonic_oscillator|Radial harmonic oscillator]]
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| ==Notes==
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| {{reflist}}
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| ==References==
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| * {{cite book | last1 = Serway | first1 = Raymond A. | last2 = Jewett|first2=John W. | title = Physics for Scientists and Engineers | publisher = Brooks/Cole | year = 2003 | isbn = 0-534-40842-7 }}
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| * {{cite book | last1 = Tipler | first1 = Paul | title = Physics for Scientists and Engineers: Vol. 1 | edition = 4th | publisher = W. H. Freeman | year = 1998 | isbn = 1-57259-492-6 }}
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| * {{cite book | last1 = Wylie | first1 = C. R. | title = Advanced Engineering Mathematics | edition = 4th | publisher = McGraw-Hill | year = 1975 | isbn = 0-07-072180-7 }}
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| ==External links==
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| * {{springer|title=Oscillator, harmonic|id=p/o070530}}
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| * [http://hypertextbook.com/chaos/41.shtml Harmonic Oscillator] from The Chaos Hypertextbook
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| * [http://phy.hk/wiki/englishhtm/Damped.htm A Java applet of harmonic oscillator with damping proportional to velocity or damping caused by dry friction]
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| [[Category:Mechanical vibrations]]
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| [[Category:Ordinary differential equations]]
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| {{Link GA|es}}
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| {{Link FA|ko}}
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