Simple harmonic motion

Simple harmonic motion

Simple harmonic motion is the motion of a simple harmonic oscillator, a motion that is neither driven nor damped. The motion is periodic, as it repeats itself at standard intervals in a specific manner - described as being sinusoidal, with constant amplitude. It is characterized by its amplitude (which is always positive), its period which is the time for a single oscillation, its frequency which is the number of cycles per unit time, and its phase, which determines the starting point on the sine wave. The period, and its inverse the frequency, are constants determined by the overall system, while the amplitude and phase are determined by the initial conditions (position and velocity) of that system.

Simple harmonic motion is defined by the differential equation mfrac{d^2 x}{dt^2} = -kx , where "k" is a positive constant, "m" is the mass of the body, and "x" is its displacement from the mean position.

In words, simple harmonic motion is "motion where the force acting on a body and thereby acceleration of the body is proportional to, and opposite in direction to the displacement from its equilibrium position" (i.e. F=-kx ).

A general equation describing simple harmonic motion is x(t) = Acos left( 2,pi ,ft+phi ight) , where x is the displacement, A is the amplitude of oscillation, f is the frequency, t is the elapsed time, and phi is the phase of oscillation. If there is no displacement at time t = 0, the phase phi= frac{pi}{2}. A motion with frequency f has period, T=frac{1}{f}.

Simple harmonic motion can serve as a mathematical model of a variety of motions and provides the basis of the characterization of more complicated motions through the techniques of Fourier analysis.


It can be shown, by differentiating, exactly how the acceleration varies with time. Using the angular frequency omega, defined as

: omega = 2 pi f, ,

the displacement is given by the function

: x(t) = Acos left( omega t +phi ight).

Differentiating once gives an expression for the velocity at any time.

: v(t) = frac{mathrm{d},x(t)}{mathrm{d}t} = - Aomega sin left( omega t+phi ight).

And once again to get the acceleration at a given time.

: a(t) = frac{mathrm{d}^2,x(t)}{mathrm{d} t^2} = - A omega^2 cos left( omega t+phi ight).

These results can of course be simplified, giving us an expression for acceleration in terms of displacement.

: a(t) = -omega^2 x(t).

a(t) = -left( 2pi f ight)^2 x(t)

When and if total energy is constant and kinetic, the formula E = frac{kA^2}{2} applies for simple harmonic motion, where E is considered the total energy while all energy is in its kinetic form. A representing the mean displacement of the spring from its rest position in MKS units.


spring-mass system undergoes simple harmonic motion.] Simple harmonic motion is exhibited in a variety of simple physical systems and below are some examples.

Mass on a spring

A mass "M" attached to a spring of spring constant "k" exhibits simple harmonic motion in space with

:omega=2 pi f = sqrt{frac{k}{M.,

Alternately, if the other factors are known and the period is to be found, this equation can be used:

: T= frac{1}{f} = 2 pi sqrt{frac{M}{k.

The total energy is constant, and given by E = frac{kA^2}{2}, where E is the total energy.

Uniform circular motion

Simple harmonic motion can in some cases be considered to be the one-dimensional projection of uniform circular motion. If an object moves with angular frequency omega around a circle of radius R centered at the origin of the x-y plane, then its motion along the x and the y coordinates is simple harmonic with amplitude R and angular speed omega.

Mass on a pendulum

In the small-angle approximation, the motion of a pendulum is approximated by simple harmonic motion. The period of a mass attached to a string of length ell with gravitational acceleration g is given by

: T= 2 pi sqrt{frac{ell}{g

This approximation is accurate only in small angles because of the expression for angular acceleration being proportional to the sine of position:

:ell m g sin( heta)=I alpha

where I is the moment of inertia; in this case I = mell^2. When heta is small, sin( heta) approx heta and therefore the expression becomes

:ell m g heta=I alpha

which makes angular acceleration directly proportional to heta, satisfying the definition of Simple Harmonic Motion.

For a solution not relying on a small-angle approximation, see pendulum (mathematics).

Useful Formulae

Given mass M attached to a spring/pendulum with amplitude A with acceleration a:

: k = frac{Ma}{A}: f = frac{A}{t} = frac{lambda}{t}: T_s = T_p = frac{1}{f} = frac{t}{A} = 2 pi sqrt{ frac{M}{k = 2 pi sqrt{ frac{A}{g = 2 pi sqrt{ frac{ell}{g.: E_{tot} = frac{kA^2}{2} = frac{MaA}{2}.


:k is the spring constant.:M is the mass (usually in kilograms):a is the acceleration.:A is the amplitude OR lambda is the wavelength.:f is the frequency (usually in hertz).:t is the time in seconds to complete one cycle.:T_s or T_p is the period of the spring or pendulum.:g is the acceleration due to gravity (On Earth at sea level: 9.81 m/s²).:ell is the length of the pendulum.:E_{tot} is the total energy.

See also

*Uniform circular motion
*Complex harmonic motion

External links

* [ Java simulation of spring-mass oscillator]

Wikimedia Foundation. 2010.

См. также в других словарях:

  • simple harmonic motion — Physics. vibratory motion in a system in which the restoring force is proportional to the displacement from equilibrium. Abbr.: S.H.M., s.h.m. * * * Repetitive back and forth movement through a central, or equilibrium, position in which the… …   Universalium

  • simple harmonic motion — noun periodic motion in which the restoring force is proportional to the displacement • Hypernyms: ↑harmonic motion …   Useful english dictionary

  • simple harmonic motion — noun a form of vibratory motion which may be represented by projecting on to the diameter of a circle the uniform motion of a point round its circumference. Abbrev.: SHM …   Australian-English dictionary

  • simple harmonic motion — noun Physics oscillatory motion under a retarding force proportional to the amount of displacement from an equilibrium position …   English new terms dictionary

  • Harmonic motion — Harmonic Har*mon ic (h[aum]r*m[o^]n [i^]k), Harmonical Har*mon ic*al ( [i^]*kal), a. [L. harmonicus, Gr. armoniko s; cf. F. harmonique. See {Harmony}.] 1. Concordant; musical; consonant; as, harmonic sounds. [1913 Webster] Harmonic twang! of… …   The Collaborative International Dictionary of English

  • Harmonic motion — can mean: The motion of a Harmonic oscillator (in physics), which can be: Simple harmonic motion Complex harmonic motion Keplers laws of planetary motion (in physics, known as the harmonic law) Quasi harmonic motion: Unit… …   Wikipedia

  • harmonic motion — noun a periodic motion in which the displacement is either symmetrical about a point or is the sum of such motions • Hypernyms: ↑periodic motion, ↑periodic movement • Hyponyms: ↑simple harmonic motion …   Useful english dictionary

  • harmonic motion — /haˌmɒnɪk ˈmoʊʃən/ (say hah.monik mohshuhn) noun Physics See simple harmonic motion …   Australian-English dictionary

  • Harmonic motion — Motion Mo tion, n. [F., fr. L. motio, fr. movere, motum, to move. See {Move}.] 1. The act, process, or state of changing place or position; movement; the passing of a body from one place or position to another, whether voluntary or involuntary;… …   The Collaborative International Dictionary of English

  • harmonic motion — noun Date: 1867 a periodic motion (as of a sounding violin string or swinging pendulum) that has a single frequency or amplitude or is composed of two or more such simple periodic motions …   New Collegiate Dictionary

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