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In numerous fields of study, the component of **instability** within a system is generally characterized by some of the outputs or internal states growing without bounds.^{ [1] } Not all systems that are not stable are unstable; systems can also be marginally stable or exhibit limit cycle behavior.

A **system** is a group of interacting or interrelated entities that form a unified whole. A system is delineated by its spatial and temporal boundaries, surrounded and influenced by its environment, described by its structure and purpose and expressed in its functioning.

In quantum physics, a **bound state** is a special quantum state of a particle subject to a potential such that the particle has a tendency to remain localised in one or more regions of space. The potential may be external or it may be the result of the presence of another particle; in the latter case, one can equivalently define a bound state as a state representing two or more particles whose interaction energy exceeds the total energy of each separate particle. One consequence is that, given a potential vanishing at infinity, negative-energy states must be bound. In general, the energy spectrum of the set of bound states is discrete, unlike free particles, which have a continuous spectrum.

In mathematics, **stability theory** addresses the stability of solutions of differential equations and of trajectories of dynamical systems under small perturbations of initial conditions. The heat equation, for example, is a stable partial differential equation because small perturbations of initial data lead to small variations in temperature at a later time as a result of the maximum principle. In partial differential equations one may measure the distances between functions using Lp norms or the sup norm, while in differential geometry one may measure the distance between spaces using the Gromov–Hausdorff distance.

- Instability in control systems
- Instability in solid mechanics
- Fluid instabilities
- Plasma instabilities
- Instabilities of stellar systems
- Joint instabilities
- Notes
- External links

In structural engineering, a structure can become unstable when excessive load is applied. Beyond a certain threshold, structural deflections magnify stresses, which in turn increases deflections. This can take the form of buckling or crippling. The general field of study is called structural stability.

**Structural engineering** is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and muscles' that create the form and shape of man made structures. Structural engineers need to understand and calculate the stability, strength and rigidity of built structures for buildings and nonbuilding structures. The structural designs are integrated with those of other designers such as architects and building services engineer and often supervise the construction of projects by contractors on site. They can also be involved in the design of machinery, medical equipment, and vehicles where structural integrity affects functioning and safety. See glossary of structural engineering.

In engineering, **deflection** is the degree to which a structural element is displaced under a load. It may refer to an angle or a distance.

In science, **buckling** is an instability that leads to structural failure. The failure modes can in simple cases be found by simple mathematically solutions. For complex structures the failure modes are found by numerical tools.

Atmospheric instability is a major component of all weather systems on Earth.

**Atmospheric instability** is a condition where the Earth's atmosphere is generally considered to be unstable and as a result the weather is subjected to a high degree of variability through distance and time. Atmospheric stability is a measure of the atmosphere's tendency to discourage or deter vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity. In unstable conditions, a lifted thing, such as a parcel of air will be warmer than the surrounding air at altitude. Because it is warmer, it is less dense and is prone to further ascent.

In the theory of dynamical systems, a state variable in a system is said to be unstable if it evolves without bounds. A system itself is said to be unstable if at least one of its state variables is unstable.

A **state variable** is one of the set of variables that are used to describe the mathematical "state" of a dynamical system. Intuitively, the state of a system describes enough about the system to determine its future behaviour in the absence of any external forces affecting the system. Models that consist of coupled first-order differential equations are said to be in state-variable form.

In continuous time control theory, a system is unstable if any of the roots of its characteristic equation has real part greater than zero (or if zero is a repeated root). This is equivalent to any of the eigenvalues of the state matrix having either real part greater than zero, or, for the eigenvalues on the imaginary axis, the algebraic multiplicity being larger than the geometric multiplicity.^{[ clarification needed ]} The equivalent condition in discrete time is that at least one of the eigenvalues is greater than 1 in absolute value, or that two or more eigenvalues are equal and of unit absolute value.

**Control theory** in control systems engineering is a subfield of mathematics that deals with the control of continuously operating dynamical systems in engineered processes and machines. The objective is to develop a control model for controlling such systems using a control action in an optimum manner without *delay or overshoot* and ensuring control stability.

In mathematics, the **characteristic equation** is an algebraic equation of degree n upon which depends the solution of a given nth-order differential equation or difference equation. The characteristic equation can only be formed when the differential or difference equation is linear and homogeneous, and has constant coefficients. Such a differential equation, with y as the dependent variable and *a*_{n}, *a*_{n − 1}, ..., *a*_{1}, *a*_{0} as constants,

- Buckling
- Elastic instability
- Drucker stability of a nonlinear constitutive model
- Biot instability (surface wrinkling in elastomers)
- Baroclinic instability
^{ [2] }

**Elastic instability** is a form of instability occurring in elastic systems, such as buckling of beams and plates subject to large compressive loads.

**Drucker stability** refers to a set of mathematical criteria that restrict the possible nonlinear stress-strain relations that can be satisfied by a solid material. The postulates are named after Daniel C. Drucker. A material that does not satisfy these criteria is often found to be unstable in the sense that application of a load to a material point can lead to arbitrary deformations at that material point unless an additional length or time scale is specified in the constitutive relations.

Fluid instabilities occur in liquids, gases and plasmas, and are often characterized by the shape that form; they are studied in fluid dynamics and magnetohydrodynamics. Fluid instabilities include:

- Ballooning mode instability (some analogy to the Rayleigh–Taylor instability); found in the magnetosphere
- Atmospheric instability
- Bénard instability
- Drift mirror instability
- Kelvin–Helmholtz instability (similar, but different from the diocotron instability in plasmas)
- Rayleigh–Taylor instability
- Plateau-Rayleigh instability (similar to the Rayleigh–Taylor instability)
- Richtmyer-Meshkov instability (similar to the Rayleigh–Taylor instability)
- Shock Wave Instability
- Benjamin-Feir Instability (also known as modulational instability)

Plasma instabilities can be divided into two general groups (1) hydrodynamic instabilities (2) kinetic instabilities. Plasma instabilities are also categorised into different modes – see this paragraph in plasma stability.

Galaxies and star clusters can be unstable, if small perturbations in the gravitational potential cause changes in the density that reinforce the original perturbation. Such instabilities usually require that the motions of stars be highly correlated, so that the perturbation is not "smeared out" by random motions. After the instability has run its course, the system is typically "hotter" (the motions are more random) or rounder than before. Instabilities in stellar systems include:

- Bar instability of rapidly rotating disks
- Jeans instability
- Firehose instability
^{ [4] } - Gravothermal instability
- Radial-orbit instability
- Various instabilities in cold rotating disks

The most common residual disability after any sprain in the body is instability. Mechanical instability includes insufficient stabilizing structures and mobility that exceed the physiological limits. Functional instability involves recurrent sprains or a feeling of giving way of the injured joint.^{ [5] } Injuries cause proprioceptive deficits and impaired postural control in the joint. Individuals with muscular weakness, occult instability, and decreased postural control are more susceptible to injury than those with better postural control. Instability leads to an increase in postural sway, the measurement of the time and distance a subject spends away from an ideal center of pressure. The measurement of a subject’s postural sway can be calculated through testing center of pressure (CoP), which is defined as the vertical projection of center of mass on the ground. Investigators have theorized that if injuries to joints cause deafferentation, the interruption of sensory nerve fibers, and functional instability, then a subject’s postural sway should be altered.^{ [6] } Joint stability can be enhanced by the use of an external support system, like a brace, to alter body mechanics. The mechanical support provided by a brace provides cutaneous afferent feedback in maintaining postural control and increasing stability.

- ↑ "Definition of INSTABILITY".
*www.merriam-webster.com*. Retrieved 23 April 2018. - ↑ "Definition of BAROCLINIC INSTABILITY".
*www.merriam-webster.com*. Retrieved 23 April 2018. - ↑ Shengtai Li, Hui Li. "Parallel AMR Code for Compressible MHD or HD Equations". Los Alamos National Laboratory.
- ↑ Merritt, D.; Sellwood, J. (1994), "Bending Instabilities of Stellar Systems",
*The Astrophysical Journal*,**425**: 551–567, Bibcode:1994ApJ...425..551M, doi:10.1086/174005 - ↑ Guskiewicz, K. M.; Perrin, David H. (1996). "Effect of Orthotics on Postural Sway Following Inversion Ankle Sprain".
*Journal of Orthopedic and Sports Physical Therapy*.**23**(5): 326–331. doi:10.2519/jospt.1996.23.5.326. PMID 8728531. - ↑ Pintsaar, A.; Brynhildsen, J.; Tropp, H. (1996). "Postural Corrections after Standardised Perturbations of Single Limb Stance: Effect of Training and Orthotic Devices in Patients with Ankle Instability".
*British Journal of Sports Medicine*.**30**(2): 151–155. doi:10.1136/bjsm.30.2.151. PMC 1332381. PMID 8799602.

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The **stability of a plasma** is an important consideration in the study of plasma physics. When a system containing a plasma is at equilibrium, it is possible for certain parts of the plasma to be disturbed by small perturbative forces acting on it. The stability of the system determines if the perturbations will grow, oscillate, or be damped out.

In fluid dynamics, the **baroclinity** of a stratified fluid is a measure of how misaligned the gradient of pressure is from the gradient of density in a fluid. In meteorology a baroclinic atmosphere is one for which the density depends on both the temperature and the pressure; contrast this with a barotropic atmosphere, for which the density depends only on the pressure. In atmospheric terms, the barotropic zones of the Earth are generally found in the central latitudes, or tropics, whereas the baroclinic areas are generally found in the mid-latitude/polar regions.

The **Kelvin–Helmholtz instability** can occur when there is velocity shear in a single continuous fluid, or where there is a velocity difference across the interface between two fluids. An example is wind blowing over water: The instability manifests in waves on the water surface. More generally, clouds, the ocean, Saturn's bands, Jupiter's Red Spot, and the sun's corona show this instability.

The **Rayleigh–Taylor instability**, or **RT instability**, is an instability of an interface between two fluids of different densities which occurs when the lighter fluid is pushing the heavier fluid. Examples include the behavior of water suspended above oil in the gravity of Earth, mushroom clouds like those from volcanic eruptions and atmospheric nuclear explosions, supernova explosions in which expanding core gas is accelerated into denser shell gas, instabilities in plasma fusion reactors and inertial confinement fusion.

The **potential temperature** of a parcel of fluid at pressure
is the temperature that the parcel would attain if adiabatically brought to a standard reference pressure
, usually 1000 millibars. The potential temperature is denoted
and, for a gas well-approximated as ideal, is given by

In fluid dynamics, the **Taylor number** (**Ta**) is a dimensionless quantity that characterizes the importance of centrifugal "forces" or so-called inertial forces due to rotation of a fluid about an axis, relative to viscous forces.

In fluid dynamics, the **Taylor–Couette flow** consists of a viscous fluid confined in the gap between two rotating cylinders. For low angular velocities, measured by the Reynolds number *Re*, the flow is steady and purely azimuthal. This basic state is known as circular Couette flow, after Maurice Marie Alfred Couette, who used this experimental device as a means to measure viscosity. Sir Geoffrey Ingram Taylor investigated the stability of Couette flow in a ground-breaking paper. Taylor's paper became a cornerstone in the development of hydrodynamic stability theory and demonstrated that the no-slip condition, which was in dispute by the scientific community at the time, was the correct boundary condition for viscous flows at a solid boundary.

The **Richtmyer–Meshkov instability (RMI)** occurs when two fluids of different density are impulsively accelerated. Normally this is by the passage of a shock wave. The development of the instability begins with small amplitude perturbations which initially grow linearly with time. This is followed by a nonlinear regime with bubbles appearing in the case of a light fluid penetrating a heavy fluid, and with spikes appearing in the case of a heavy fluid penetrating a light fluid. A chaotic regime eventually is reached and the two fluids mix. This instability can be considered the impulsive-acceleration limit of the Rayleigh–Taylor instability.

**Standing**, also referred to as **orthostasis**, is a human position in which the body is held in an upright ("**orthostatic**") position and supported only by the feet.

In biomechanics, **balance** is an ability to maintain the line of gravity of a body within the base of support with minimal postural sway. Sway is the horizontal movement of the centre of gravity even when a person is standing still. A certain amount of sway is essential and inevitable due to small perturbations within the body or from external triggers. An increase in sway is not necessarily an indicator of dysfunctional balance so much as it is an indicator of decreased sensorimotor control.

In physics and engineering, **kinetics** is the branch of classical mechanics that is concerned with the relationship between motion and its causes, specifically, forces and torques. Since the mid-20th century, the term "dynamics" has largely superseded "kinetics" in physics textbooks, though the term is still used in engineering.

In fluid dynamics, **hydrodynamic stability** is the field which analyses the stability and the onset of instability of fluid flows. The study of hydrodynamic stability aims to find out if a given flow is stable or unstable, and if so, how these instabilities will cause the development of turbulence. The foundations of hydrodynamic stability, both theoretical and experimental, were laid most notably by Helmholtz, Kelvin, Rayleigh and Reynolds during the nineteenth century. These foundations have given many useful tools to study hydrodynamic stability. These include Reynolds number, the Euler equations, and the Navier–Stokes equations. When studying flow stability it is useful to understand more simplistic systems, e.g. incompressible and inviscid fluids which can then be developed further onto more complex flows. Since the 1980s, more computational methods are being used to model and analyse the more complex flows.

**Energy dissipation and entropy production extremal principles** are ideas developed within non-equilibrium thermodynamics that attempt to predict the likely steady states and dynamical structures that a physical system might show. The search for extremum principles for non-equilibrium thermodynamics follows their successful use in other branches of physics. According to Kondepudi (2008), and to Grandy (2008), there is no general rule that provides an extremum principle that governs the evolution of a far-from-equilibrium system to a steady state. According to Glansdorff and Prigogine, irreversible processes usually are not governed by global extremal principles because description of their evolution requires differential equations which are not self-adjoint, but local extremal principles can be used for local solutions. Lebon Jou and Casas-Vásquez (2008) state that "In non-equilibrium ... it is generally not possible to construct thermodynamic potentials depending on the whole set of variables". Šilhavý (1997) offers the opinion that "... the extremum principles of thermodynamics ... do not have any counterpart for [non-equilibrium] steady states ." It follows that any general extremal principle for a non-equilibrium problem will need to refer in some detail to the constraints that are specific for the structure of the system considered in the problem.

The **Taylor–Goldstein equation** is an ordinary differential equation used in the fields of geophysical fluid dynamics, and more generally in fluid dynamics, in presence of quasi-2D flows. It describes the dynamics of the Kelvin–Helmholtz instability, subject to buoyancy forces, for stably stratified fluids in the dissipation-less limit. Or, more generally, the dynamics of internal waves in the presence of a (continuous) density stratification and shear flow. The Taylor–Goldstein equation is derived from the 2D Euler equations, using the Boussinesq approximation.

**Rayleigh–Bénard convection** is a type of natural convection, occurring in a plane horizontal layer of fluid heated from below, in which the fluid develops a regular pattern of convection cells known as **Bénard cells**. Rayleigh–Bénard convection is one of the most commonly studied convection phenomena because of its analytical and experimental accessibility. The convection patterns are the most carefully examined example of self-organizing nonlinear systems.

In fluid dynamics, **Rayleigh's equation** or **Rayleigh stability equation** is a linear ordinary differential equation to study the hydrodynamic stability of a parallel, incompressible and inviscid shear flow. The equation is:

The **interchange instability** is a type of plasma instability seen in magnetic fusion energy that is driven by the gradients in the magnetic pressure in areas where the confining magnetic field is curved. The name of the instability refers to the action of the plasma changing position with the magnetic field lines without significant disturbance to the geometry of the external field. The instability causes flute-like structures to appear on the surface of the plasma, and thus the instability is also known as the **flute instability**. The interchange instability is a key issue in the field of fusion energy, where magnetic fields are used to confine a plasma in a volume surrounded by the field.

**Combustion instabilities** are physical phenomena occurring in a reacting flow in which some perturbations, even very small ones, grow and then become large enough to alter the features of the flow in some particular way.