Auxetic Materials
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Materials which, when stretched, become thicker perpendicularly to the applied force, i.e. they have a negative Poisson's ratio. Such materials have interesting mechanical properties such as high energy absorption and fracture resistance. This may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material and sponge mops.
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Boyle's Law
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(also known as the the Boyle-Mariotte law). At constant temperature, the absolute pressure and the volume of a gas are inversely proportional. The law can also be stated in a slightly different manner: that the product of absolute pressure and volume is always constant.
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Centrifugal Force
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An outward force associated with rotation. Centrifugal force is one of several so-called pseudo-forces (or inertial forces), so named because, unlike real forces, they do not originate in interactions with other bodies situated in the environment of the particle upon which they act. Instead, centrifugal force originates in the rotation of the frame of reference within which observations are made.
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Compression
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The result of the subjection of a material to compressive stress, resulting in reduction of volume. The opposite of compression is tension. In simple terms, compression is a pushing force.
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Condensation
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The change of the physical state of aggregation (or simply state) of matter from gaseous phase into liquid phase.
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Converse Piezoelectric Effect
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Materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied). For example, lead zirconate titanate crystals will exhibit a maximum shape change of about 0.1% of the original dimension.
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Indirectly via solid undergoing the converse Piezoelectric effect
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Cooling
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The act of reducing temperature.
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Electrostriction
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A property of all electrical non-conductors, or dielectrics, that causes them to change their shape under the application of an electric field. All dielectrics exhibit some electrostriction, but certain engineered ceramics, known as relaxor ferroelectrics, have extraordinarily high electrostrictive constants,e.g. Lead magnesium niobate (PMN), Lead magnesium niobate-lead titanate (PMN-PT), Lead lanthanum zirconate titanate (PLZT).
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Entrainment
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The movement of one fluid due to the motion of another.
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Entropic Explosion
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An explosion in which the reactants undergo a large change in volume without releasing a large amount of heat.
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Explosion
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A sudden increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases.
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Ferromagnetism
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The mechanism by which certain materials (such as iron) form permanent magnets and/or exhibit strong interactions with magnets. Responsible for commonly observed magnetism phenomena, e.g. 'fridge magnets. A material is 'ferromagnetic' only if all its magnetic ions add a positive contribution to the net magnetisation. If some of them subtract from the net magnetisation (i.e. are partially anti-aligned), then the material is 'ferrimagnetic'.
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Fluid Hammer
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A pressure surge or wave resulting when a fluid in motion is forced to stop or change direction suddenly (Momentum Change). Water hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe.
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Force
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That which can cause an object with mass to accelerate. Force has both magnitude & direction, i.e. it's a vector quantity. An object with constant mass will accelerate in proportion to the net force acting upon it and in inverse proportion to its mass (or the net force on an object is equal to the rate of change of momentum it experiences). Forces acting on objects may cause them to rotate or deform, or result in a change in pressure.
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Gravitation
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A natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass.
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Ground Effect
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Aircraft may be affected by a number of ground effects, or aerodynamic effects due to a flying body's proximity to the ground.
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Harmonic Oscillator
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A system which, when displaced from its equilibrium position, experiences a restoring force proportional to the displacement. Mechanical examples include pendula (with small angles of displacement) and masses connected to springs.
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For part of the cycle.
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Hydraulic Press
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A hydraulic mechanism for applying a large lifting or compressive force. It is the hydraulic equivalent of a mechanical lever.
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Impact Force
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A high force or shock applied over a short time period. Such a force or acceleration can sometimes have a greater effect than a lower force applied over a proportionally longer time period.
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Jet
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A coherent stream of fluid (e.g. gas or liquid) that is projected into a surrounding medium, usually from some kind of a nozzle or aperture. Jets can travel long distances without dissipating.
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Length Contraction
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The physical phenomenon of a decrease in length detected by an observer of objects that travel at any non-zero velocity relative to that observer. This contraction is usually only noticeable at a substantial fraction of the speed of light; the contraction is only in the direction parallel to the direction in which the observed body is travelling.
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Lever
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A rigid object that is used with an appropriate fulcrum or pivot point to multiply the mechanical force that can be applied to another object.
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Magnetism
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One of the phenomena by which materials exert attractive or repulsive forces on other materials. Some well-known materials that exhibit easily detectable magnetic properties (called magnets) are nickel, iron, cobalt, and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field.
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Magnetoelastic Effects
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A group of effects including Magnetostriction (or Joule Magnetostriction), Delta-E Effect, Wiedemann Effect, Magnetovolume Effect, and their Inverses: Villari Effect, Delta-E Effect, Matteucci Effect and the Nagaoka-Honda Effect.
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Magnetostriction
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A property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetisation. The variation of material's magnetisation due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value. This effect can cause losses due to frictional heating in susceptible ferromagnetic cores.
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Magnus Effect
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The phenomenon whereby a spinning object flying in a fluid creates a whirlpool of fluid around itself, and experiences a force perpendicular to the line of motion and away from the direction of spin. The overall behaviour is similar to that around an aerofoil with a circulation which is generated by the mechanical rotation, rather than by aerofoil action.
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Mechanical Force
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A mechanical influence that causes a free body to undergo an acceleration.
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Oblique Shock Wave
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A shock wave inclined with respect to the incident upstream flow direction. Occurs when a supersonic flow encounters a corner that effectively turns the flow into itself and compresses. Can be made by placing a wedge into supersonic, compressible flow. Consists of a very thin region across which nearly discontinuous changes in the thermodynamic properties of a gas occur.
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Osmotic Pressure
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Osmotic pressure is the pressure applied by a solution to prevent the inward flow of water across a semipermeable membrane.
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Pascal's Law
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(or the Principle of transmission of fluid-pressure) Pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains same.
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Phase Change
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The transformation of a thermodynamic system from one phase to another. Most commonly used to describe transitions between solid, liquid and gaseous states of matter, in rare cases including plasma.
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Poisson's Effect
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When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression.
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By indirect action of an intermediary material expanding in accordance with Poisson's Effect.
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Pressure Increase
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Pressure is an effect which occurs when a force is applied on a surface. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point.
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Pressurisation
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The application of pressure in a given situation or environment; and more specifically refers to the process by which atmospheric pressure is maintained in an isolated or semi-isolated atmospheric environment (for instance, in an aircraft, or whilst scuba diving).
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Pseudoelasticity
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(or superelasticity) An elastic (impermanent) response to relatively high stress caused by a phase transformation between the austenitic and martensitic phases of a crystal. Exhibited in Shape memory alloys. Superelastic alloys belong to the larger family of shape memory alloys. Unlike shape-memory alloys, no change in temperature is needed for the alloy to recover its initial shape.
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Indirectly via a pseudoelastic solid
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Shock Wave
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A type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma) or through a field such as the electromagnetic field. Characterized by an abrupt, nearly discontinuous change in the characteristics of the medium. Associated with rapid rise in pressure, temperature and density of the flow. A shock wave travels through most media at a higher speed than an ordinary wave.
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Surface Acoustic Wave
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(SAW) An acoustic wave traveling along the surface of a material having some elasticity, with an amplitude that typically decays exponentially with the depth of the substrate. This kind of wave is commonly used in devices called SAW devices in electronics circuits.
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Temperature Gradient
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The variation in temperature over distance.
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Tension
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The pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression.
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Tension in one system can be used to induce compression in another.
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Thermal Contraction
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The tendency of matter to reduce in volume (contract) in response to a change in temperature or when a substance is cooled.
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Thermal Expansion
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The tendency of matter to change in volume in response to a change in temperature or when a substance is heated.
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Expansion in one system can be used to induce compression in another.
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Tidal Force
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A situation in which a body or material (for example, tidal water, or the Moon) is mainly under the gravitational influence of a second body (for example, the Earth), but is also perturbed by the gravitational effects of a third body (for example, by the Moon in the case of tidal water, or by the Sun in the case of the Moon).
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For example, in a tethered diving bell during a rising tide.
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Ultrasound
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Cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, 20 kHz serves as a useful lower limit in describing ultrasound.
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Wind
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The flow of air or other gases that compose an atmosphere
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