Alfred Marshall , p. He elaborated these ideas in his Principles , book 3, chapter 3 and the concept has since been adopted by the profession. Prior to Marshall, there had been no generally accepted terminology or measure of the relationship between price and quantity, although concepts similar to elasticity had been suggested. For example, Mill , book 3, chapter 2, sec. Cournot , chapter 4, sec. Elasticity of demand.
Elasticity of demand is defined as the percentage change in the quantity demanded, divided by the percentage change in price. It is derived from the demand curve, which shows the absolute quantity demanded as a function of price. Since the rate of demand rises if the price falls, and vice versa, the elasticity of demand is actually negative, except that it is common to ignore the algebraic sign and use the absolute value.
By definition, demand is inelastic if the elasticity is less than one and is elastic if the elasticity is greater than one. An elasticity of one is called unit , or unitary, elasticity. Elastic demand. If demand is elastic, a given percentage increase in price causes a larger percentage decline in the quantity demanded, or a given percentage reduction in price causes a larger percentage increase in quantity demanded. In the limiting case of perfectly infinitely elastic demand, which is illustrated by the horizontal line segment DE in Figure 1 if it is visualized as extended horizontally so as to constitute the entire demand curve , any increase in price, no matter how small, will cause the quantity demanded to decline to zero; and any reduction in price, again no matter how small, will cause the quantity demanded to increase without limit.
It is part of the definition of perfect competition that there must be so many firms in the industry producing such excellent substitutes that the demand facing the firm is perfectly infinitely elastic, even though the demand facing the industry may be inelastic. Inelastic demand. Conversely, if demand is inelastic the percentage change in quantity is smaller than the percentage change in price.
An increase in price therefore causes a proportionately smaller decline in the quantity demanded, and a reduction in price causes a proportionately smaller increase in the quantity demanded. In the limiting case of zero elasticity perfectly inelastic demand , which is illustrated by the vertical line segment AB in Figure 1 if it is visualized as extended vertically so as to constitute the entire demand curve , neither an increase nor a reduction in price has any effect upon the quantity demanded. Relation to total revenue. Elasticity of demand also shows how the total dollar value of sales of the commodity responds to a change in its price.
For example, if the price of a commodity with inelastic demand is increased, the total revenue of the seller will increase even though the quantity demanded declines somewhat, since the increase in price more than offsets the reduction in volume. This creates a presumption that a seller faced with an inelastic demand will raise his price and will continue to do so until demand becomes elastic.
Similar reasoning shows that a reduction in price reduces total revenue if demand is inelastic but increases total revenue if demand is elastic, since the increase in volume more than offsets the reduction in price. Finally, an increase in price reduces total revenue if demand is elastic and does not change total revenue if demand has unit elasticity. Relation to marginal revenue. There is also a logical relationship between the elasticity of demand and marginal revenue.
Marginal revenue is defined as the change in the total revenue per unit change in quantity sold, if the relationship between the price and the quantity sold is given by the demand curve. Positive marginal revenue therefore implies that total revenue increases if the quantity sold increases, or declines if the quantity sold declines. Since the relation between price and quantity is given by the demand curve, an increase in sales volume must be accompanied by a decline in price.
Thus, a positive marginal revenue means that total revenue increases as volume increases and price declines. Since a decline in price increases total revenue only if demand is elastic, positive marginal revenue must be associated with elastic demand. Furthermore, since profit maximization requires that marginal cost equal marginal revenue, and marginal cost is always positive, it follows that the marginal revenue of the profit-maximizing firm must be positive and the demand elastic. Similar reasoning shows that negative marginal revenue is associated with inelastic demand: if demand is inelastic, an increase in price reduces volume and increases total revenue, so that marginal revenue is negative.
The intermediate case of unit elasticity of demand implies zero marginal revenue, since any change in price is just balanced by a change in volume, and total revenue is the same at any price or at any volume. Relation to price.
Elasticity of demand usually changes with the price of the product. Thus, it is incorrect to say that the demand for cigarettes is inelastic or that the demand for butter is elastic. The correct statement is that these demands are elastic or inelastic at the prices that are currently charged. At higher prices the demand for cigarettes might well become elastic, while the demand for butter might well become inelastic if its price fell far enough below the price of oleomargarine.
In the particular case of the straight-line demand curve, which is so frequently used for illustrative purposes in economics, the demand is elastic and marginal revenue is positive throughout the upper half of the possible price range, while demand is inelastic and marginal revenue negative throughout the lower half of the price range. Elasticity of supply. Defined as the percentage change in the quantity offered for sale divided by the percentage change in price, elasticity of supply is derived from the supply curve, which shows the absolute amount producers wish to sell as a function of price.
Since sellers wish to sell more at a higher price, the elasticity of supply is positive. If the supply curve is inelastic, a relatively larger increase in price is required to produce a given percentage increase in quantity supplied. If the supply curve is elastic, the sellers are more responsive to a change in price, and the increase in the quantity supplied is larger, on a percentage basis, than the increase in price.
Other elasticities. Marshall originally introduced the concept of elasticity in connection with demand and supply curves, but economists soon started to use the ratio of the percentage changes to describe the relationship between any two variables, provided only that the variables are functionally related. By , Johnson p. The most important elasticities, other than demand and supply, are income elasticity of demand and cross-elasticity of demand.
Income elasticity of demand. Quantity demanded is therefore functionally related to price, if incomes and other prices do not change; and the price elasticity of demand is a property of this function. Quantity demanded is also functionally related to income, if the price of the commodity and other prices do not change. The income elasticity of demand is a property of this function and is defined as the percentage change in the quantity demanded divided by the percentage change in income. If income elasticity of demand is negative, consumers purchase less of the commodity as their incomes rise, and the commodity is, by definition, an inferior good.
If income elasticity is positive, but less than one, consumers buy more of the product as their incomes rise if prices do not change , but spend a smaller proportion of their incomes on the commodity. For commodities with income elasticities greater than one, consumers spend a higher proportion of a higher income on the commodity, and the quantity demanded grows at a faster percentage rate than income grows. Cross-elasticity of demand. Cross-elasticity of demand is a property of this function and is defined as the percentage change in the quantity demand of commodity A divided by the percentage change in the price of commodity B.
The commodities A and B are, by definition, sub stitutes if the cross-elasticity is positive, since an increase in the price of B then causes a reduction in the demand for B and an increase in the demand for A. Conversely, if cross-elasticity is negative, the commodities are complements, since an increase in the price of B causes consumers to demand less of both commodities. Mathematical definitions. Point elasticity. Arc elasticity. Sometimes the functional relationship between p and q is unknown, but two points, B and D , with coordinates q 1 , p 1 and q 2 , p 2 as in Figure 1, are known; and the arc elasticity between the two points is desired.
The usual practice is to assume that the demand curve passing through these points is a straight line, HBIDJ , and to calculate the point elasticity at I , the point midway between B and D. The formula for the arc elasticity, so defined, is. While this procedure is arbitrary, it has two advantages over calculating the elasticity at either B or D. First, the elasticity has the same value whether the change is from B to D or from D to B.
More important, however, is the fact that the relationships between elasticity, total revenue, and marginal revenue, which were discussed above, are valid if elasticity is calculated at I , but not if it is calculated at either B or D. New York : Kelley. London: Stanford. Marshall, Alfred Principles of Economics. New York and London: Macmillan. The eighth edition is more convenient for normal use. Edited by W. New York: Kelley. Elasticity is a measure of the responsiveness of one economic variable to another. For example, advertising elasticity is the relationship between a change in a firm's advertising budget and the resulting change in product sales.
Economists are often interested in the price elasticity of demand, which measures the response of the quantity of an item purchased to a change in the item's price. A good or service is considered to be highly elastic if a slight change in price leads to a sharp change in demand for the product or service. Products and services that are highly elastic are usually more discretionary in nature — readily available in the market and something that a consumer may not necessarily need in his or her daily life. On the other hand, an inelastic good or service is one for which changes in price result in only modest changes to demand.
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These goods and services tend to be necessities. Elasticity is usually expressed as a positive number when the sign is already clear from context. Elasticity measures are reported as a proportional or percent change in the variable being studied.
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The general formula for elasticity, represented by the letter "E" in the equation below, is:. Elasticity can be zero, one, greater than one, less than one, or infinite. When elasticity is equal to one there is unit elasticity. This means the proportional change in one variable is equal to the proportional change in another variable, or in other words, the two variables are directly related and move together. When elasticity is greater than one, the proportional change in x is greater than the proportional change in y and the situation is said to be elastic. Inelastic situations result when the proportional change in x is less than the proportional change in y.
Perfectly inelastic situations result when any change in y will have an infinite effect on x. Finally, perfectly elastic situations result when any change in y will result in no change in x.
A special case known as unitary elasticity of demand occurs if total revenue stays the same when prices change. Economists compute several different elasticity measures, including the price elasticity of demand, the price elasticity of supply, and the income elasticity of demand.
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Elasticity is typically defined in terms of changes in total revenue since that is of primary importance to managers, CEOs, and marketers. For managers, a key point in the discussions of demand is what happens when they raise prices for their products and services. It is important to know the extent to which a percentage increase in unit price will affect the demand for a product.
With elastic demand, total revenue will decrease if the price is raised. With inelastic demand, however, total revenue will increase if the price is raised. The possibility of raising prices and increasing dollar sales total revenue at the same time is very attractive to managers. This occurs only if the demand curve is inelastic. Here total revenue will increase if the price is raised, but total costs probably will not increase and, in fact, could go down. Since profit is equal to total revenue minus total costs, profit will increase as price is increased when demand for a product is inelastic.
It is important to note that an entire demand curve is neither elastic nor inelastic; it only has the particular condition for a change in total revenue between two points on the curve and not along the whole curve. Demand elasticity is affected by three things: 1 availability of substitutes; 2 the urgency of need, and 3 the importance of the item in the customer's budget.
Substitutes are products that offer the buyer a choice. For example, many consumers see corn chips as a good or homogeneous substitute for potato chips, or see sliced ham as a substitute for sliced turkey. The more substitutes available, the greater will be the elasticity of demand. If consumers see products as extremely different or heterogeneous, however, then a particular need cannot easily be satisfied by substitutes. In contrast to a product with many substitutes, a product with few or no substitutes — like gasoline — will have an inelastic demand curve.
Similarly, demand for products that are urgently needed or are very important to a person's budget will tend to be inelastic. It is important for managers to understand the price elasticity of their products and services in order to set prices appropriately to maximize firm profits and revenues. Hodrick, Laurie Simon. May Montgomery, Alan L. November Perreault, William E. Jerome McCarthy. McGraw-Hill, Hillstrom, Northern Lights.
In economics, elasticity measures a response of one variable to changes in the other variable. The concept of elasticity can be applied to any two variables, but the most commonly used are price elasticity of demand and elasticity of substitution between factors of production, consumer goods, or bundles of consumption in different periods of time elasticity of intertemporal substitution. Elasticity measures the percentage change in variable Y in response to a 1 percent change in variable X.
Formally, the elasticity of Y with respect to X is defined as. The concept of the price elasticity of demand, which was introduced in by Alfred Marshall , measures the percentage change in the quantity of a good demanded when the price of this good changes by 1 percent. The demand is elastic if price changes lead to large changes in quantity demanded. The demand is inelastic if the quantity demanded does not respond much to changes in price.
If demand is perfectly elastic, even a small increase in price will send the quantity demanded to zero. If demand is perfectly inelastic, the quantity demanded will be constant regardless of the price. Price elasticity of demand for goods depends on the characteristics of the goods — that is, whether or not they are necessities and whether or not there are close substitutes for these goods.
Price elasticity of demand also varies with the time horizon in consideration.
For example, demand for gasoline is very inelastic in the short run, because people have to fill up their gas tanks, but in the long run, if prices remain high, people will switch to more gasoline-efficient cars and will demand less gasoline. By the estimates of the Mackinac Center for Public Policy , a 10 percent increase in the price of gasoline will lower the demand for it by 2 percent in the short run elasticity of Price elasticity of demand also depends on how narrowly a good is defined.
For example, demand for milk in general is quite inelastic, because there are no close substitutes. However, demand for milk of a specific brand is very elastic because there are many close substitutes i. The concept of the elasticity of substitution was introduced independently by John Hicks and Joan Robinson in and , respectively. It is used to measure how easily factors of production can be substituted for one another.
For example, how much will the ratio of capital input to labor input in production increase if the ratio of capital cost to labor cost falls by 1 percent? The concept of the elasticity of substitution can also be applied to consumption of goods, to measure how relative consumption of two goods is affected by their relative price. Elasticity of intertemporal substitution is estimated to be close to zero, indicating that consumers are not sensitive in their intertemporal consumption decisions to the changes in the interest rate.
Anderson, Patrick L. McLellan, Joseph P. Overton, and Gary L. Price Elasticity of Demand. Mackinac Center for Public Policy. Yogo, Motohiro. Review of Economics and Statistics 86 3 : — In essence he described the procedure he had introduced in his paper on the "contra-indications of the active technique" , in which he recommended using relaxation to reduce tension in certain difficult cases.
In two other articles from the same period "Family Adaptation to the Child" and "The Problem of the End of Analysis" he dealt with difficulties in the educational environment. The question became one of how far the idea of elasticity could be taken. In , Michael Balint would write on Ferenczi's problem, "His earlier experiences had familiarized him with two models: one was the classic technique with its objective and benevolent passivity, and apparently imperturbable and unlimited patience; the other was the active technique with its well-directed interventions founded on attentive observation and empathy.
In the paper, Ferenczi developed the technical importance of tact in deciding on the right moment to communicate to the patient any conjectures the analyst may have made, "based essentially on the dissection of our own Self. Ferenczi proposed "a perpetual oscillation between feeling-with, self-observation and judgment activity. He concluded this reflection on the counter-transference with a "metapsychology of the technique," denouncing the "fanaticism of interpretation as an infantile disease of analysis" because, in order for patients to become free of all emotional binds, they must "abandon, at least provisionally, all sorts of superegos, including that of the analyst.
Balint, Michael. Paris: Payot, Contre-indications de la technique active. Paris, Payot. Paris: Payot. The elasticity of psycho-analytic technique. Bergmann and F. Hartman Eds. New York : Basic Books.
Many materials will noticeably deviate from Hooke's law well before those elastic limits are reached. On the other hand, Hooke's law is an accurate approximation for most solid bodies, as long as the forces and deformations are small enough. For this reason, Hooke's law is extensively used in all branches of science and engineering, is the foundation of many disciplines such as seismology , molecular mechanics and acoustics , it is the fundamental principle behind the spring scale , the manometer , the balance wheel of the mechanical clock.
The modern theory of elasticity generalizes Hooke's law to say that the strain of an elastic object or material is proportional to the stress applied to it. However, since general stresses and strains may have multiple independent components, the "proportionality factor" may no longer be just a single real number, but rather a linear map that can be represented by a matrix of real numbers.
In this general form, Hooke's law makes it possible to deduce the relation between strain and stress for complex objects in terms of intrinsic properties of the materials it is made of. For example, one can deduce that a homogeneous rod with uniform cross section will behave like a simple spring when stretched, with a stiffness k directly proportional to its cross-section area and inversely proportional to its length.
Consider a simple helical spring that has one end attached to some fixed object, while the free end is being pulled by a force whose magnitude is F s. Suppose that the spring has reached a state of equilibrium, where its length is not changing anymore. Let x be the amount by which the free end of the spring was displaced from its "relaxed" position.
Moreover, the same formula holds when the spring is compressed, with F s and x both negative in that case. According to this formula, the graph of the applied force F s as a function of the displacement x will be a straight line passing through the origin, whose slope is k.
Hooke's law for a spring is stated under the convention that F s is the restoring force exerted by the spring on whatever is pulling its free end. Hooke's spring law applies to any elastic object, of arbitrary complexity, as long as both the deformation and the stress can be expressed by a single number that can be both positive and negative. For example, when a block of rubber attached to two parallel plates is deformed by shearing, rather than stretching or compression, the shearing force F s and the sideways displacement of the plates x obey Hooke's law.
Hooke's law applies when a straight steel bar or concrete beam, supported at both ends, is bent by a weight F placed at some intermediate point. The displacement x in this case is the deviation of the beam, measured in the transversal direction, relative to its unloaded shape. The law applies when a stretched steel wire is twisted by pulling on a lever attached to one end.
In this case the stress F s can be taken as the force applied to the lever, x as the distance traveled by it along its circular path. Or, one can let F s be the torque applied by the lever to the end of the wire, x be the angle by which that end turns. In either case F s is proportional to x In the case of a helical spring, stretched or compressed along its axis, the applied force and the resulting elongation or compression have the same direction which is the directi.
Plasma physics Plasma is one of the four fundamental states of matter , was first described by chemist Irving Langmuir in the s. Plasma can be artificially generated by heating or subjecting a neutral gas to a strong electromagnetic field to the point where an ionized gaseous substance becomes electrically conductive, long-range electromagnetic fields dominate the behaviour of the matter.
Plasma and ionized gases have properties and display behaviours unlike those of the other states, the transition between them is a matter of nomenclature and subject to interpretation. Based on the surrounding environmental temperature and density ionized or ionized forms of plasma may be produced. Neon signs and lightning are examples of ionized plasma; the Earth's ionosphere is a plasma and the magnetosphere contains plasma in the Earth's surrounding space environment. The interior of the Sun is an example of ionized plasma, along with the solar corona and stars.
Positive charges in ions are achieved by stripping away electrons orbiting the atomic nuclei, where the total number of electrons removed is related to either increasing temperature or the local density of other ionized matter. This can be accompanied by the dissociation of molecular bonds, though this process is distinctly different from chemical processes of ion interactions in liquids or the behaviour of shared ions in metals.
The response of plasma to electromagnetic fields is used in many modern technological devices, such as plasma televisions or plasma etching. Plasma may be the most abundant form of ordinary matter in the universe, although this hypothesis is tentative based on the existence and unknown properties of dark matter. Each of these nuclei are suspended in a movable sea of electrons.
Plasma was first identified in a Crookes tube , so described by Sir William Crookes in ; the nature of this " cathode ray" matter was subsequently identified by British physicist Sir J. Thomson in ; the term "plasma" was coined by Irving Langmuir in Lewi Tonks and Harold Mott-Smith, both of whom worked with Irving Langmuir in the s, recall that Langmuir first used the word "plasma" in analogy with blood.
Mott-Smith recalls, in particular, that the transport of electrons from thermionic filaments reminded Langmuir of "the way blood plasma carries red and white corpuscles and germs. We shall use the name plasma to describe this region containing balanced charges of ions and electrons. The plasma state can be contrasted with the other states: solid and gas. Plasma is an electrically neutral medium of unbound negative particles. Although these particles are unbound, they are not "free" in the sense of not experiencing forces.
Moving charged particles generate an electric current within a magnetic field, any movement of a charged plasma particle affects and is affected by the fields created by the other charges. In turn this governs collective behaviour with many degrees of variation. Bulk interactions: The Debye screening length is short compared to the physical size of the plasma; this criterion means that interactions in the bulk of the plasma are more important than those at its edges, where boundary effects may take place. When this criterion is satisfied, the plasma is quasineutral. Plasma frequency: The electron plasma frequency is large compared to the electron-neutral collision frequency.
When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics. Plasma temperature is measured in kelvin or electronvolts and is, informally, a measure of the thermal kinetic energy per particle. High temperatures are needed to sustain ionisation , a defining feature of a plasma; the degree of plasma ionisation is determined by the electron temperature relative to the ionization energy, in a relationship called the Saha equation.
At low temperatures and electrons tend to recombine into bound states—atoms—and the plasma will become a gas. In most cases the electrons are close enough to thermal equilibrium that their temperature is well-defined; because of the large difference in ma. Fracture mechanics Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material's resistance to fracture.
In modern materials science, fracture mechanics is an important tool used to improve the performance of mechanical components, it applies the physics of stress and strain behavior of materials, in particular the theories of elasticity and plasticity, to the microscopic crystallographic defects found in real materials in order to predict the macroscopic mechanical behavior of those bodies. Fractography is used with fracture mechanics to understand the causes of failures and verify the theoretical failure predictions with real life failures; the prediction of crack growth is at the heart of the damage tolerance mechanical design discipline.
The processes of material manufacture, processing and forming may introduce flaws in a finished mechanical component. Arising from the manufacturing process and surface flaws are found in all metal structures. Not all such flaws are unstable under service conditions. Fracture mechanics is the analysis of flaws to discover those that are safe and those that are liable to propagate as cracks and so cause failure of the flawed structure.
Despite these inherent flaws, it is possible to achieve through damage tolerance analysis the safe operation of a structure. Fracture mechanics as a subject for critical study has been around for a century and thus is new. Fracture mechanics should attempt to provide quantitative answers to the following questions: What is the strength of the component as a function of crack size?
What crack size can be tolerated under service loading, i. How long does it take for a crack to grow from a certain initial size, for example the minimum detectable crack size, to the maximum permissible crack size? What is the service life of a structure when a certain pre-existing flaw size is assumed to exist?
During the period available for crack detection how should the structure be inspected for cracks? Griffith — thus the term Griffith crack — to explain the failure of brittle materials. Griffith's work was motivated by two contradictory facts: The stress needed to fracture bulk glass is around MPa; the theoretical stress needed for breaking atomic bonds of glass is 10, MPa. A theory was needed to reconcile these conflicting observations. Experiments on glass fibers that Griffith himself conducted suggested that the fracture stress increases as the fiber diameter decreases.
Hence the uniaxial tensile strength, used extensively to predict material failure before Griffith, could not be a specimen-independent material property. Griffith suggested that the low fracture strength observed in experiments, as well as the size-dependence of strength, was due to the presence of microscopic flaws in the bulk material.
To verify the flaw hypothesis, Griffith introduced an artificial flaw in his experimental glass specimens. Linear elasticity theory predicts that stress at the tip of a sharp flaw in a linear elastic material is infinite. To avoid that problem, Griffith developed a thermodynamic approach to explain the relation that he observed; the growth of a crack, the extension of the surfaces on either side of the crack, requires an increase in the surface energy.
Griffith found an expression for the constant C in terms of the surface energy of the crack by solving the elasticity problem of a finite crack in an elastic plate; the approach was: Compute the potential energy stored in a perfect specimen under a uniaxial tensile load. Fix the boundary so that the applied load does no work and introduce a crack into the specimen. The crack hence reduces the elastic energy near the crack faces. On the other hand, the crack increases the total surface energy of the specimen.
Compute the change in the free energy as a function of the crack length. Failure occurs when the free energy attains a peak value at a critical crack length, beyond which the free energy decreases as the crack length increases, i. Griffith's criterion.
From Wikipedia, the free encyclopedia. The deformation of a solid material undergoing non-reversible changes of shape in response to applied forces. For the material used in manufacturing, see Plastic. See also: Deformation mechanics and Deformation engineering. True elastic limit Proportionality limit Elastic limit Offset yield strength. Solid mechanics. Fluid mechanics. Surface tension Capillary action. Main article: critical state soil mechanics.
Main article: rock mass plasticity. Main article: Flow plasticity theory. Main article: Yield engineering. Main article: Von Mises yield criterion. Plasticity theory. Cambridge University Press. Inelastic analysis of structures. John Wiley and Sons. Limit Analysis and Soil Plasticity. Ross Publishing. Generalized Plasticity. Plasticity in Reinforced Concrete. Skeletal Injury in the Child. Aging skin: Properties and Functional Changes. Marcel Dekker. Urbassek: Reversible Plasticity in fcc metals. In: Philosophical Magazine Letters. January Acta Materialia. AIP Conf. AIP Conference Proceedings.
The Mathematical Theory of Plasticity. Oxford University Press. Mathematisch-Physikalische Klasse. Czasopismo Techniczne. Archives of Mechanics. History of Strength of Materials. New York: McGraw-Hill. Categories : Plasticity physics Solid mechanics Deformation mechanics. Revision History.
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Related Images. YouTube Videos. Household items made of various types of plastic. Residual stress es inside a plastic protractor are revealed by the polarized light. Roman -era bridge in Switzerland. Inca bridge on the Apurimac River. Cupola of the Pantheon , Rome. In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time.
Gottfried Leibniz. Gaspard-Gustave Coriolis. James Prescott Joule. Contact mechanics is the study of the deformation of solids that touch each other at one or more points. Stresses in a contact area loaded simultaneously with a normal and a tangential force. Stresses were made visible using photoelasticity. Read More on This Topic. Learn More in these related Britannica articles:. A stress is expressed as a quotient of a force divided by an area.
The deformation, expressed by strain, arises throughout the material as the particles molecules, atoms, ions of which the material is composed are slightly displaced from their normal position. History at your fingertips. Sign up here to see what happened On This Day , every day in your inbox!