Just about every solid material possesses some degree of elasticity, as do most liquids. The key to determining whether a substance is elastic is to apply a force to it; with sufficient force, the substance should change its size, shape, or volume. If, when the force is removed, the sample returns to its original state, then it is elastic; if the substance returns only partially (or not at all) to its original state, it is called inelastic. If too much force is applied, the material is in danger of reaching its elastic limit --that is, the point at which the material is bent beyond its ability to return to its original shape. Once the elastic limit is passed, the material will experience permanent reshaping, called plastic deformation, and will no longer act as an elastic substance. This is easily evidenced by hanging a weight from a spring: if the weight is within the spring's capacity, the spring will bounce back (in an elastic manner); however, if the weight is too heavy for the spring, the weight will pull the spring straight, making it inelastic.
Elasticity works because of two fundamental molecular forces known as the attracting and repelling forces. When at rest, these forces within the molecules balance each other. By adding a compressing force (say, by squeezing a spring), the repelling force increases in an attempt to once again balance the system. Similarly, by adding a stretching force (as in a weight pulling a spring) the attracting force increases, causing the elastic material to bounce back.
The first scientist to conduct in-depth research into the behavior of elastic materials was the eminent English physicist Robert Hooke. Through experimentation Hooke discovered that the relationship between tension (the force applied) and extension (the amount of bending that is produced) is directly proportional; for example, a weight will stretch a spring, while a weight twice as heavy will stretch it twice as much. Hooke's research has since been combined into a series of mathematical principles known as Hooke's law.
More than one hundred years after Hooke's studies, another English scientist, Thomas Young, discovered that different elastic materials bend to different degrees when a force is applied; for example, brass bends more than lead but less than rubber. The characteristic elasticity of a particular material, Young found, can be expressed as a constant called Young's modulus. Knowledge of Young's modulus is essential to modern architects who must anticipate the behavior of construction material stress.
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