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In physics, elasticity is the ability of a solid to return to its initial shape after having an outside force applied to it and then removed. An object with a high level of elasticity is able to have its shape changed a great deal, while still being able to return to its original form. Solids with little or no elasticity either become permanently deformed or break when a force is applied to them. The term elasticity can also be used to describe the ability of processes or systems to stretch or be flexible.
Due to the molecular make up of solids, liquids, and gases, they all react differently to outside stresses. The molecules that make up a solid are very close together and are found in a precise arrangement. This means that there is little room for give when a force is applied to a solid. The molecules of liquids and gases are spread further apart, and move more freely than those of solids. When a force is applied to liquids and gases, they can either flow away from or around the force, or be compressed a great deal, unlike most solids.
There are three different classes of force, or stresses, that can affect solid objects. The first is tension, also called stretching, which occurs when equal but opposite forces are applied to either end of the object. Compression is the second type of stress, which occurs when an object is put under pressure, or the force pushing on the solid is at 90 degrees to its surface. Imagine crushing an empty paper towel roll between your hands with your hands at either end. The final type of stress is shear, which happens when the force is parallel to the surface of the object.
Initially, when any force is applied to a solid, it will resist and remain in its original shape. As the force is increased, the solid will not be able to keep up the resistance and will start to change shape, or become deformed. Just as different types of solids have different elastic properties, they can also withstand different levels of force before being affected. Eventually, if the force is strong enough, the deformed shape will become permanent or the solid will break.
It is the amount of force that is applied to an object, not duration, that will determine if it can return to its initial form. When the solid cannot return to its original shape, it is said to have passed its elastic limit. The elastic limit is the maximum amount of stress that can be endured by a solid that will allow it to return back to normal. This limit depends on the type of material being used. Rubber bands have high elasticity, for example, and thus a high elastic limit compared to a concrete brick, which is almost inelastic and has a very low elastic limit.
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