Elastic Energy
A material is said to be elastic if it returns to its original shape after being deformed. Elastic energy is energy contained by an object as a result of deforming it from its relaxed position. A rubber band used to hold a stack of papers together and a trampoline are elastic. Deforming something requires application of a force. A person pulls on a rubber band to stretch it; a gymnast pushes down on a springy trampoline to deform it. The act of doing work to deform an elastic material produces elastic energy in the material. This elastic energy can then be used to do work as the deformed material returns to its original shape.
Springs made from coils of wire are used to store elastic energy which is a form of potential energy. The most common type of spring is cylindrically shaped, with coils evenly spaced and of the same diameter. A ballpoint pen uses a coil spring to hold the point in place for writing and to return it to the case for protection.
The energy stored in a spring depends on the strength of the spring and the deformation that may be either an extension or compression from its relaxed condition. For many springs, the deforming force is directly proportional to the deformation; doubling the force doubles the deformation. For example, if a 10 N force stretches a spring 0.01 m then a 20 N force will stretch it 0.02 m. Such a spring is said to obey Hooke's law, and the relation between force and deformation is written F = kx, where F is the deforming force in newtons (N), x is the deformation in meters (m), and k is the spring constant in newtons/meter (N/m). The stiffer the spring, the larger the spring constant. The elastic energy of a spring obeying Hooke's law is given by E = 1½kx2. The energy depends strongly on the deformation because doubling the deformation (x) quadruples the energy. In other words, a spring stretched 0.1 m will have four times the elastic energy as when it was stretched 0.05 m.
Some watches and clocks, not powered by batteries, use springs to store energy for their mechanical mechanism. Usually, the springs are made by winding wire in a flat spiral. Rather than storing energy by stretching or compressing, the spirals wound tighter. Rewinding a watch by turning a knob amounts to tightening the spirals of the spring inside the watch. Energy released when the spring unwinds is used to run the clock mechanism. The design is such that it takes only a few seconds to store the energy that is then released over twenty-four hours or more.
Elastic energy is not limited to springs. In fact, it is an important property that material scientists consider in selecting materials for products. A bent diving board, a drawn bow, and a planted vaulting pole are just a few of the many examples of elastic materials. It is not easy to calculate the elastic energy for these materials applications, but it is easy to see how energy is involved. A diver does work on the board by jumping on it, a hunter does work on the bow by pulling on the strings, and a vaulter does work on the pole by bending it. Elastic energy is stored as a result. The diver recovers the elastic energy of the board and rises upward when the board relaxes. The arrow recovers the elastic energy of the bow when the hunter releases the string. And the vaulter recovers the elastic energy of the pole and rises upward.
Potential Energy.
Bibliography
Hobson, A. (1995). Physics: Concepts and Connections. Upper Saddle River, NJ: Prentice-Hall.
Serway, R. A., and Faughn, J. S. (1995). College Physics. Philadelphia, PA: Saunders College Publishing.
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