# Elastic collision

There are separate articles about elasticity in economics, and about British rubber bands.

In solid mechanics, the adjective elastic characterises both collisions between, and deformations of, physical objects.

A collision is perfectly elastic if the total kinetic energy of the colliding objects is conserved as kinetic energy (overall energy is conserved in all collisions, whether elastic or not). No macroscopic collisions are truly elastic, although some come close. A frequent cause of highly inelastic collisions occurs when one or more of the objects suffers a plastic deformation.

The deformation of a solid is part of the study of solid mechanics. In solid mechanics, a material is elastic if it changes shape due to an applied load, but that when the load is removed, recovers its original shape. Further, the deformation of an elastic material is linearly related to the applied load.

The elasticity of a solid is inversely proportional to the strength of the material. If the material is isotropic, its strength is characterized by Young's modulus. Furthermore, if the material is isotropic, and a force is applied in one direction, the deformation in the other directions is governed by Poisson's ratio.

The word elastomer refers to a material which is very elastic, like rubber. The word elastic is often used colloquially to refer to an elastomeric material such as rubber or cloth/rubber combinations.

## Applications of elastomeric materials

• holding things together, e.g. with a rubber band
• moving something over a thick to a thin part, and/or to keep it in place, e.g. clothing put on over the head, hips, hands, feet, and a necklace, bracelet, watch, a condom. An alternative in many of these cases (not for socks or a condom) is a closing device such as buttons, a zipper, laces, Velcro, a buckle, a snap
• a hose
• airtight or watertight connections
• cushioning (a shock absorber) as with tires and for the soles of shoes, and under the legs of chairs, under vibrating equipment, on a door or door-post, on bumper cars, etc.
• for balloons, as decoration or entertainment, and for use in medicine
• in bungee jumping, for reducing downward speed gradually, and then pulling one up again
• slingshot, bow (weapon)

For some applications a spring is an alternative.

In some of the aplications the flexibility is needed but not the elastic force. In that case, when a large flexibility is needed for a large surface, a folding wall is an alternative connections between railroad cars and in articulated buses, and in an accordion.

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