An elastic material pushes you back, when you try to squish it by applying force. These materials return to their actual shape when you stop applying force on it. If the deformation of the material and the force you apply on it follow a linear relationship, such materials are called linear elastic materials. So, a force of two pounds can cause a deformation equivalent to double the deformation caused by a force of one pound.

## Elastic Limit

Most of solids -- like plastics and metals -- are elastic in nature. These materials go through some deformation, based on the force applied to the material. But, there is a limit to the deformation -- up to which the material can retain its original size and shape -- after the force is removed. This limit is the "elastic limit" of the material. The elastic limit is denoted by the maximum force that can be applied per unit area before complete deformation. Applying a force greater than the elastic limit of the material would cause the material to bend permanently or to crack.

## Elastic Collision

A collision that happens between two bodies is an elastic collision -- or a completely elastic collision -- if it satisfies two conditions.

The first condition is that the total kinetic energy that the bodies possess after collision should be equal to the total kinetic energy that they have before collision.

The second condition is that the difference between the velocities of the bodies after collision should be equal in value -- and of opposite sign -- to the difference in velocities before collision. The difference in velocities is called "coefficient of restitution."

## Hooke’s Law and Elasticity

Hooke’s law states that the amount of elongation of a spring is proportional to the force that is acting on it. These materials are called elastic materials. The characteristic of the elastic materials is such that they go back to their true shape or actual shape after the removal of the force acting on them. So, every elastic material would require the application of some force to stretch it. The force required to stretch an elastic material depends on the stiffness of the material; termed the "elastic modulus."

## Elastic Bands in Fitness

Elastic resistance training has been recently introduced into aerobic classes. These instruments take after the springs that were used earlier and they help to improve muscular strength. The introduction of elastic bands into aerobics is based on the principles of Hooke’s law. The more you stretch the elastic band, the greater the resulting tension. So, you can find out the level of resistance by determining how far you stretch the band.