Have you seen a stretched slingshot? You surely must have played with it, haven’t you? What happens when you release it? This is an important concept of elasticity and the Elastic behaviour of substances. It finds various applications in our day to day lives. Let us look at this concept in a greater detail.

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## Elastic Behaviour of Materials

A slingshot deforms when you stretch it. However, it regains its original shape when you stop applying a force. But let us say that you take a thin steel rod and try to bend. You manage to bend it a little and then stop applying force. Does the rod regain its original shape? No, it doesn’t. This difference in the behaviour of the material is based on their elastic and plastic nature.

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### Why does this happen?

The rubber strip of the slingshot has high elasticity. Elasticity is the ability of a body to resist any permanent change to it when stress is applied. When stress application ceases, the body regains its original shape and size.

Different materials show different elastic behaviour. The study of the elastic behaviour of a material is of much importance. Almost every engineering design requires knowledge of the elastic behaviour of materials.

### Applications of this Concept

In the construction of various structures like bridges, columns, pillars, beams, etc. Knowledge of the strength of the materials used in the construction is of prime importance.

For example: While constructing a bridge, a load of traffic that it can withstand should be adequately measured beforehand. Or while constructing a crane used to lift loads, it is kept in mind that the extension of the rope does not exceed the elastic limit of rope. To overcome the problem of bending under force the elastic behaviour of the material used must be considered primarily.

To study the elastic behaviour of materials let us consider a beam of length l, breadth b and depth d supported at the ends and loaded at the centre by load W.

In this case, it is given as; δ=Wl³/4bd³Y, where δ is the sag or the measure of bending, Y is Young’s modulus of elasticity. Study of beams is very useful in civil engineering and other such avenues. Using the above equation we can easily say that to reduce the amount of bending for a certain load, Young’s modulus of elasticity of the material used must be large.

Also, the depth d must be considered since sag is inversely proportional to the cube of depth. But the problem faced on increasing the depth is that bending increases and this is known as buckling. Therefore, a compromise is made between the different cross-sectional shapes.

## Solved Example For You

Q: Why is steel used in the construction of bridges?

Answer: Amongst bridge materials steel has the highest and most favourable strength qualities, and it is, therefore, suitable for the most daring bridges with the longest spans. Normal building steel has compressive and tensile strengths of 370 N/sq mm, about ten times the compressive strength of a medium concrete and a hundred times its tensile strength. A special merit of steel is its ductility due to which it deforms considerably before it breaks because it begins to yield above a certain stress level.