8.1. INTRODUCTION

In Chapter 6, we studied the rotation of the bodies and then realised that the motion of a body depends on how mass is distributed within the body. We restricted ourselves to simpler situations of rigid bodies. A rigid body generally means a hard solid object having a definite shape and size.

In this chunk, we revisit our understanding of rigid bodies, which are defined as objects that maintain their shape and size. When studying the mechanics of these bodies, we find that the distribution of mass within the body significantly influences how they move. In earlier discussions, we simplified our analysis by focusing only on these rigid bodies.

Think of a solid cube of wood. When you push it, it doesn't change its shape, which is why we consider it a rigid body. However, if you tried to push a sponge, its shape would change because it is not rigid.

But in reality, bodies can be stretched, compressed and bent. Even the appreciably rigid steel bar can be deformed when a sufficiently large external force is applied on it.

This chunk discusses the fact that, unlike ideal rigid bodies, real-world materials can undergo deformation. For example, when a hefty weight is placed on a steel bar, it may bend or stretch, which implies that no material is perfectly rigid. The extent of deformation depends on the type of material and the magnitude of the applied force.

Imagine a metal ruler. If you gently bend it, it may remain slightly bent, but if you apply too much force, it can become permanently deformed, similar to how a rubber band can stretch but eventually return to its original form if not overstretched.

A solid has definite shape and size. In order to change (or deform) the shape or size of a body, a force is required. If you stretch a helical spring by gently pulling its ends, the length of the spring increases slightly.

Here, the text emphasizes the characteristics of solids, specifically their definite shape and size, and how any alteration to these characteristics necessitates the application of force. The example of a helical spring illustrates elastic deformation, where the material stretches under force and returns to its original shape once the force is removed, showcasing the concept of elasticity.

Consider a painted spring in a toy. When you pull on it gently, it stretches and then comes back to its original shape once you let it go, just like how we can define back the original shape of a ball made of rubber when we compress it and then release it.

The property of a body, by virtue of which it tends to regain its original size and shape when the applied force is removed, is known as elasticity and the deformation caused is known as elastic deformation.

This chunk defines elasticity, which is the capability of a material to return to its original shape after deformation once the applied stress is removed. Elastic deformation is temporary, and when the force is removed, the object reverts to its initial form. In contrast, materials like putty or mud do not return to their original shape, exhibiting plastic behavior instead.

Picture a balloon. When you inflate it, the balloon stretches, but when you let the air out, it goes back to its original shape. On the other hand, if you take some playdough and mold it, it will stay in the new shape because it is plastic.

The elastic behaviour of materials plays an important role in engineering design. For example, while designing a building, knowledge of elastic properties of materials like steel, concrete etc. is essential.

This chunk highlights the significance of understanding elastic properties in engineering and design. Engineers must consider how materials behave under stress to ensure structures can withstand forces such as weight, wind, and other stresses without failing or deforming excessively.

Think of constructing a bridge. If engineers do not consider how the materials will behave, the bridge could bend under the weight of traffic or crumble in strong winds, which could lead to catastrophic failure.

--