Introduction

One of the oldest surviving dams in the world is the Kallanai, built across the river Kaveri near Tiruchirappalli to hold back and direct floodwater for irrigation. Even when the Kaveri runs at its highest flood velocity, the dam has stayed stable for centuries -- a stability that reflects a genuinely sound, intuitive grasp of material behaviour by the Tamil engineers who built it as far back as the 2nd century AD. The pyramids of Egypt show the same instinct at work elsewhere in the ancient world: structures that have survived millennia because their builders understood, long before any formal theory of matter existed, how to make a structure withstand load. The same challenge is everywhere in modern engineering too -- every flyover and bridge today carries a constant stream of heavy vehicles and is therefore permanently under stress, and without choosing the right material and the right design for that stress, such structures simply would not stay stable. In a real sense, the growth of human civilization has depended, at every stage, on a progressively better understanding of the different forms matter can take -- solid, liquid and gas.

That same understanding matters just as much today when a material has to be picked for a very specific job: a spacecraft component must be light yet strong; an artificial hip or heart valve must be biocompatible with the human body; an artificial body fluid used as a tissue substitute in radiotherapy testing must mimic the mechanical response of real tissue; a lubricant or a fuel must flow with just the right amount of resistance. Every one of these macroscopic, everyday properties is ultimately decided by what is happening at the microscopic -- atomic and molecular -- level inside the material. This unit is built around that idea: it develops the laws that govern the mechanical behaviour of solids -- through the study of elasticity, stress, strain and the elastic moduli -- and the mechanical behaviour of fluids -- through pressure, buoyancy, viscosity, surface tension and the flow of liquids culminating in Bernoulli's theorem. Studying these laws is what lets an engineer or a scientist predict, rather than merely observe, how a chosen material will behave under load, in motion, or at a free surface, and hence choose the right material for the right job.