Introduction

In XIth Std we studied how a solid behaves under the action of a force — it nearly keeps its own shape and volume no matter how large a force is applied to it. Liquids and gases behave quite differently: neither has a shape of its own, and both simply take up the shape of whatever container holds them. This happens because a liquid or a gas, unlike a solid, flows under the action of an external force rather than resisting and holding its shape. A fluid is exactly this: any substance that can flow. Liquids and gases, taken together, are therefore both called fluids, and what makes a substance a fluid is that it has no rigidity at all, or at most a very low rigidity.

We experience the effects of fluids constantly in daily life — the pressure a fluid at rest exerts on whatever contains it, and the very different pressure and behaviour of a fluid once it is set in motion. Two further properties, viscosity (a fluid's internal friction) and surface tension (a special property of a liquid's free surface), also play an important role in how fluids behave in nature. This chapter develops all of these properties — pressure, surface tension, and viscosity/flow — building on the mechanical-properties framework already studied for solids.