NCERT Book Class 11 Physics Chapter 9 Mechanical Properties of Fluids

CHAPTER NINE

MECHANICAL PROPERTIES OF FLUIDS

9.1 INTRODUCTION
In this chapter, we shall study some common physical
properties of liquids and gases. Liquids and gases can flow
9.1 Introduction and are therefore, called fluids. It is this property that
9.2 Pressure distinguishes liquids and gases from solids in a basic way.
9.3 Streamline flow Fluids are everywhere around us. Earth has an envelop of
9.4 Bernoulli’s principle air and two-thirds of its surface is covered with water. Water
9.5 Viscosity is not only necessary for our existence; every mammalian
9.6 Surface tension
body constitutes mostly of water. All the processes occurring
in living beings including plants are mediated by fluids. Thus
Summary understanding the behaviour and properties of fluids is
Points to ponder important.
Exercises How are fluids different from solids? What is common in
Additional exercises liquids and gases? Unlike a solid, a fluid has no definite
Appendix shape of its own. Solids and liquids have a fixed volume,
whereas a gas fills the entire volume of its container. We
have learnt in the previous chapter that the volume of solids
can be changed by stress. The volume of solid, liquid or gas
depends on the stress or pressure acting on it. When we
talk about fixed volume of solid or liquid, we mean its volume
under atmospheric pressure. The difference between gases
and solids or liquids is that for solids or liquids the change
in volume due to change of external pressure is rather small.
In other words solids and liquids have much lower
compressibility as compared to gases.
Shear stress can change the shape of a solid keeping its
volume fixed. The key property of fluids is that they offer
very little resistance to shear stress; their shape changes by
application of very small shear stress. The shearing stress
of fluids is about million times smaller than that of solids.

9.2 PRESSURE
A sharp needle when pressed against our skin pierces it. Our
skin, however, remains intact when a blunt object with a
wider contact area (say the back of a spoon) is pressed against
it with the same force. If an elephant were to step on a man’s
chest, his ribs would crack. A circus performer across whose