NCERT Book Class 11 Physics Chapter 8 Mechanical Properties of Solids

CHAPTER EIGHT

MECHANICAL PROPERTIES OF SOLIDS

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
8.1 Introduction distributed within the body. We restricted ourselves to simpler
8.2 Stress and strain situations of rigid bodies. A rigid body generally means a
8.3 Hooke’s law hard solid object having a definite shape and size. But in
8.4 Stress-strain curve reality, bodies can be stretched, compressed and bent. Even
8.5 Elastic moduli the appreciably rigid steel bar can be deformed when a
8.6 Applications of elastic sufficiently large external force is applied on it. This means
behaviour of materials that solid bodies are not perfectly rigid.
A solid has definite shape and size. In order to change (or
Summary
deform) the shape or size of a body, a force is required. If
Points to ponder
you stretch a helical spring by gently pulling its ends, the
Exercises
length of the spring increases slightly. When you leave the
ends of the spring, it regains its original size and shape. 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. However, if you apply force to a lump
of putty or mud, they have no gross tendency to regain their
previous shape, and they get permanently deformed. Such
substances are called plastic and this property is called
plasticity. Putty and mud are close to ideal plastics.
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. The same is true in the design of
bridges, automobiles, ropeways etc. One could also ask —
Can we design an aeroplane which is very light but
sufficiently strong? Can we design an artificial limb which
is lighter but stronger? Why does a railway track have a
particular shape like I? Why is glass brittle while brass is
not? Answers to such questions begin with the study of how
relatively simple kinds of loads or forces act to deform
different solids bodies. In this chapter, we shall study the