Lesson 1 of 9 · 6 min
Elasticity and plasticity
NCERT §8.1
Meera spends her holidays at the flyover site where her uncle Ravi is the site engineer. A steel girder hangs from the crane; next to it a labourer presses a lump of wet mortar into a gap. One of these will spring back if you push it, the other will not.
The story this chapter follows: Meera at the flyover site
The lesson in notes
In short
A rigid body is an idealisation. Every real solid, even a thick steel bar, changes length, shape or volume when a large enough force acts on it.
Elasticity: the property by which a body tries to get back its original size and shape once the deforming force is taken away. The change it undergoes meanwhile is an elastic deformation, as in a gently stretched helical spring.
Plasticity: the property of bodies such as putty or mud, which show no real tendency to recover and stay deformed for good. Putty and mud come close to ideal plastic bodies.
Engineers need the elastic behaviour of steel, concrete and other materials to design buildings, bridges, automobiles and ropeways, and to make aircraft or artificial limbs that are light yet strong.
Questions such as why a rail has an I-shaped section, or why glass is brittle while brass is not, start from how simple loads deform solids.