1. Deforming force

A force applied on body which produces a change in
shape or size of body (length, volume) is called
deforming force

2. Perfectly elastic body

A body which regains its original configuration (size or
shape) immediately and completely after the removal
of deforming force is called a perfectly elastic body


Example Quartz and phosphor bronze are nearly
perfectly elastic body.


3. Perfectly plastic body

A body which does not regain its original configuration
(size and shape) at all after the removal of deforming
force is called a perfectly plastic body.

Example Putty , lump of gum, mud, paraffin wax are
nearly perfectly plastic body.


NOTE :


1. No material which in practice behaves as perfectly
elastic or perfectly plastic.

2. Only the degree of elasticity or plasticity differs from
body to body.

Atoms and molecules of matter are held together by
interatomic or intermolecular forces.

They are electrical in nature and hence can be
attractive or repulsive.

The mutual potential energy (U) corresponding to this
force between two molecules or atoms as a function of
their separation(r) is shown.

The corresponding force between two molecules

` F = - (dU)/(dr)`

The potential energy is a minimum at the equilibrium
separation `r_0` , at which point the force is zero.
As the separation `(r)` decreases, the attractive force
first increases and then decreases to zero at `r = r_0` .
For smaller distances force is repulsive.