Elasticity Class 11 Notes
· Elasticity
Elasticity is the property of a material
by virtue of which it regains its original position after removing deforming forces.
The objects which obey the property of elasticity are known as elastic objects.
Elastic objects are suitable to use in the construction of buildings and bridges. Nobody in nature is perfectly elastic.
·Plasticity
Plasticity is the property of a material by virtue
of which it does not regain its original position after removing deforming forces.
The objects which obey plastic behavior are known as plastic objects. They are
not suitable to use in the construction of buildings and bridges. Nobody in nature
is also perfectly plastic.
· Stress
The restoring force per unit area of the cross-section of the body is called stress.
Stress = deforming force / cross-section
area
It’s is a scalar quantity and the unit is Nm-2
in the SI system.
Types of
Stress :
1)
Normal Stress
when the external deforming forces
are applied perpendicular to the surface of a body, the restoring force developed per unit area of
that body is called normal stress.
2) Tangential | shearing Stress
when the external
deforming forces act tangentially to the surface of the body, the restoring force
developed per unit area of the body is called tangential stress
3) Bulk | volume Stress
when the external deforming
forces are applied normally through the entire surface of the body such that there
is a change in volume, then the restoring force developed per unit area of the
body is called bulk stress.
· Strain
Strain is defined as the ratio of change
in the configuration of a body to its original configuration.
Strain = change in configuration /
original configuration
Types of Strain
1) Longitudinal Strain
It is defined as the ratio of change
in length of a body to its original length.
2) Volumetric Strain
It is defined as the ratio of
change in volume of a body to its original volume.
3) Shear Strain
It is defined as the angle by which
the face originally perpendicular to the fixed face has been turned. It is expressed
in terms of angle.
Elastic limit:
The maximum or
upper limit of deforming force up to which when the force is applied, the body
regains its original configuration is known as an elastic limit. Beyond the upper
limit, if deforming force is applied then the body losses the property of
elastic permanently and never retains back.
'Hooke's law:
It states that, within the elastic limit, the deforming
force is directly proportioned to the extension produced i.e.
F∝e ------
(i) →→ where F is the deforming force and e is the extension
produced.
Hooke's law can be verified experimentally by using vernier
apparatus as follows:
Now, two wires A and B are taken in which wire A is
connected to the main scale and wire B is connected to the venire scale. Wire A is the reference wire whereas wire B is the experimental wire. Initially, equal
weights are kept on scale pans S1 and S so that both wires
A and B become from kinks and then become ready for the experiment. In this
condition, the main scale reading and vernier scale reading l0 are
found.
Now, a load of 0.5 kg is added on scale pan S1 and
it is left for two minutes. After that main scale, reading, and venire scale
reading are noted. The difference between this reading and the previous reading gives
the extension produced. This process is repeated by taking other weights of
1kg, 1.5kgs, 2 kgs....... and then the corresponding extension is noted.
