NEET Physics Notes Mechanics-Properties of Matter-Elastic Behaviour
Elasticity is the property of body by virtue of which a body regains or tends to regain its original configuration (shape as well as size), when the external deforming forces acting on it, is removed.
The internal restoring force per unit area of cross-section of the deformed body is called stress. Thus
SI unit of stress is Nm-2 or pascal (Pa).
Stress are of two types normal stress or longitudinal stress and tangential stress or shearing stress.
Also,the longitudinal stress can be of two types.
- If a solid in the form of a wire/rod is stretched by applying a force normally, then the stress is called tensile stress (first type of longitudinal stress).
- If the solid wire/rod is compressed, then the stress is called compressive stress (other type of longitudinal stress).
- For a bulk solid or liquid or gas if the deforming force is applied normal to the surface such that magnitude of force on any small area is proportional to the area, the stress is called volumetric stress.
Strain is the ratio of change in configuration to the original configuration of the body. Being the ratio of two similar quantities, strain is a unitless and dimensionless quantity,
- For a wire or rod, longitudinal strain is defined as the ratio of change in length to the original length.
- When the deforming force causes a change in volume, the strain is called volumetric strain.
- When the deforming force, applied tangentially to a surface, produces a change in shape of the body, the strain developed is called shearing strain or shear.
Whenever a material is stressed, there are basically two different regimes, elastic and the non-elastic. The latter one is difficult to describe while in the former one stress is directly proportional to the strain.
If large deformation takes place between points b and c material is ductile and for small deformation between c and d material is said to brittle.