On the application of stress on an elastic body, the strain experienced by the body is directly proportional to the stress applied. Hooke’s law is the basic law in elasticity, it states that the extension produced in a wire directly proportional to the load attached to it. Thus, according to Hooke’s law:
Extension α Load.
This proportionality holds good up to certain limit called the elastic limit. Hooke’s law can be easily verified suspending a long metallic wire of uniform area of cross section from a rigid support and noting the extension on loading it. The extension can be measured accurately within a Vernier arrangement. As load is increased in steps, it is found the extension is always directly proportional to the load.
The English physicist Thomas Young pointed out that the load and the extension are more schematically described in terms of stress and strain respectively. Thus, Hooke’s law may be stated that stress is directly proportional to strain and according to modified form of Hooke’s law.
Hooke’s law states that for metals within elastic limit, stress applied to a body is directly proportional to the resulting strain.
Stress α Strain
Stress = Constant x Strain
[Stress/ Strain] = Constant.
Tis constant of proportionality is called Modulus of Elasticity or Coefficient of Elasticity of the material, its value depends upon the nature of the material of the body. There are three modulus of elasticity corresponding to the three types of the strain namely, Young’s Modulus (Y), Modulus of Rigidity (η) and Bulk Modulus (K).