The fundamental law of elasticity was propounded by Robert Hooke in 1676. Later Thomas Young expressed this law in the following way:
Within the elastic limit, stress is directly proportional to strain.
Hence, stress ∝ strain
or stress / strain = constant
This constant is called the modulus of elasticity for the material of the body. Therefore, the stress developed for unit strain is defined as the modulus of elasticity. This modulus depends on the material of the body. the modulus of elasticity changes with temperature. Usually, its value decreases with an increase in temperature.