A body is said to be rigid if the distance between any two points on the body will remain constant. In reality, it is impossible to have a perfectly rigid body. Every object gets deformed under the action of external force, some to a lesser extent and some to a larger extent. When the external force causing deformation is removed, there is usually some tendency for the body to recover its original shape. The property of the body by virtue of which it tends to regain its original shape or size after the removal of deforming force is called elasticity.
Some of the body will completely regain its original shape and size by removing the deforming force. Such bodies are called perfectly elastic. On the other hand, some of the bodies will not regain its modified form. Such bodies are called perfectly inelastic or perfectly plastic. In practice, no body is perfectly elastic or perfectly plastic. However, quartz is regarded as almost perfectly elastic and wet clay is regarded as almost perfectly plastic. The elastic bodies show the property of elasticity only up to a certain value of deforming force. If the deforming force is increased gradually a stage is reached when the body does not regain its initial state on removing the applied force. The maximum deforming force up to which a body retains its property of elasticity is known as the limit of elasticity of the material of the body.
Elastic Stress and Strain
When the body is deformed by the application of the external force, the forces are brought into the play within the body tending to counteract the external forces. The external and the internal forces are equal in magnitude but opposite in direction. If the external force F is applied uniformly over a surface of area A then the stress is defined as the force per unit area.
Stress = F/A. The SI unit of stress is N/m2.
The fractional change in the dimension of the body produced by the external force acting on it is known as the strain.
Hooke’s law is the fundamental law of elasticity. It was enunciated by Robert Hooke. According to Hooke’s law, stress is directly proportional to strain, provided strain is small. From Hooke’s law, it follows that if the strain is small the ratio between the stress and strain is constant. The constant E is called the modulus of elasticity.
The purpose of the vernier caliper is to measure the external/internal dimensions of the objects and also to measure the length of the object accurately. The measurement taken using a vernier caliper is more accurate than a ruler. The vernier calliper is mainly of two types: manual vernier calliper that has a main scale and a vernier scale and the other one is the digital vernier calliper which has an LCD display. The manual vernier calliper is still much preferred by the people. The least count of vernier callipers is also known as the vernier constant. It is defined as the difference between one main scale division and one vernier scale division.