Question

What do you understand about perfectly elastic, plastic, and rigid bodies? Discuss their limits.

Answer 1

In order to solve this question we need to understand young modulus. So young modulus is defined as the ratio of stress and strain. Stress is defined as magnitude of deforming force per unit area where strain is defined as the change in length per unit length of body. Actually each material is made up of atoms so when we apply external force against the body then the deformation produced in the body but when we apply external force, the body opposes this behavior and hence after deformation it tries to regain its shape and size.

Complete answer:
Perfectly elastic body is a body whose after deformation returns to its original shape and size, or body perfectly opposes the deforming force. But no material could be perfectly elastic; however some materials tend to be, like a spongy ball which regains its shape after deformation.

Perfectly plastic bodies are those bodies whose after deformation could not regain their original shape and size and the material left in that state where after deformation it was. So no material could be perfectly plastic. Rigid bodies are those bodies whose atoms are closely packed with each other leaving very less interspace between them, so when deforming force acts they strongly oppose this behavior and do not deform at all.

Limits: Perfectly elastic body there is no deformation so strain is zero which in turn makes young modulus infinite and perfectly plastic body has zero young modulus while perfectly rigid body strain is zero so young modulus is infinite.

Note: It should be remembered that Bulk modulus is defined as the material ability to withstand the deformation force from all sides of material. It is mathematically defined as the ratio of applied pressure to the relative deformation. Bulk modulus for a perfectly elastic body is infinite as it is not compressible.