Question

What is the difference between a solid and fluid?

Answer 1

Hint: In order to solve this question, we are going to first define the solid and fluids and then, step by step the difference in the properties shown by the solids and the fluids is discussed by giving the examples for the solids and the fluids also.

Complete step-by-step answer:
The Solid is defined as one of the four fundamental states of matter in which the molecules are closely packed together and contain the least amount of kinetic energy while a fluid is defined as a substance that deforms continuously when acted on by a shearing stress of any magnitude.
A solid is characterized by structural rigidity and resistance to a force applied to the surface while the fluids are not rigid at all and are deformed easily.
When common solids such as steel or other metals are acted on by a shearing stress, they will initially deform (usually a very small deformation), but they will not continuously deform (flows). However, common fluids such as water, oil and air satisfy the definition of a fluid-that is, they will flow when acted on by a shearing stress.
When we describe the behavior of the fluids, then it is not possible to classify and note the behavior of each and every molecule however, the geometry, structure and properties of a solid are known by their constituent molecules only.

Note: Some materials, such as slurries, tar, putty, toothpaste, and so on, are not easily classified since they will behave as a solid if the applied shearing stress is small, but if the stress exceeds some critical value, the substance will flow. The study of such materials is called Rheology and does not fall within the province of classical fluid mechanics.