When a force is applied parallel to the top surface of a solid, it resists the applied force due to the internal frictional forces between the layers of the solid known as shearing resistance. However, when the force is removed, the shearing forces restore the original shape of the solid. Other deformation forces also cause temporary changes in shape if the forces are not beyond a threshold magnitude. Solids tend to retain their shape, making the study of their rest and motion easier. Beyond the threshold magnitude of the force, the solids undergo permanent changes.
Unlike solids, liquids and gases do not resist external forces in the same way. Due to the external forces, the liquids and gases flow. As a result, liquids and gases are collectively called fluids. The study of fluids is called fluid mechanics, which is categorized into fluid statics and fluid dynamics—the study of fluids at rest and motion, respectively.
The essential difference between solids and fluids is their molecular structure. The molecules in a solid are tightly held together by intermolecular forces, allowing solids to retain their original shape when an external force is withdrawn. The intermolecular forces are much weaker in fluids compared to solids. Fluid molecules easily slide over each other or get completely separated and they do not retain their shape. Fluids take the shape of the container in which they are put. Furthermore, gases are highly compressible—their density changes significantly due to the change in volume.
