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Why do solids resist forces that push or slide across their surfaces?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

Solids resist forces that push or slide across their surfaces—known as shear stress—because of their internal atomic structure. Unlike liquids, where atoms are mobile and continually wander, atoms in a solid are not mobile; they oscillate rapidly about fixed points in a durable, organized structure.12

This resistance to deformation is a defining characteristic of the solid state of matter. When a shearing force is applied, the material's internal structure resists the sliding of its internal planes. If the force is within the material's elastic limit, the solid will "spring back" to its original shape once the force is removed, a property described by the shear modulus.13

The strength of this resistance depends on the nature of the atomic bonding, such as the rigid covalent networks found in diamond or the intermolecular forces in other solids. While some materials may flow or deform permanently under extreme stress or over long time scales, their ability to support shear stress is what fundamentally distinguishes them as solids.4523