Solidbodies

A solid body, in physics and engineering, is a macroscopic object whose shape and volume remain effectively constant under normal external forces. Its internal structure resists deformation, so, within the limits of material strength, it does not flow or change shape like a liquid or gas.

In classical mechanics, problems are often simplified by treating a solid body as rigid: its geometry is fixed, its mass is distributed according to a density function, and its motion can be described by the translation of its center of mass and rotation about that point. This rigid-body approximation underpins many analyses of motion, forces, and energy.

Solids can be crystalline, consisting of a regularly ordered lattice, or amorphous, lacking long-range order. They may be isotropic, having the same properties in all directions, or anisotropic, with direction-dependent properties. Their mechanical behavior is described by elasticity, plasticity, and failure criteria; properties include stiffness (Young's modulus), strength, density, and thermal expansion.

Rigid-body dynamics uses concepts such as moment of inertia, angular velocity, and torque to predict motion. When deformations are non-negligible, continuum mechanics models the solid as a deformable continuum, using stress–strain relations and constitutive laws to describe elastic, plastic, or viscoelastic response.

Solid bodies are central to engineering disciplines, appearing in statics, dynamics, vibration analysis, and structural design. The term is often contrasted with fluids and with deformable bodies, with the rigid-body approximation serving as a common theoretical idealization.