internalmotion

Internal motion is a term used in physics and engineering to describe the motion of parts within a system that occurs without a corresponding motion of the system’s center of mass. It encompasses vibrational, rotational, and conformational motions that change the internal configuration of a body while the overall object translates or rotates as a whole. In many frameworks, total motion is decomposed into external motion (translation and rotation of the entire system) and internal motion (the relative motion among constituents).

At the molecular level, internal motion refers to vibrational and rotational modes of molecules and the ongoing fluctuations of their geometry. For larger assemblies, it includes bending, twisting, and folding of components. In solids, collective internal motions give rise to phenomena such as phonons, the quantized lattice vibrations that underpin many thermal and acoustic properties. In mechanical and structural systems, internal motion can describe deformation, flexing, and damping within an assembly.

Kinematically and thermodynamically, internal motion contributes to the internal degrees of freedom of a system. The kinetic energy of a composite system is often partitioned into translational, rotational, and internal components. The number and character of internal modes depend on flexibility and internal structure; rigid bodies possess no internal motion, while deformable bodies do.

Measurement and analysis of internal motion employ methods such as normal mode analysis, molecular dynamics simulations, and spectroscopy or scattering techniques to probe vibrational frequencies and internal fluctuations. Applications span materials science, molecular spectroscopy, thermodynamics, and the study of conformational dynamics in biomolecules. See also internal energy, degrees of freedom, vibrational mode, phonon, and normal mode analysis.