Rigidity

Rigidity is one of the properties of matter which sharply differentiate solids from fluids. In abstract dynamics a rigid system is a collocation of particles which, however much they may move as a whole, never alter their mutual relative positions. Such a system has no true physical existence, since there is no known substance which can resist deformation. Nevertheless, those substances which yield but slightly to deforming stresses—all solids practically—are regarded as possessing a certain rigidity, which is measured by the ratio of the deforming stress to the deformation produced. The greater this ratio is, the more nearly does the substance approach the condition of the ideal rigid body, whose behaviour under the action of given forces forms the subject-matter of what it is usual to call Rigid Dynamics. The property of rigidity itself, as described above, falls to be discussed under the general subject of elasticity. Of ordinary substances steel possesses the highest rigidity. See ELASTICITY, ETHER, MATTER.