]> 1. The ratio of the mass of any substance to the volume occupied by it (usually expressed in kilograms per cubic meter, but any other unit system may be used); the reciprocal of specific volume. Energy per unit area. A form of energy arising from the motion of a system against a force, existing only in the process of energy conversion. A single point in space emitting radiation, mass, or some other quantity. Mechanics is the branch of physics concerned with the behaviour of physical bodies when subjected to forces or displacements, and the subsequent effect of the bodies on their environment.