Dynamics treats of the nature and laws of motion. In this sense it is used in opposition to Statics. A single force, always producing motion, must 6e considered under the head of Dynamics; but two or more forces, accordingly as they result in producing motion or not, constitute cases in Statics or Dynamics. These two terms are frequently used to designate the two great divisions of mechanics - Statics, including all cases where equilibrium is produced, and Dynamics, those in which motion results. The term dynamics is, however, not confined exclusively to the motions of solid bodies. When applied to describe the laws of motion in fluids, it is termed hydrodynamics, and electrodynamics when used to comprehend the phenomena of electricity in motion. See Forces, Motion, etc.