The Foundations of Dynamics

Abstract

DYNAMICAL investigations are often made to depend upon the following three propositions, viz., the principle of linear momentum, the principle of angular momentum, and the principle of energy. In treatises which are concerned with ordinary mechanical systems, such as rigid bodies, elastic solids and frictionless fluids, the word energy is generally employed in the restricted sense of mechanical energy, and is confined to two particular species, viz. kinetic energy arising from such motions of the system as can be controlled by ordinary mechanical agencies, and potential energy arising from the configuration of the system or from its position in space. All other forms of energy, such as molecular kinetic energy arising from the production of heat by friction, and chemical potential energy contained in fuel, explosive compounds, &c., are excluded from consideration. In systems of this kind the sum of the kinetic and potential energies is an invariable quantity, and this proposition may be termed the principle of the conservation of mechanical energy.