DYNAMICS

is the science which deals with the laws of force in their relation to matter at rest or in motion, and as such it is differentiated from kinematics, which considers motion mathematically, and apart from the forces producing it. Dynamics is divided into two great branches: statics, which treats of solid bodies at rest under the action of forces; and kinetics, which treats of the action of forces in producing motion in solid bodies. Formerly the latter alone was called dynamics, and to this, in conjunction with statics, the general name mechanics was given. In the wide sense dynamics includes also hydrodynamics.: Primary laws of force.—It is to Newton that we owe the clear statement of the three primary laws of force on which the science of dynamics is based. These are: (1) that every body remains in a state of rest, or of uniform motion along a straight line, unle it is compelled by force to change that state; (2) that rate of change of momentum is in proportion to the force employed, and occurs along the straight line in which the force acts; (3) that, as the result of every action, there is always an equal 25 DYNAMICS and opposite reaction. These laws, which were formulated from experiment, involve the conception of force ag a primary influence or action expre ed in terms of space, time, and matter. Now, in dealing with the laws of force, a standard of measurement is required which shall be applicable to all forces at all times, and we therefore require to begin by establishing units of space, time, and ma . ‘There are two systems of units in use, the one British, the other French. In the British system the foot is taken as the unit of length, and the second as the unit of time. In the French the centimetre is the unit of tength, the second the unit of time; the unit velocity in the one case being that of one foot per second, in the other one centimetre per second. The British unit of ma is the pound (the ma of a certain lump of platinum deposited in the Exchequer Office, London); the French the gramme; and accordingly the French units of space, ma , and time are commonly known as the C.G.S. (centimetre, gramme, second) units. As the weight ofa pound (or a gramme) is not the same at all parts of the earth’s surface, it cannot give us of itself an absolute or dynamical unit of force, that is, an invariable unit; but taking it in conjunction with unit time and unit velocity. we do obtain such a unit. Two absolute units of force are in common use in dynamics, the poundal and the dyne; the latter being the absolute unit in the C.G.S. system. The former is that force which, acting on the ma of one pound for one second, generates in that ma a velocity of one foot per second. The latter is that force which, acting on the ma of one gramme for one second, generates in that ma a velocity of one centimetre per second. It is important in dynamics to distinguish between ma and weight. The ma of one pound is the quantity of matter equal to a certain standard quantity (a certain lump of metal) and is quite independent of force. The weight of one pound is the force with which the ma of one pound is attracted to the earth’s surface by the force of gravity. Another important term is momentum: the momentum of a body in motion at any instant is the product of the ma of the body and the velocity at that instant. See CouUPLE; ELASTICITY; ENERGY; FORCE; HyDRODYNAMICS; KINEMATICS; KINETICS; STATICS; THERMODYNAMICS; WAVES.—BIBLIOGRAPHY: Kelvin and Tait, Natural Philosophy; A. Gray, Dynamics; P. G. Tait, Dynamics; 8S. L. Loney, Mechanics and Hydrostatics for Beginners. 22 DYSODILE> DYN’AMITE. See EXPLOSIVES, DY’NAMO. See GENERATOR.