The New Student's Reference Work/Statics
Statics (stăt′ĭks), a branch of mechanics. In pure mechanics (or dynamics, as it is frequently called), the effects of forces upon bodies are considered. If the effect of forces upon a body is to change the rate of motion of the body, the forces acting are said not to be in equilibrium. Forces and effects of this kind are studied under the head of kinetics. In all other cases forces are said to be in equilibrium and are studied under the head of statics. If one hold a steel-spring in his hands and bend it, he recognizes that there are forces acting upon the spring by the fact that the shape of the spring is temporarily changed. These forces do not produce any motion in the spring; they are in equilibrium. It is this group of forces which is considered under the head of statics. For another illustration allow a locomotive to stand at rest on a steel bridge. The bridge sags slightly as the locomotive takes up its position. But the upward force with which the bridge tends to regain its original shape is exactly equal and opposite to the weight of the locomotive. These stresses produced by the locomotive and the strains which result from them illustrate the kind of problems studied under the title of statics.
Statics may, therefore, be defined as the science of the equilibrium of forces. It is very important in this connection for the student to distinguish carefully between a body which is at rest and one which is in equilibrium. A pendulum bob at the end of its swing is in rest; but it is not in equilibrium, for at that very instant it is being set in motion by its weight. The two general conditions of equilibrium for any body are that the sum of all the forces acting upon the body shall be zero and that the sum of all the moments of force acting upon the body shall be zero.
The application of these general principles to the cases of simple machines, as the lever, inclined plane etc., is the portion of statics ordinarily studied in physics; while the application of these principles to various structures, as bridges, roofs etc., is the part of statics which is pursued by engineers. The conditions under which fluids remain in equilibrium form the science of hydrostatics.
Stevinns (1548-1603) showed that a force can be completely represented by a straight line. Upon this simple fact a branch of mathematics called graphical statics has been recently founded, and has furnished many elegant geometrical solutions of engineering problems. For an unusually interesting history of statics see Mach's Science of Mechanics. Minchin's Treatise on Statics (Clarendon Press) contains an extraordinarily clear presentation of the entire subject.