Stability, the name given to the property possessed by all material systems whose configuration remains permanent or never departs far from a permanent average type. There are two kinds—static and kinetic. Of static stability, or stable equilibrium, we have numerous examples of a simple character. A pendulum or any body hanging under the influence of gravity by a point which is not its centre of mass; a ball resting inside a basin; any object resting on supports in such a way that a vertical line through its centre of mass falls well within the polygon formed by joining the points of support—all these are familiar instances. If any displacement (within certain limits) is given to the body, it will, when released, tend to recover its original condition. In dynamic language the forces brought into play by the displacement resist it. If, however, the ball is placed on the top of a convex surface, or if a chair, for example, is tilted until the vertical line through its centre of mass falls outside the original area of its base, then the configuration is no longer stable. Both bodies will fall away from these positions until a new configuration of stable equilibrium is reached. In general, stability is proved by a system recovering its configuration after a slight displacement. Instability is demonstrated when any slight displacement is followed by a complete change of configuration, forces being brought into existence which assist the displacement. When a displacement brings into play no forces, so that the system tends neither to recover nor to fall away from its original configuration, the equilibrium is said to be neutral or labile. A uniform sphere resting on a plane is a simple example of this kind of equilibrium.
In kinetic stability, or stability of steady motion, a new factor comes into play. Neither a spinning-top nor a bicycle can rest upright unless it is in more or less rapid motion. The moon would fall into the earth, and the earth into the sun, if it were not for the orbital velocity sustaining each in its path. The perturbations produced by the planets cause the earth to be constantly deviating from its mean orbit; yet in virtue of kinetic stability this deviation is never large, and takes place now in one direction, now in another. If no frictional effects existed in the solar system, all the planetary orbits would never vary beyond certain assignable limits.