Note:  Do not rely on this information. It is very old.


Stability is of two kinds, static and kinetic. A body possesses static stability or is in stable equilibrium when, after any slight displacement, it tends to return to its original position. A weight hanging at the end of a string is in this state. We may give it a push in any direction, but it will sooner or later return to its lowest position. Any body is in stable equilibrium when a vertical line through its mass centre falls within the figure obtained by joining the points of support. A three-legged table may be tilted up and will resume its own place again so long as this vertical line remains within the triangle formed by the three feet. A body possesses instability when any slight displacement causes it to completely change its position. An egg strnding on its end is obviously in this condition. Certain bodies may be displaced without afterwards recovering their original position or departing farther from it. These are said to be in neutral equilibrium, and the best example is that of a sphere on a flat surface; the sphere may be moved and will remain, wherever it is placed. A body possesses kinetic stability when it tends to remain in a steady state of motion. A hoop remains erect while in motion, and the planets remain in their orbits in virtue of their velocity. A small displacement of either the hoop or planet causes no permanent change in their behaviour; they both maintain a definite average position.