Stress
a convenient term used to express the mutual action between any two por portions of matter. Thus the pressure between a table and a book resting on it is of the nature of a stress, which has two aspects, according as we fix our attention on the table or the book. With reference to the former the pressure is downward, with reference to the latter upward, and these two forces, which according to Newton's third law are equal and opposite, form when regarded as a whole the stress. Similarly the attraction of the earth for the sun and the attraction of the sun for the earth are opposite aspects of the same stress, and the same can be said of all forces which obey Newton's third law, i. e., of all known forces which invariably exist in pairs. The so-called centrifugal force is merely the one aspect of the stress whose other aspect is the force which acts upon the body normally to the path. See STRAIN: STRENGTH OF MATERIALS.