A Brief Course in Elementary Dynamics; For Students of Engineering (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1909 Excerpt: ...necessary to impart to him the downward acceleration of the elevator. Thus, his apparent weight is now R' = tng--mAi If Ad = g, that is, if the elevator were falling freely, the man would exert zero pressure on the floor of the elevator. 71. Density and Specific Gravity.--The density of a substance is the ratio of its mass to its volume. The number which expresses the density of any substance depends upon the units in terms of which mass and volume are measured. For instance, the density of copper is 8.92 grams per cubic centimeter, or 557 pounds per cubic foot. The relative density, or specific gravity, of a substance is the ratio of its density to the density of some standard substance. In other words, the specific gravity of a body is the ratio of its mass to the mass of an equal volume of a standard substance. Specific gravity is an abstract number which is independent of the units employed. In the case of solids and liquids, water at the temperature of its maximum density (40 C. or 39.2 F.) is arbitrarily taken as the substance with which the densities of other substances are compared. Since the gram is the mass of one cubic centimeter of water at the temperature of its maximum density, it follows that the number which expresses the density of a substance in grams per cubic centimeter is the same number which expresses its specific gravity. 72. The Center of Mass.--If a body or system of bodies be conceived to be divided into particles of equal mass, then that point whose distance from any given plane is equal to the average distance from that plane of all the constituent particles, is termed the center of mass, or center of inertia, of the body or system of bodies. In case the bodies composing the system have incommensurable masses, then they may be c...