Abstract
II. A MONGST the elementary principles of mechanics which are capable of easy illustration without special apparatus is that of the centre of gravity. In every solid mass a point can be found such that the resultant of all parallel forces acting on the individual particles passes through it, and such forces balance themselves around this point. The gravitation-force of the earth is exerted towards its centre, but this being 4,000 miles away, the individual forces acting on the separate particles of a body on the earth's surface may be regarded as parallel forces. Hence the centre of the parallel gravitation-forces is termed the centre of gravity. If the centre of gravity be supported, that is to say, if the resultant force be met by an equal and opposite force of resistance, then the body will not fall. The leaning tower of Pisa does not fall because, in the first place, the mortar is strong enough to bind the masonry into a substantial whole, and, in the second place, because the obliquity of the inclination of the tower is not so great as to throw the centre of gravity beyond the supporting base. A vertical plumb-line dropped down from the centre of gravity of the tower would meet the ground inside the base. It is very easy to imitate the leaning tower by taking a common wooden roller and sawing off a piece with oblique ends. The toys which are sold under the name of the Toy Blondin also illustrate the principle of the centre of gravity. A metal figure slides or walks down a stretched string, being kept upright by means of a weight fixed to the end of the rod held in the hand of the figure, thus causing the centre of gravity of the whole to fall below the point of support. A simple way of showing the same thing with improvised material is illustrated in Fig. 3. A couple of forks are stuck into a cork. Their weight being considerable, the centre of gravity of the combination is below the cork, and if the 'cork be placed on the tip of the figure or on the lip of a wine-bottle, it will stand there securely even while the bottle is emptied. M. Tissandier has revived another illustration of the same principle which is capable of evoking roars of laughter at a dinner-table. If a dish of snipe has been served up the head with its long beak may be fixed in a cork; and then, two forks being thrust into the sides of the cork and a needle having been fixed into the lower end of it, the cork can be balanced upon a coin laid on the top of a wine-bottle, and can be spun slowly round while the snipe's head nods at the various members of the company in turn, and finally stops opposite one of them (Fig. 4).
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Physics without Apparatus 1 . Nature 22, 343–345 (1880). https://doi.org/10.1038/022343b0
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DOI: https://doi.org/10.1038/022343b0