Page:Elementary Text-book of Physics (Anthony, 1897).djvu/84

 CHAPTER II.

MASS ATTRACTION.

58. Mass Attraction.—The law of mass attraction was the first generalization of modern science. It may be stated as follows:—

Between every two material particles in the universe there is a stress, tending to move them toward each other, which varies directly as the product of the masses of the particles, and inversely as the square of the distance between them. This law is sometimes called the law of universal attraction and sometimes the law of gravitation.

Some of the ancient philosophers had a vague belief in the existence of an attraction between the particles of matter. This hypothesis, however, with the knowledge which they possessed, could not be proved. The geocentric theory of the planetary system, which obtained almost universal acceptance, offered none of those simple relations of the planetary motions upon which the law was finally established. It was not until the heliocentric theory of Copernicus had been established by the discoveries of Galileo, and the labors of Kepler, that the discovery of the law became possible.

In particular, the three laws of planetary motion published by Kepler in 1609 and 1619 laid the foundation for Newton's demonstrations. The laws are as follows:—

I. The planets move in ellipses of which one focus is situated as the sun. II. The radius vector drawn from the sun to the planet sweeps out equal areas in equal times.