Isaac Newton’s ‘Mathematical Principles of Natural Philosophy’ (or ‘Principia’) was published in 1687. This work dealt with observed properties of bodies and their motions from which could be derived certain general laws relating to the forces of nature, general laws that could be used to predict other natural phenomena. The ‘Principia’ was divided into three parts, the first two dealing with the general principles of motions and forces, the third, his System of the World. In the third, Newton propounded the mutual forces of gravity between the planets and the Sun, and from these forces, deduced their motions. All the planetary motions, described empirically by Kepler’s three laws, could be deduced from one fundamental law, the LAW OF UNIVERSAL GRAVITATION: the force between two bodies is directed along a line connecting them, is directly proportional to the product of their masses, and is inversely proportional to the square of their separation. Astronomy. the most ancient of sciences, and dynamics, were united by this law.

The concept of a mutual attraction between objects, along with ideas on the nature of motion of terrestrial bodies, had been developing throughout the seventeenth century. Kepler and other physicists mentioned the possibility of an inverse square attraction between objects, although none of them provided any test for it. Galileo, one of the pioneers in dynamics, had built up a theory of falling objects in which the notion of uniform acceleration occurred for the first time and he formulated an example of the law of inertia (a more general form of which became Newton’s first law). In his ‘scientific method’, his emphasis on the necessity for repeated careful experiment. Galileo could be called one of the first moderns. Important work was also done by Christiaan Huygens (1629 -95) on the theory of the pendulum and allied problems describing the motion of a body in a circle or other curve. One of the principles which he introduced and examined was that of centripetal force, the force which holds a body moving in a circle in that motion, a very useful concept for celestial as well as terrestrial mechanics. Newton’s greatness lay partly in his ability to synthesize all the important results of his predecessors, but also in his insight into the real nature of the problem; none of his predecessors saw as clearly as he the universality of gravity.

Although the Principia’ was not published until 1687, Newton had tested the validity of the law of gravitation as early as 1666. Using the known distance to the Moon and calculating its rate of fall’, he determined that the force maintaining it in its orbit was the game as the force of gravity observed in terrestrial bodies. The
delay in publication can be partly explained by a natural reticence on Newton’s part, but also by certain inherent difficulties in the theory. The exact-relationship governing the attraction of a sphere on an external point, and the search for a general solution to the three-body problem (i.e. Moon-Earth-Sun) occupied Newton and many mathematicians after him.

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