The story that Newton's hypothesis of universal gravitation was prompted by the fall of an apple, is probably true because several reliable accounts from the last few years of his life which record him describing such an event. Even Voltaire, who was in London for the last year of Newton's life, stated in one of his essays : "Sir Isaac Newton walking in his garden had the first thought of his System of Gravitation, upon seeing an apple falling down from the tree.". From this event Newton asked himself : why should that apple always descend perpendicularly to the ground ? not sideways ? or upwards ? but constantly to the earth's centre ? And that power of gravity, bringing the apple from the tree to the ground, is it limited to a certain distance from the earth or does it extend much farther around the Earth and in the whole universe ? Why not as high as the moon ? and if so this power must influence her motion and perhaps retain her in her orbit !
To develop his theory of gravitation, Newton first had to develop the science of forces and motion called mechanics. Newton proposed that the natural motion of an object is motion at a constant speed on a straight line, and that it takes a force to slow down, speed up, or change the path of an object. Because Newton also invented calculus, this important tool was a fundamental help in the calculations of his theory of gravitation.
During the two plague years of 1665-1666, Newton laid the foundations of all his major discoveries - on infinite series, the calculus, optics and gravitation. From his own recollections in old age we read : "I began to think of gravity extending to the Orb of the Moon, and having found out how to estimate the force with which a globe revolving within a sphere presses the surface of the sphere. From Kepler's Rule of the periodical times of the Planets, I deduced that the forces which keep the Planets in their Orbs must be reciprocally as the squares of their distances from the centres about which they revolve : and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth, and found them to answer pretty nearly".
By the 1670s, the mechanics of the uniform circular motion were well known and the inverse square law of gravity was widely discussed by Hooke and others. However Newton was the only man who could give a full mathematical analysis of the dynamics of the Solar System, including an explanation of Kepler's three laws!
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687 Newton proposed his law of gravitation in the Principia and stated that every particle in the universe attracts every other particle in the universe with a force that depends on the product of the two particles' masses divided by the square of the distance between them. According to Newton, the force acts along a line between the two particles. In the case of two spheres, it acts along the line between their centers. The attraction between objects with irregular shapes is more complicated. Every bit of matter in the irregular object attracts every bit of matter in the other object. A simpler description is possible near the surface of the earth where the pull of gravity is approximately uniform in strength and direction. In this case there is a point in an object (even an irregular object) called the center of gravity, at which all the force of gravity can be considered to be acting.Newton's law affects all objects in the universe, from raindrops in the sky to the planets in the solar system. lt is therefore known as the universal law of gravitation.
" I don't know what I may seem to the world, but, as to myself, I seem to have been only like a boy playing on the sea shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me "
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