
Everyone has heard the story of Isaac Newton and the apple tree, but the truth behind his laws of gravitation are more complex than a simple story.
Sir, Isaac Newton (1642  1727) was born in a small Lincolnshire village. At the age of 19 he was sent to Trinity College Cambridge and by the age of 24 he had made important discoveries in mathematics, optics and mechanics. His book Philosophiae Naturalis Principa Mathematica, published in 1687, helped to explain many mysteries about the universe such as the tides.
Newton\'s work describing the laws of motion were seen as a great advance in the understanding of the physical world. His theories still stand valid today for velocities and dimensions within a person\'s normal experience, but fail for velocities approaching the speed of light or subatomic dimensions.
Newton\'s main laws cover kinematics, motion, gravitation and momentum. Kinematics (or the science of motion) is covered by four equations that cover the motion of any body moving in a straight line with uniform acceleration. These equations can be adapted to apply to rotational movement and Newton developed the concept of centrifugal force.
Newton\'s three laws of motion are as follows:
(1) A body will stay at rest or travel in a straight line at constant speed unless acted upon by an external force.
(2) The resultant force exerted on a body is directly proportional to the acceleration produced by the force.
(3) To every action there is an equal and opposite reaction.
The law concerning the conservation of momentum follows from the third law. The principle states
(4) that before and after a collision, two bodies will have the same momentum, but there is an important rider that there will always be a change in the total energy of the two bodies.
The gravitational force that Newton discovered is one of the four forces that occur in nature. (The others are electromagnetic force and the strong and weak interatomic forces.) Newton discovered that every body in the universe attracts every other body. This law applies to the stars in space, but can also be demonstrated by setting up a pendulum beside a large mountain. After thousands of swings the path of the pendulum can be measured to have approached the mountain by a small amount.
Newton\'s laws have been the basis of most analysis in mechanics and engineering from his time to the present. AA
Further reading H. Goldstein, Classical Mechanics. 
