Isaac Newton (1642 - 1727)

Isaac Newton was born on Christmas Day 1642 (actually, January 4, 1643 according to the Gregorian calendar) in Woolsthorpe, Lincolnshire, England. He was educated at Cambridge University and did most of his groundbreaking work in gravity and motion while he was in his twenties. He worked out his law of universal gravity and three laws of motion back home in Woolsthorpe while Cambridge was closed during an outbreak of the Black Plague. However, he set this work aside for a number of years and did not prepare it for publication until he was urged to do so by members of the Royal Society, especially Edmond Halley. The work was finally published in 1687 under the title Philosophiae Naturalis Principia Mathematica (The Mathematical Principles of Natural Philosophy). In this publication he presented compelling mathematical and physical arguments as to why planets move around the sun (and why moons move around planets) in elliptical orbits, as discovered by Kepler. In addition to his work on the mechanics of planetary motion, Newton did pioneering work in optics, demonstrating that white sunlight is composed of all of visible colors and inventing the reflecting telescope.

Law of Universal Gravitation

Every material body in the universe is attracted to every other material body in the universe by a gravitational force proportional to the product of the masses divided by the square of the separation between the centers. The force of gravitational attraction F between any two masses is given by:

F = G(m₁m₂)/r²

Here, m₁ is the mass of one body, m₂ is the mass of the second body, r is the separation between their centers, and G is the gravitational constant.

Newton’s Three Laws of Motion

First Law

A body continues doing what it is already doing, either remaining at rest or moving with a uniform (constant) velocity until acted upon by an external force. This is also known as the Law of Inertia.

Second Law

The force acting on a body is always proportional to the mass of the body times the acceleration (change of velocity) experienced by that body.

F = ma

Here, m is the mass of the body and a is the acceleration experienced by that body. Remember, acceleration can be either a change in speed, or a change in direction, or both.

Third Law

For every action (or force) there is an equal and opposite reaction (a force of equal magnitude directed in the opposite direction). If you push on a wall, the wall pushes back on you with exactly the same amount of force as you apply to the wall.