Newton

Isaac Newton was born in Woolsthorpe, England in 1643. His father died before he was born. He attended The King's School in Grantham and then Trinity College, Cambridge. In 1665 and 1666, Cambridge closed due to the Great Plague, and Newton returned home. During these two years—his 'year of wonders'—he made most of the discoveries he would spend the rest of his life publishing and defending.

Newton realized that gravity was a force—something that pulled objects downward. But gravity also had to be the force holding the Moon in orbit. The same force that made an apple fall held the Moon in its path. He calculated that gravity's strength decreased with distance (by the inverse square law), and that this single principle could explain both terrestrial and celestial motion. He invented calculus to work through the mathematics.

In 1687, Newton published Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), known as the Principia. It was the most important scientific book ever written. Newton showed that mathematics could describe the physical universe. His three laws of motion and his law of universal gravitation became the foundation of physics for 200 years.

Newton lived until 1727, revising and defending his work. He was elected President of the Royal Society. He was made a knight. In 1948, the SI unit of force was adopted and named the newton. It is defined as the force required to accelerate one kilogram at one meter per second squared. The unit is small—one newton is the force of a 100-gram apple pressing on your hand. Newton discovered forces. We measure them in newtons.