By the early seventeenth century, many forces had been identified: friction, gravity, air resistance, electrical, forces people exerted, etc.
Newton’s mathematical concept of gravity was the first step in joining these seemingly different forces into a single, unified concept. An apple fell; people had weight; the moon orbited Earth, all for the same reason. Newton’s law of gravity was a giant, simplifying concept.
Newton’s concept of, and equations for, gravity stand as one of the most used concepts in all science. Most of our physics has been built upon Newton’s concept of universal gravitation and his idea that gravity is a fundamental property of all matter.
In 1666, Isaac Newton was a 23-year-old junior fellow at Trinity College in Cambridge. With his fair complexion and long blond hair, many thought he still looked more like a boy. His small, thin stature and shy, sober ways reinforced that impression. His intense eyes and seemingly permanent scowl pushed people away.
In London, the bubonic plague ravaged a terrified population. Universities were closed, and eager academics like Isaac Newton had to bide their time in safe country estates waiting for the plague to loosen its death grip on the city. It was a frightening time.
In his isolation, Newton was obsessed with a question: What held the moon circling the earth, and what held the earth in a captive orbit around the sun? Why didn’t the moon fall down to the earth? Why didn’t the earth fall down to the sun?
In later years Newton swore that this story actually happened. As he sat in the orchard at his sister’s estate, he heard the familiar soft “thunk” of an apple falling to the grass-carpeted ground, and turned in time to see a second apple fall from an overhanging branch and bounce once before settling gently into the spring grass. It was certainly not the first apple Isaac Newton had ever seen fall to the ground, nor was there anything at all unusual about its short fall. However, while it offered no answers to the perplexed young scientist, the falling apple did present Isaac with an important new question, “The apple falls to Earth while the moon does not. What’s the difference between the apple and the moon?”
Next morning, under a clearing sky, Newton saw his young nephew playing with a ball. The ball was tied to a string the boy held tight in his fist. He swung the ball, slowly at first, and then faster and faster until it stretched straight out.
With a start Newton realized that the ball was exactly like the moon. Two forces acted on the ball, its motion (driving it outward) and the pull of a string (holding it in). Two forces acted on the moon. Its motion and the pull of gravity, the same pull (force) that made the apple fall.
For the first time, Newton considered the possibility that gravity was a universal attractive force instead of a force that applied only to planets and stars. His deep belief in alchemy and its concept of the attraction of matter led him to postulate that gravitational attraction force did not just apply to heavenly objects, but to all objects with any mass. Gravity pulled apples to earth, made rain fall, and held planets in their orbits around the sun.
Newton’s discovery of the concept of universal gravitation was a major blow to the prevalent belief that the laws of nature on Earth were different from those that ruled the heavens. Newton showed that the machinery that ruled the universe and nature is simple.
Newton developed universal gravitation as a property of all matter, not just of planets and stars. Universal gravitation and its mathematical expression lie at the foundation of all modern physics as one of the most important principles in all science.
The Flower of Kent is a large green variety of apple. According to the story, this is the apple Isaac Newton saw falling to ground from its tree, inspiring his discovery of universal gravitation.
