Universal Gravitation
Year of Discovery: 1666
What Is It? Gravity is the attractive force exerted by all objects on all other
objects.
Who Discovered It? Isaac Newton
Why Is This One of the 100 Greatest?
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.
How Was It Discovered?
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?”
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