Sir Isaac Newton introduced his Universal Law of Gravitation to the world in his book, the Philosophiæ Naturalis Principia Mathematica, published in July 1687. This law explains why an apple falls from a tree, why you stay firmly on the ground, and what keeps the Earth revolving around the Sun. It allows us to predict with remarkable precision the positions of planets, moons, and even comets across the Solar System and beyond.
Newton’s law of gravitation captures the idea that every single piece of matter in the universe attracts others through gravity. This law tells us how strong this attractive force is. The equation states that the force (F) between two objects is proportional to the product of their masses (m1 and m2), divided by the square of the distance between them. It also involves the gravitational constant (G), which scientists have determined through experiments.
When you use the equation with your mass and the Earth’s mass, you can figure out your weight in Newtons. Weight, scientifically speaking, is the pull of gravity on your mass at any particular time. Your mass stays the same no matter where you are in the universe, but your weight changes depending on the mass and position of objects around you.
If I have seen further it is by standing on the shoulders of Giants.
Astronomers, including Tycho Brahe, dedicated centuries to documenting the positions of stars and planets in the nighttime sky. With this precious data, Johannes Kepler, a German mathematician, described how planets traveled around the Sun. However, he didn’t explain the reasons behind their movements. That’s when Newton stepped in and proposed the existence of a force between them that drives their motion.
According to the story, Newton became curious when he saw an apple fall straight down to the ground. He wondered why it didn’t go left or right. Based on his own laws of motion, Newton knew that when something starts moving from a still position, it means there must be a force. Since the apple started in the tree and landed on the Earth, there must be a force pulling the apple toward the Earth.
Portrait of Isaac Newton at the age of 46 by Godfrey Kneller, 1689
Newton also thought that even if the apple was higher up in the tree, it would still feel this force pulling it toward the Earth. He believed that this force doesn’t stop at the top of the tree but continues upward into the sky. This made him question why the Moon doesn’t fall and crash onto our planet like the apple did.
Newton came to the conclusion that the Moon does experience the Earth’s gravitational force and is, in fact, falling toward it. However, there’s a reason why it doesn’t crash down. To explain his idea, Newton proposed a thought experiment: picture yourself firing a cannonball horizontally from the top of a mountain. As the ball moves forward, it follows a curved path due to gravity pulling it toward the ground. If you give the cannonball more energy, it will land farther away from the mountain, but it will still follow a curved trajectory.
An illustration of Newton’s cannonball
Now, Newton suggested that if the initial velocity of the cannonball is high enough, the curvature of the Earth effectively becomes negligible compared to the cannonball’s trajectory, resulting in the cannonball continuously falling but “missing” the Earth as it falls. In other words, the ball would orbit the Earth.
The same principle applies to the moon—it is essentially in freefall around the Earth but moves at such a high speed that the Earth’s surface never quite catches up with it.
Nowadays, the Universal Law of Gravitation still stands as one of the most fundamental principles in physics. By establishing a mathematical relationship between masses and distances, Newton elegantly explained why objects fall to the ground, how planets orbit the Sun, and even the interactions between galaxies.
Words of Wisdom
“Nature is pleased with simplicity. And nature is no dummy” —Isaac Newton
“Two things are infinite: the universe and human stupidity; and I’m not sure about the universe.” —Albert Einstein
“An expert is a person who has made all the mistakes that can be made in a very narrow field.” —Niels Bohr
“I’m sure the universe is full of intelligent life. It’s just been too intelligent to come here.” —Arthur C. Clarke
Bibliography
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