Gravity Equation
F = force
G = gravitational constant
m1 = mass of object 1
m2 = mass of object 2
r = distance between centers of the masses

Isaac Newton is said to have grasped the nature of gravity when he saw an apple fall from a tree at his birthplace, Woolsthorpe Manor. He then wondered why apples fall to the ground instead of floating upward. His subsequent reflections on gravity led him to believe that gravity is not only a force that pulls objects to the ground but is also the force that keeps the planets in orbit around the sun.

Newton explained the connection between apples and planets in his book, Mathematical Principles of Natural Philosophy. He says that the force of gravity between two massive objects is stronger compared to the force of gravity between two less massive objects. He adds that a massive object has a stronger gravitational force than a less massive object. 

Earth is much more massive than an apple and exerts a greater gravitational force, so apples fall to the ground instead of floating in mid-air or upwards to the sky.

Gravity, according to Newton, is also the reason planets orbit the sun. The sun is the most massive body in the solar system, and so it exerts the strongest gravitational pull on the planets. The nearer an object is to the sun, the stronger the sun’s pull on that object. Because Mercury is the nearest planet to the sun, it is the planet that experiences the strongest pull. As a result of this pull, Mercury is the planet that goes the fastest along its orbit. 

Consequently, Newton says that the reason the planets do not fall into the sun is that gravity weakens as the distance between two objects increases. Mercury, despite being the nearest planet to the sun, is still 70 million kilometers away from it. This distance, combined with the planet’s momentum, is enough to weaken the sun’s gravity and prevent Mercury from falling into the ball of fire. For this same reason, the farther away a planet is from the sun, the slower it travels along its orbit. Pluto, being the outermost planet in the solar system, experiences a weaker pull and so takes the longest time to complete one revolution.

Newton, who had arrived at his discoveries by mathematical analysis, also pointed out that the same gravitational forces act between the moon and Earth and between the sun and comets.

While Newton may not be the first person to think about gravity, he was the first to develop a sweeping explanation about it using a new form of mathematics called calculus that he himself developed.