The History Of Gravitational Theory

The word gravity has its origin in the Latin word “gravitas”. Moreover, “gravis” means “heavy” and dates back to the 15th century. At the same time, “gravitas” is defined as the weight whose invention dates back to the 17th century. 

Aristotle said there is motion in the universe without a cause. He states that the downward movement of heavy objects is due to their nature, and lighter objects move upward due to their nature. So, we can say that this is where the history of gravitational theory begins.

History of gravitational theory by Aristotle

 Archimedes discovered the centre of gravitation of a triangle. He said the centres of gravity of two objects of equal weights may not lie at the exact same points on their respective surfaces because the shapes of the objects can be different. But, if we join both objects with a straight line. Then, the centre of gravity will lie in the middle of line.

For example, if we take two objects with identical mass but of different shapes, their mass distribution will also be different. So, the points where all the mass will concentrate will also lie on the different points. But if we place them both at the opposite ends of a stick or rod, then the centre of gravitation will lie at the exact middle of the stick or rod. This theory added more to the history of gravitational theory. 

John Philoponus’s contribution to the history of gravitational theory

In the 6th century CE, John Philoponus gave the theory of impetus. He had modified the theory proposed by Aristotle, saying that continuity of motion depends upon the continued action of force.

 History of gravitational theory before the 17th century

 The history of gravitational theory before the 17th century is related more to planetary movements than to gravity itself. But it has helped a lot in the later discoveries about gravitation. Before the 17th century, There was the belief that the earth was the centre of the universe. Hence, everything moves around the planet. If you think about it, you will find that this idea willfully disturbs the formulas and theories of gravitation which we use today.

 But in the later 17th century, Johannes Kepler was the first philosopher who stated that the earth is not the centre of the universe. It looks pretty acceptable today, but back then, Kepler had to leave his town for this statement. Later, he also stated that the force of gravity only depends upon the mass of an object.

History of gravitational theory in the modern era 

One of the well-known experiments was Galileo Galilei’s experiment at the Leaning Tower of Pisa. In the investigation, Galileo Galilei free-felled two objects of different masses to show that time of the freefall is equal for both and independent of the mass. After that, many discoveries about gravitation began taking place.

Newton’s law of gravitation

 After discovering the planetary movements by Kepler and Galilei, Newton established his law of gravitation, which states that any particle of matter in this universe creates attraction to the other particle of value with a force directly proportional to the product of the masses of both objects and inversely proportional to the square of their centres. After that, this definition was presented mathematically by Johannes Kepler as

F = G (m1× m2)/R²

Where,

● F = force of gravitation

● G = Gravitational constant

● m1 = mass of the first object

● m2 = mass of the second object

● R = distance between their centres

Newton’s law of gravitation is very similar to Coulomb’s law of electrical forces. Moreover, you can quickly determine how identical these two theories are.

The first-ever testing of Newton’s law of gravitation took place in 1789. The British scientist Henry Cavendish did the test. The experiment was named after him as “the Cavendish experiment”. Moreover, this test took place after 111 years of publishing Newton’s Principia and about 71 years after Newton’s death.

In the modern era, Albert Einstein came up with his revolutionary theory of relativity. The idea of relativity explains many terms of the universe. Also, the theory of relativity explained the gravitational law more precisely. Newton’s law of gravitation has come to be used everywhere because the theory provides more precise calculations, which are not very useful in the case of gravitation.

Newton’s Principia dispute

This dispute was one of the most famous disputes of its time. The plagiarism disputes of that time happened to be very serious. Then Newton published his book Principia and presented it to the royal society. Robert Hooke, another very famous philosopher, accused Newton of plagiarism. He accused Newton of plagiarising content from his book “System of the World” published in the 1660s.

Derivation of Newton’s law of gravitation from Kepler’s law

Newton’s law of gravitation has a simple derivation from Kepler’s law of planetary motion.

Assume a planetary object of mass m revolves around the sun of mass M in a circular orbit of radius r, with a constant angular velocity ω. T is the time taken by the planetary object for one revolution around the sun.

Then,

ω = 2π/T … (1)

The centripetal force acting on the planet for its circular motion is:

F = mrω² = mr (2π/T)² = mr4π²/T²

According to Kepler’s Third Law:

T² α r³ (Or) T² = Kr³

Where K = Constant of proportionality

Therefore,

F = 4π²mr/Kr³ = 4π²m/Kr² = (4π²/K) × (m/r²)… (2)

F ∝ mr²(∵4π²K is a constant) … (3)

The centripetal force to the planetary object is due to the sun’s gravitational attraction. According to Newton’s law, the gravitational attraction between the sun and the planetary object is mutual. If the force F ∝ mass of the planetary object, then it should also be directly proportional to the sun’s mass.

Hence, the factor.

4π²/K∝M

 (Or) 4π²/K = GM … (4)

If we substitute equation (4) in equation (2), we get:

F = GMm/r², which is Newton’s law of gravitation. 

Conclusion

This article was a brief introduction to the history of gravitational theory. Gravitation or other topics were not always the same as today. This is proof of the hard work and struggle of our great scientists in discovering such excellent concepts. Moreover, this article will help you understand gravitation and the derivation of Newton’s law of gravitation from Kepler’s law. It will also help you know about more topics, such as Coulomb’s law of electrical forces.