Certain examples would suggest Aristotle’s idea of gravity originating from elements nature to be true, such as dropping an rock and a leaf at the same time. The rock would fall to the ground faster, since it is heavy like the core of the earth. However, Italian scientist Galileo had suspicions about the veracity of this experiment and decided to test it out himself. He experimented by rolling balls of different masses down sloped planes and found that it was possible to have two balls of different masses reach the bottom of the plane at the same time. This discovery suggested that the rate of acceleration to the earth is universal, a novel concept that contradicted Aristotle’s theory of gravity.
Physicist Isaac Newton built off of this idea and made a great leap in the theory, shortly after Galileo’s experimentation. He made a proposition in 1687 that was tremendously successful in predicting the strength of gravity. Isaac believed that the force of gravity that causes a ball thrown into the air to retreat back to the earth was the same force that caused the planets to orbit around the sun. His "Law of Universal Gravitation" states that a particle attracts every other particle in the universe. The degree of this attraction is proportional to the product of their masses and is inversely proportional to the square of the distance between their centers. It is defined by the equation F=(G*m1*m2)/r^2^, where m1 and m2 are the masses of the particles, r is the distance between them, and G is the “gravitational constant." It took another 250 years before this a new theory suggested the Law of Universal Gravitation to be incorrect, due primarily to the fact that this theory works for most practical purposes.
Referencing scientists by name
This is a good history an your second paragraph introduces Isaac Newton. It is okay to reference the full name and use the first names when that is what is best known(e.g. Aristotle and Galileo) but after naming Isaac Newton it would be more professional to refer to him by his surname Newton. You start a sentence with "Isaac believed" which gives a cavalier impression. You did not know Newton and it would be best to refer to him by the name that people associate with the figure.
In the last sentence there
In the last sentence there seems to be an extra "this" which should probably be removed. Likely just an oversight. I would suggest splitting this into to sentences though. I would instead say "It took another 250 years before a new theory suggested the Law of Universal Gravitation to be incorrect. However, it still works for most practical purposes."