what discovery did sir isaac newton make about force?
He assumed that the rate of fall is proportional to the force of gravity and that this force is inversely proportional to the square of the distance from the center of the Earth.
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 Newton 's contribution to gravitational theory was to unify the motions of heavenly bodies , which Aristotle had assumed were in a natural state of constant motion , with falling motion observed on the Earth . He proposed a law of gravity that could account for the celestial motions that had been described earlier using Kepler 's laws of planetary motion . 

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 Newton 's derivation is found in Section IX of his Principia , specifically Propositions 43 – 45 . His derivations of these Propositions are based largely on geometry . 

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 An experiment to test Newton 's idea would both provide supporting evidence for his law of universal gravitation , and estimates of the mass and density of the Earth . Since the masses of astronomical objects were known only in terms of relative ratios , the mass of the Earth would provide reasonable values to the other planets , their moons , and the Sun . The data were also capable of determining the value of Newton 's gravitational constant G , though this was not a goal of the experimenters ; references to a value for G would not appear in the scientific literature until almost a hundred years later . 

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 Newton 's work has been said " to distinctly advance every branch of mathematics then studied " . His work on the subject usually referred to as fluxions or calculus , seen in a manuscript of October 1666 , is now published among Newton 's mathematical papers . The author of the manuscript De analysi per aequationes numero terminorum infinitas , sent by Isaac Barrow to John Collins in June 1669 , was identified by Barrow in a letter sent to Collins in August of that year as : 

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 It was while Galileo was under house arrest that he dedicated his time to one of his finest works , Two New Sciences . Here he summarised work he had done some forty years earlier , on the two sciences now called kinematics and strength of materials , published in Holland to avoid the censor . This book has received high praise from Albert Einstein . As a result of this work , Galileo is often called the " father of modern physics " . He went completely blind in 1638 and was suffering from a painful hernia and insomnia , so he was permitted to travel to Florence for medical advice . 
