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== Gravity ==

== Gravity ==

Newton developed the [[Theory of Universal Gravitation]], more commonly known as gravity, when, according to legend, a falling apple struck him on the head. He realized that the same force that pulled the apple to the ground could be what keeps [[Jupiter]]'s moons in orbit. His theory stated that all matter is attracted to matter by a force, whose strength increases proportionally to the mass of the objects involved, and inversely proportional to the squared distance between them. Newton's theory is based on action-at-a-distance which has always been opposed by some scientists, and now most physicists endorse the very different [[theory of relativity]].  Both theories predict identical results at small scales, similar results at the scale of the [[solar system]] and very different results at [[Cosmology|cosmological]] scales beyond the [[solar system]].

Newton developed the [[Theory of Universal Gravitation]], more commonly known as gravity, when, according to Newton himself, he wastched an apple fall one day. It fell straight down--and why was that? He realized that the same force that pulled the apple to the ground could be what keeps [[Jupiter]]'s moons in orbit. His theory stated that all matter is attracted to matter by a force, whose strength increases proportionally to the mass of the objects involved, and inversely proportional to the squared distance between them. Newton's theory is based on action-at-a-distance which has always been opposed by some scientists, and now most physicists endorse the very different [[theory of relativity]].  Both theories predict identical results at small scales, similar results at the scale of the [[solar system]] and very different results at [[Cosmology|cosmological]] scales beyond the [[solar system]].

Newton had already made great progress in his devising "method of fluxions" (the infinitesimal calculus). During the plague years he recorded his first thoughts on gravitation, inspired by watching an apple fall. It fell straight down--why was that? He was trying at that time to determine what type of force could hold the moon in its path. The fall of the apple led him to think that it might be the same gravitational force, suitably diminished by distance, that had acted on the apple.  Thereby he discovered the law of [[gravitation]] (attraction is proportional with [[inverse-square law|inverse distance squared]]). He verified his conjecture approximately by a numerical calculation. He did not, at the time, pursue the matter, because the problem of calculating the combined attraction of the whole earth on a small body near its surface was obviously one of great difficulty.<ref> The problem was enormously simplified when he later used his calculus to prove that, for purposes of gravity, a uniform sphere of any size can be considered as a single mass located at one point, the center.</ref>   

Newton had already made great progress in his devising "method of fluxions" (the infinitesimal calculus). During the plague years he recorded his first thoughts on gravitation, inspired by watching an apple fall. It fell straight down--why was that? He was trying at that time to determine what type of force could hold the moon in its path. The fall of the apple led him to think that it might be the same gravitational force, suitably diminished by distance, that had acted on the apple.  Thereby he discovered the law of [[gravitation]] (attraction is proportional with [[inverse-square law|inverse distance squared]]). He verified his conjecture approximately by a numerical calculation. He did not, at the time, pursue the matter, because the problem of calculating the combined attraction of the whole earth on a small body near its surface was obviously one of great difficulty.<ref> The problem was enormously simplified when he later used his calculus to prove that, for purposes of gravity, a uniform sphere of any size can be considered as a single mass located at one point, the center.</ref>