Newton's law of universal gravitationstates that any two bodies in the universe attract each other with aforcethat isdirectly proportionalto the product of their masses and inversely proportional to the square of the distance between them.[note 1]This is a generalphysical lawderived fromempiricalobservationsby whatIsaac Newtoncalledinduction.[2]It is a part ofclassical mechanicsand was formulated in Newton's workPhilosophi Naturalis Principia Mathematica("thePrincipia"), first published on 5 July 1687. (When Newton's book was presented in 1686 to theRoyal Society,Robert Hookemade a claim that Newton had obtained the inverse square law from him; see theHistorysection below.)In modern language, the law states: Everypointmassattracts every single other point mass by aforcepointing along thelineintersecting both points. The force isproportionalto theproductof the two masses andinversely proportionalto thesquareof the distance between them.[3]The first test of Newton's theory of gravitation between masses in the laboratory was theCavendish experimentconducted by theBritishscientistHenry Cavendishin 1798.[4]It took place 111 years after the publication of Newton'sPrincipiaand 71 years after his death.Newton's law of gravitation resemblesCoulomb's lawof electrical forces, which is used to calculate the magnitude of electrical force arising between two charged bodies. Both areinverse-square laws, where force is inversely proportional to the square of the distance between the bodies. Coulomb's law has the product of two charges in place of the product of the masses, and theelectrostatic constantin place of thegravitational constant.Newton's law has since been superseded by Einstein's theory ofgeneral relativity, but it continues to be used as an excellent approximation of the effects of gravity in most applications. Relativity is required only when there is a need for extreme precision, or when dealing with very strong gravitational fields, such as those found near extremely massive and dense objects, or at very close distances (such asMercury's orbit around the sun).