Physics Guru’s notepad: how New­ton dis­cov­ered grav­ity

Guru Magazine - - Physics -

Even if New­ton did ob­serve a fall­ing ap­ple in 1666, it’s highly un­likely that he de­vel­oped his the­ory of grav­ity right there on the spot. At some point, though – and this is what fits in nicely with the ap­ple story – he came to re­alise that the same force which gov­erns the ac­cel­er­a­tion of ob­jects to­wards the ground also reaches far out into space. His univer­sal law of grav­i­ta­tion was pub­lished some 20 years af­ter the al­leged ap­ple in­ci­dent. In Philosophiæ Nat­u­ralis Prin­cipia Math­e­mat­ica (1687), New­ton ex­plains that the grav­i­ta­tional force be­tween two ob­jects is in­versely pro­por­tional to the square of the dis­tance be­tween them, and di­rectly pro­por­tional to the prod­uct of their masses. In other words, if you dou­ble the dis­tance be­tween two ob­jects you re­duce the grav­i­ta­tional force by a fac­tor of four; triple the dis­tance and you re­duce it by a fac­tor of nine. Us­ing this law, New­ton was able to show that the or­bits of the plan­ets around the Sun – and the Moon around the Earth – can be ex­plained by the bod­ies’ mu­tual grav­i­ta­tional at­trac­tion. It’s the very same ef­fect that pulls a fall­ing ap­ple to­wards the ground.

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