Changes

He used inclined planes as a way to study the acceleration of gravity in, as it were, slow motion.

He used inclined planes as a way to study the acceleration of gravity in, as it were, slow motion.

This book contains his famous discussion and thought experiment, in which he shows both by experiment and by logic that light and heavy objects fall at the same speed (contradicting Aristotle). One of his discussants says "But I, Simplicio, who have made the test can assure you that a cannon ball weighing one or two hundred pounds, or even more, will not reach the ground by as much as a span ahead of a musket ball weighing only half a pound, provided both are dropped from a height of 200 cubits."<ref>Crew, Henry and Alfonso de Salvio (1914), ''Dialogues Concerning Two New Sciences by Galileo Galilei,'' Northwestern University, 1939 reprint of 1914 Macmillan edition, p. 62</ref>.  

This book contains his famous discussion and thought experiment, in which he shows both by logic and by appeal to experiment that light and heavy objects fall at the same speed (contradicting Aristotle). One of his discussants says "But I, Simplicio, who have made the test can assure you that a cannon ball weighing one or two hundred pounds, or even more, will not reach the ground by as much as a span ahead of a musket ball weighing only half a pound, provided both are dropped from a height of 200 cubits."<ref>Crew, Henry and Alfonso de Salvio (1914), ''Dialogues Concerning Two New Sciences by Galileo Galilei,'' Northwestern University, 1939 reprint of 1914 Macmillan edition, p. 62</ref>.  

Another discussant points out that this result is logical, by positing a large stone that moves at a speed of eight and a smaller one which moves at a speed of four, and showing that this leads to a contradiction if the stones are tied together. Since the smaller stone tends to fall more slowly, it ought to retard the motion of the larger one, so the combination should fall more slowly than the large stone by itself. However, "the two stones when tied together make a stone larger than before," and hence ought to fall more quickly than the large stone by itself.

Another discussant points out that this result is logical, by positing a large stone that moves at a speed of eight and a smaller one which moves at a speed of four, and showing that this leads to a contradiction if the stones are tied together. Since the smaller stone tends to fall more slowly, it ought to retard the motion of the larger one, so the combination should fall more slowly than the large stone by itself. However, "the two stones when tied together make a stone larger than before," and hence ought to fall more quickly than the large stone by itself.