From: long@next3 (Richard Long)
Newsgroups: sci.physics
Subject: Re:  Gallileo and falling bodies
Date: 6 Apr 92 17:58:19 GMT
Organization: University of Central Florida

In article <1992Apr1.220153.8855@morgan.ucs.mun.ca> court@newton.physics.mun.ca  
(J. Lewis) writes:
> It is worth consulting what Galileo himself wrote.  "Dialogues Concerning Two
> New Sciences" is perfectly readable, in translation;  in fact it sounds not
> unlike a tidied-up sci.physics flame war!  E.g.
> 
> First Day, [106]
> "	SALVIATI: ...  And as to the first, I greatly doubt that Aristotle ever
> tested by experiment whether it be true that two stones, one weighing ten
> ties as much as the other, if allowed to fall, at the same instant, from a
> height of, say, 100 cubits, would so differ in speed that when the heavier
> had reached the ground, the other would not have fallen more than 10
> cubits.
> 	SIMPLICIO:  His language would seem to indicate that he had tried
> the experiment, because he says: *We see the heavier*; now the word *see*
> shows that he had made the experiment.
> 	SAGREDO:  But I, Simplicio, who have made the test can assure
> 				[107]
> 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.
> 
> ....
> 	SALV. If then we take two bodies whose natural speeds are different,
> it is clear that on uniting the two, the more rapid one will be partly  
retarded
> by the slower, and the slower will be somewhat hastened by the swifter.  Do
> you not agree with me in this opinion?
> 	SIMP.  You are unquestionably right.
> 	SALV.  But if this is true, and if a large stone moves with a speed
> of, say, eight while a smaller moves with a speed of four, then when they
> are unitied, the system will move with a speed less than eight;  but the
> two stones when tied together make a stone larger than that which before
> moved with the speed of eight.  Hence the heavier body moves with less
> speed than the lighter; an effect which is contrary to your supposition.
> Thus you see 
> 				[108]
> how, from your assumption that the heavier body moves more rapidly than the
> lighter one, I infer that the heavier body moves more slowly.
> 	SIMP.  I am all at sea because it appears to me tha the smaller
> stone when added to the larger increases its weight and by adding wight I
> do not see how it can fail to increase its speed or, at least, not to
> diminish it.
> 	SALV.  Here again you are in error, Simplicio ... "
> [Dover edition, trans. Henry Crew and Alfonso de Salvio]
> 
> We see that Galileo is actually attacking the idea that the speed of a
> falling body is directly *proportional* to mass, and he does so primarily by
> means of a gedanken experiment.  He is well aware that bodies do fall at
> somewhat different speeds in air.  He questions whether *Aristotle* ever
> performed the experiment.  Galileo suggests that he, Galileo, had done so, or
> had seen it done, but the details are vague: Sagredo is not identical with
> Galileo.  There is no mention of Pisa, or of its famous belltower, and indeed
> the beginning of the "Dialogues" suggests that they are set in Venice.
> 
> John Lewis
> Physics Dept.
> Memorial University of Newfoundland
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