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Re: [HM] Galileo
Posted:
Mar 21, 2004 8:32 AM
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Dear All,
I thought that the following email from Tom about the Galilean
"experiment" should be of general intellectual interest, particularly
when the whole bunch of exchanges on this thread was a bit saltless
to my taste. Mind that what I paste below and forward to you has been
originally sent for my private eyes only, and never intended to be
put out on HM whatsoever -- I've taken the liberty of scissoring a
few personal comments. Others remarks should also be trimmed, I know,
but they are far too juicy to ignore them ;)
Saludos, Julio
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From: Tom Whiteside
Date: Thu, 4 Mar 2004 17:30:38 -0000
Dear Julio,
[...] The Galilean "experiment" -- a thought one which seems "obvious"
should work to anyone who reads the "Two New Sciences" (and is unaware of
what it leaves out) -- does not BEGIN to do so, yielding even under ideal
conditions only loosely half of what it should (as Gordon Fisher notices,
but fails correctly to explain why). A ball does not slide down an inclined
plane -- or if it did there would be a massive force of friction totally
negating the experiment. So we tolerate that it rolls down the plane, blind
to the fact that this can only be if its initial state of rotary rest be
converted into an ever faster spinning one in tune with its ever
accelerating roll down the plane. To impress that rotary spin on the ball
takes up about half the pull of gravity imprest in the direction of fall,
but WORSE it is mass-dependent : it takes less energy to spin a light ball
round than a heavy one (as you will have noticed if ever as a child you
played whip-and-top). On the other hand a lighter ball "grips" the plane
beneath less than a heavier one. So out goes not merely any accurate
measure of "g" from rolling a ball down a tilted table, but even that
"experimentum crucis" where you start off two balls, one of wood, one of
metal, and say "Hey presto, see how they hit the bottom together!".
Because they don't, not by a long way. Some twenty years ago when the Open
University here was doing a programme on "g" I had a panicky 'phone call
from the organisers of the Unit to ask if I would go along. When I arrived
(at their studio in Alexandra Palace I found that they had set up a highly
polished wood plane, some 15 metres long, and with two grooves(!) to guide
the balls down incised in it to ensure that they did not deviate from a
straight path down : the plane, which could be raised or lowered at one end,
was inclined at some 30-40 degrees to the horizontal when I got there.
Pretending innocence, I asked what the matter was. One took two balls, one
of steel, the other a white snooker one (which if anything should have done
better...), up a ladder and released them, while a second showed me the
script where it read "Look, they reach the bottom together" -- which would
have been true if you overlooked that they did not by several seconds in
time. They thought there was something wrong in the design of their
experiment, but I left them with the worry that it can't work. In the
programme transmitted, which I took special care to watch, they scrabbled
over what really happened to their balls at the bottom, skating over that
they hit one thud for the metal ball first to arrive, then a slight pause,
before a second, duller thud for the plastic one.
Such be history of science as taught to the masses. Stephen Brush once
dubbed the search after what really happened in the past in science the most
demanding of intellectual pursuits, one where it takes your first
half-lifetime in apprenticeship to unlearn the falsities.
One of these is that Galileo achieved great things in dynamics. But who
among the members of [HM] knows that Thomas Harriot, had gone far beyond him
and with deeper clarity forty years earlier ?
[...]
Best wishes to you, Tom
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