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 ;)
From: Tom Whiteside Date: Thu, 4 Mar 2004 17:30:38 -0000
[...] 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 ?