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Galileo Galilei - and his mechanics and motion

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picture of Galileo inclined plane motion

Galileo Galilei (1564-1642), was a good mathematician and early astronomer and in physics he mainly experimented in mechanics and in the workings of gravity. He was put under stong religious and political pressure to restrict his publications, and put under house arrest. His published works, in Latin, included his 1632 'Dialogues concerning Two New Sciences' which was in an ancient-Greek logical reasoning argument dialogue style - little theory but with experimental proofs and with some sections more in a Euclid or Newton style.

Galileo produced little useful theory - at times taking a Newton black-box position as in saying the cause of gravity was of no immediate importance, maybe simply down to fear of persecution or maybe to failure to produce a satisfying gravity theory - but he like many early scientists considered experiment more important. Galileo was an outstanding early experimental scientist, but maybe worse than useless on theory.

"Dialogues concerning Two New Sciences" (1632),Translated by H Crew and A De Salvio

Galileo stated that he himself had not sought the publishing of this his major work on mechanics and motion. It largely deals with mechanical strength under gravity, motion generally and especially motion under gravity including motion on inclined planes, projectile motion and pendulum motion. Its dialogue style (between Salviati, Sagredo and Simplicio) made the presentation of its science somewhat more difficult for readers, but partly suited the requirement Galileo was under to publish his ideas as 'only ideas'. Some brief extracts follow ;

So on motion, Page 154 ;

"Uniform Motion.
In dealing with steady or uniform motion, we need a single definition which I give as follows.
DEFINITION. By steady or uniform motion, I mean one in which the distances traversed by the moving particle during any equal intervals of time, are themselves equal."

And Page 169 ;

"Sagrado.
... it would seem that up to the present we have established the definition of uniformly accelerated motion, which is expressed as follows ;
A motion is said to be equally or uniformly accelerated when, starting from rest, its momentum receives equal increments in equal times."

And Page 215 ;

"... Furthermore we may remark that any velocity once imparted to a moving body will be rigidly maintained as long as the external causes of acceleration or retardation are removed ..."

And on others theories on gravity, Pages 166-167 ;

"Salviati.
The present does not seem to be the proper time to investigate the cause of the acceleration of natural motion concerning which various opinions have been expressed by various philosophers, some explaining it by attraction to the centre, others to repulsion between the very small parts of the body, while still others attribute it to a certain stress in the surrounding medium which closes in behind the body and drives it from one of its positions to another. Now all these fantasies, and others too, ought to be examined ; but it is not really worth while. At present it is the purpose of our author merely to demonstrate and to investigate some of the properties of accelerated motion (whatever the cause of this acceleration may be) - meaning thereby a motion, such that the speed goes on increasing after departure from rest, in simple proportionality to the time ..."

And supporting the existance of a vacuum, Page 81 ;

"... in the previous experiment we weighed the air in vacuum and not in air or other medium."

Galileo claimed to have a Universal Law of Gravitation covering both terrestrial gravity and the motion of planets which he was afraid to discuss. But this looks more an inspired aspiration than a reality, as he seems not to have considered gravitational force as decreasing with distance from its source, which was central to Newton's later Universal Law of Gravitation and had been considered earlier by others and was demonstrated earlier by Gilbert for magnetic and electrical forces at least. But Earth's gravitational force does not decrease much if the highest you test from is the top of a Pisa tower, and worse was his testing gravity by the acceleration it produced on bodies using a gravity clock to measure it. He used a water version of the sand hour-glass or egg-timer - but if gravity was weaker and actually produced less acceleration, then his gravity clock would run proportionately slower so that the acceleration, and gravity strength, would appear constant. Clearly a clock must be independent of the event it is measuring, so Galileo should maybe have used iron filings and a magnet horizontally for a magnetic clock - he is known to have certainly have been aquainted with magnetism if not with all of Gilbert's work. But many kinds of clock are of course possible using astronomical, physical, chemical, biological or possibly even mental processes of determinate duration, and this was perhaps also an issue even later for Einstein ?

Yet on Pages 261-262 ;

"Sagrado.
(if each planet had started from rest at particular heights under gravity and) fall with naturally accelerated motion along a straight line, and were later to change the speed thus acquired into uniform motion, the size of its orbit and its period of revolution would be those actually observed.
Salviati.
I think I remember him having told me that he once made the computation and found a satisfactory correspondence with observation. But he did not wish to speak of it ..."

In his polemical 'Il saggiatore' (The Assayer) 1623, Galileo supported mechanical push physics against Gilbert attraction physics in claiming that science should concern itself only with the size, shape and relative motion of objects - a clearly unreasonable narrow view taken up by Rene Descartes and many others. Galileo, like Kepler, praised Gilbert while opposing his attraction physics with no actual disproofs or even discussion being provided, both were capable of using bits of trickery at times and not just to try to avoid the scary anti-science Inquisition. Galileo also unreasonably rejected Kepler's proof that planet orbits are elliptical and are not circles.

NOTE :

Early science in Europe faced opposition from churches that often dominated governments, with Bruno being burnt at the stake in Rome in 1600 (the year that Gilbert after much hesitation finally published his work in a then slightly less intolerant England under Queen Elizabeth who however died just months before Gilbert's death in 1603). Galileo faced legal restriction to his death in 1642. Churches being inclined to the view of God as the cause of everything, led many early scientists to omitting causal theory from their science. But the churches were in fact less concerned about what caused day to day events, than with science contradicting some particular words in their holy books. So their real opposition was to science claiming that the Earth is not the centre of the universe but is just one planet of several orbiting the sun, and to science claiming that humans were not specially created but evolved from apes.

So early scientists even claiming everything was caused by God, could still be in trouble. Of course Descartes did just get away with claiming that everything was caused by God AND nothing was caused by God. But in the end science survived because the power of religion in Europe gradually weakened and science was increasingly seen as being of practical use.

Galileo produced a mechanical push theory of Earth tides, as later did Descartes, and Kepler produced a mechanical-field push theory of Earth tides - all were easily disproved and Newton later developed Gilbert's attraction theory of Earth tides. (modern field or continuum mechanical push explanations of tides may well be possible but seem hard to find) Kepler, and not Galileo, correctly had gravity decrease as the square of the distance from its source and a better mathematics of planet motion within his own pseudo-Gilbert physics. But Galileo's motion under gravity experiments did crucially show how ellipse type motion in nature could derive from linear motion. He was just not himself very strong on theory.

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