## Galileo and Pisa

Welcome to Galileo Excel Baccusi, which will be replacing Newton Excel Bach for the next four weeks, while I travel in Northern Italy.

Arriving in Rome on Friday morning, we travelled to Camogli by mini-bus, stopping for a quick tour of Pisa, home of the famous Leaning Tower:

The Duomo and Leaning Tower at Pisa

and two magnificent 14th century domes; of The Baptistry:

The Baptistry at Pisa

and the Duomo:

The Dome of the Duomo

This is what Wikipedia says of Galileo’s alleged experiment with dropping balls of different mass from the leaning tower:

According to a biography by Galileo’s pupil Vincenzo Viviani, in 1589 the Italian scientist Galileo had dropped two balls of different masses from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass.[1]. Via this method, he supposedly discovered that the objects fell at the same acceleration, proving his prediction true, while at the same time proving Aristotle’s theory of gravity (which states that objects fall at speed relative to their mass) false. At the time when Viviani asserts that the experiment took place, Galileo had not yet formulated the final version of his law of free fall. He had, however, formulated an earlier version which predicted that bodies of the same material falling through the same medium would fall at the same speed.[2] This was contrary to what Aristotle had taught: that heavy objects fall faster than lighter ones, in direct proportion to weight.[3][4] While this story has been retold in popular accounts, there is no account by Galileo himself of such an experiment, and it is accepted by most historians that it was a thought experiment which did not actually take place.[5][6] An exception is Drake,[7] who argues that the experiment did take place, more or less as Viviani described it.

But whether the real experiment actually happened at Pisa or not, the really important part is the thought experiment that led to it; from Wikipedia again:

Galileo arrived at his hypothesis by a famous thought experiment outlined in his book On Motion[8]. Imagine two objects, one light and one heavier than the other one, are connected to each other by a string. Drop this system of objects from the top of a tower. If we assume heavier objects do indeed fall faster than lighter ones (and conversely, lighter objects fall slower), the string will soon pull taut as the lighter object retards the fall of the heavier object. But the system considered as a whole is heavier than the heavy object alone, and therefore should fall faster. This contradiction leads one to conclude the assumption is false.

This line of thinking led not only to Galileo’s work on falling objects, but leads directly to Newton’s Laws of Motion and Universal Gravitation, which makes it arguably the most important thought experiment in the history of science.

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### 4 Responses to Galileo and Pisa

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2. Georg says:

Welcome in Europe, Doug!! There are some spectacular bridges on the motorway/railway tracks crossing the Alps as well as the Abruzzi (Rome Pescara), must see…

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3. dougaj4 says:

Thanks Georg! I won’t be doing either of those routes, but the road through the mountains from Pisa to Camogli was spectacular enough for me.

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4. Scholars have pointed to such passages to support their argument that Galileo did not perform such experiments and that his references to experiments were only rhetorical devices. After all, we all know that in a vacuum all bodies would fall with the same speed and in a medium such as air the heavier body (assuming the two bodies are of the same shape) will fall slightly faster: at no time will the lighter body be ahead of the heavier one. But when Galileo’s supposed experiment was repeated, the results showed that he had described a real experiment. Students dropped spherical balls of wood and iron of equal diameter and the wooden balls invariably moved ahead of the iron balls. The explanation lies in the fact that the heavier iron ball must be clasped in the hand with more force and is therefore released slightly later than the wooden ball.

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