
Between 1589 and 1592, the Italian scientist
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
(then professor of mathematics at the
University of Pisa) is said to have dropped "unequal weights of the same material" from the
Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass, according to a biography by Galileo's pupil
Vincenzo Viviani, composed in 1654 and published in 1717.
[ Vincenzo Viviani (1717), ''Racconto istorico della vita di Galileo Galilei'', p. 606: [...dimostrando ciò con replicate esperienze, fatte dall'altezza del Campanile di Pisa con l'intervento delli altri lettori e filosofi e di tutta la scolaresca... [...Galileo showed this [all bodies, whatever their weights, fall with equal speeds] by repeated experiments made from the height of the Leaning Tower of Pisa in the presence of other professors and all the students...].] The basic premise had already been demonstrated by Italian experimenters a few decades earlier.
According to the story, Galileo discovered through this experiment that the objects fell with the same acceleration, proving his prediction true, while at the same time disproving
Aristotle
Aristotle (; 384–322 BC) was an Ancient Greek philosophy, Ancient Greek philosopher and polymath. His writings cover a broad range of subjects spanning the natural sciences, philosophy, linguistics, economics, politics, psychology, a ...
's theory of gravity (which states that objects fall at speed proportional to their mass). Though Viviani wrote that Galileo conducted "repeated experiments made from the height of the Leaning Tower of Pisa in the presence of other professors and all the students,"
[ most historians consider it to have been a ]thought experiment
A thought experiment is an imaginary scenario that is meant to elucidate or test an argument or theory. It is often an experiment that would be hard, impossible, or unethical to actually perform. It can also be an abstract hypothetical that is ...
rather than a physical test.
Background
The 6th-century Byzantine Greek
Medieval Greek (also known as Middle Greek, Byzantine Greek, or Romaic; Greek: ) is the stage of the Greek language between the end of classical antiquity in the 5th–6th centuries and the end of the Middle Ages, conventionally dated to the F ...
philosopher and Aristotelian commentator John Philoponus argued that the Aristotelian assertion that objects fall proportionately to their weight was incorrect. By 1544, according to Benedetto Varchi, the Aristotelian premise was disproven experimentally by at least two Italians. In 1551, Domingo de Soto suggested that objects in free fall accelerate uniformly. Two years later, mathematician Giambattista Benedetti questioned why two balls, one made of iron and one of wood, would fall at the same speed. All of this preceded the 1564 birth of Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
.
Delft tower experiment
The Nieuwe Kerk in Delft, where the experiment by Stevin and de Groot took place
A similar experiment was conducted in Delft
Delft () is a List of cities in the Netherlands by province, city and Municipalities of the Netherlands, municipality in the Provinces of the Netherlands, province of South Holland, Netherlands. It is located between Rotterdam, to the southeast, ...
in the Netherlands
, Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...
, by the mathematician and physicist Simon Stevin
Simon Stevin (; 1548–1620), sometimes called Stevinus, was a County_of_Flanders, Flemish mathematician, scientist and music theorist. He made various contributions in many areas of science and engineering, both theoretical and practical. He a ...
and Jan Cornets de Groot (the father of Hugo de Groot). The experiment is described in Stevin's 1586 book '' De Beghinselen der Weeghconst'' (''The Principles of Statics''), a landmark book on statics
Statics is the branch of classical mechanics that is concerned with the analysis of force and torque acting on a physical system that does not experience an acceleration, but rather is in mechanical equilibrium, equilibrium with its environment ...
:
Let us take (as the highly educated Jan Cornets de Groot, the diligent researcher of the mysteries of Nature, and I have done) two balls of lead, the one ten times bigger and heavier than the other, and let them drop together from 30 feet high, and it will show, that the lightest ball is not ten times longer under way than the heaviest, but they fall together at the same time on the ground. ... This proves that Aristotle is wrong.
Galileo's experiment
At the time when Viviani asserts that the experiment took place, Galileo had not yet formulated the final version of his law of falling bodies. 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. This was contrary to what Aristotle had taught: that heavy objects fall faster than the lighter ones, and in direct proportion to their weight. While this story has been retold in popular accounts, there is no account by Galileo himself of such an experiment, and many historians believe that it was a thought experiment
A thought experiment is an imaginary scenario that is meant to elucidate or test an argument or theory. It is often an experiment that would be hard, impossible, or unethical to actually perform. It can also be an abstract hypothetical that is ...
. An exception is Stillman Drake, who argues that it took place, more or less as Viviani described it, as a demonstration for students.
Galileo set out his ideas about falling bodies, and about projectiles in general, in his book '' Two New Sciences'' (1638). The two sciences were the science of motion, which became the foundation-stone of physics, and the science of materials and construction, an important contribution to engineering. Galileo arrived at his hypothesis by a famous thought experiment outlined in his book '' On Motion''. He writes:
His argument is that 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.
Later performances
Hammer and feather drop on the Moon by astronaut , David Scott, Apollo 15 (1.38 Megabyte">MB, ogg">Apollo 15">David Scott, Apollo 15 (1.38 Megabyte">MB, ogg/Theora format)
Astronaut David Scott performed a version of the experiment on the Moon during the Apollo 15 mission in 1971, dropping a feather and a hammer from his hands. Because of the negligible lunar atmosphere, there was no drag on the feather, which reached the lunar surface at the same time as the hammer.
The basic premise behind these experiments is now known as the (weak) equivalence principle
The equivalence principle is the hypothesis that the observed equivalence of gravitational and inertial mass is a consequence of nature. The weak form, known for centuries, relates to masses of any composition in free fall taking the same t ...
. Galileo's hypothesis that inertial mass (resistance to acceleration) equals gravitational mass (weight) was extended by Albert Einstein
Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...
to include special relativity
In physics, the special theory of relativity, or special relativity for short, is a scientific theory of the relationship between Spacetime, space and time. In Albert Einstein's 1905 paper, Annus Mirabilis papers#Special relativity,
"On the Ele ...
and that combination became a key concept leading to the development of the modern theory of gravity, general relativity
General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...
. Physical experiments following Galileo increased the precision of the equivalence to better than one part in a trillion.
See also
* Delft tower experiment
* Terminal velocity (An object dropped through air from a sufficient height will reach a steady speed, called the terminal velocity, when the aerodynamic drag force pushing up on the body balances the gravitational force (weight) pulling the body down.)
* Nordtvedt effect
* Newton's second law
* Law of Inertia
Notes
Further reading
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External links
Galileo experiment on the Moon
The Hammer-Feather Drop in the world’s biggest vacuum chamber
{{Galileo Galilei
History of Pisa
Physics experiments
Leaning Tower of Pisa experiment
Scientific folklore
Articles containing video clips
1589 in the Grand Duchy of Tuscany
1589 in science
Thought experiments in physics