Galileo, perhaps more than any other single person, was responsible for the birth of modern science – Steven Hawking [1]

[Galileo] stood with one foot in the neatly ordered cosmos of medieval philosophy and with the other greeted the dawn of the mechanical universe – Richard Westfall [2]

Galileo Galilei (1564-1642) was fascinated by motion.  As recounted here, to fully comprehend Galileo’s contributions to our understanding of motion, one must first recognize his early immersion in the paradigm existing at that time based largely on Aristotelian physics. As a student in Pisa and teacher in Padua, Galileo spent years absorbing the teachings of the Ancients and fully embraced Aristotle’s and his successors’ ideas about motion.  Such teachings helped set the stage for the ensuing conflict; the path blazed by Aristotle frustrated many.  But while many were thus stymied, Galileo wasn’t.  He simply created a path out.

Galileo’s break from Aristotle

Galileo most likely began his break from Aristotle in 1592 [3], his first year of teaching at Padua University, when he was exposed to, ironically enough, non-academic science.  On his way to Padua from Pisa, Galileo is suggested [4] to have visited his benefactor Guidobaldo del Monte, a man of learning and wealth, at his home.  The two had started to exchange letters in 1588, and Guildobaldo, suitably impressed, endorsed Galileo’s teaching position at Padua.

During this likely visit, historians further speculate that Galileo and Guidobaldo collaborated in an experiment concerning projectile trajectory.  The details here are unclear regarding who owned the key idea, but most believe it was Galileo.  Regardless, the outcome was clear. 

The Photograph

As summarized later by Galileo in Dialogues Concerning Two New Sciences, when one rolls a small metal ball, made warm and damp from contact with a person’s hand, or alternatively covered in ink, across an inclined metal mirror at an upward angle, beautifully symmetrical parabolas are traced.

Discovery of the parabolic shape of a projectile’s trajectory

This discovery of the parabolic shape of a projectile’s trajectory in 1592 was monumental.  Cameras obviously did not exist back then.  So Galileo’s concept of capturing such a dynamic event on such a creative “film” was truly remarkable.  It afforded him the opportunity to actually look at the shape, frozen in time, and realize its symmetry.  Many had wondered and speculated.  Galileo rolled up his sleeves and got the data point.

But what was so monumental about this symmetry?  Well, in the absence of fact, opinion dominates.  And the prevailing opinion at that time, based on Aristotle’s philosophy, was that a projectile’s trajectory could not be symmetrical since it is determined at the beginning and at the end of its path by different causes.  At the beginning, it is dominated by the “violent” impetus impressed into it, and at the end by its “natural” motion towards the center of the earth.[5]  Per this opinion, a trajectory based on two very different causes could not be symmetrical. Galileo’s data point proved this wrong.

Nowadays high school physics students learn that this parabolic shape results from the combination of a constant velocity in the horizontal direction and a downward accelerating velocity in the vertical direction. This would be an excellent moment to educate the students on who discovered this parabolic shape : )

Thank you for reading my post on Galileo’s discovery of the parabolic motion of a projectile. I go into much greater detail on Galileo’s contributions [6] to the science of motion and energy in Chapter 7 of my book Block by Block – The Historical and Theoretical Foundations of Thermodynamics.

References

[1] Hawking, Stephen W. 1988. A Brief History of Time: From the Big Bang to Black Holes. A Bantam Book. Toronto: Bantam Books, p. 179.

[2] Westfall, Richard S. 1971. Force in Newton’s Physics: The Science of Dynamics in the Seventeenth Century. American Elsevier, New York, p. 42.

[3] Renn, Jürgen, Peter Damerow, Simone Rieger, and Domenico Giulini. 2000. “Hunting the White Elephant: When and How Did Galileo Discover the Law of Fall?” Science in Context 13 (3–4): 299–419.

[4] (Renn et al., 2000) p. 299.  “In spite of being the subject of more than a century of historical research, the question of when and how Galileo made his major discoveries is…still only insufficiently answered.”  Alas, history can only be based on the fossils we find.

[5] Violent motion was thought to be any forced motion that does “violence” to the natural order.  For example, an upward motion would be violent since it violated the “natural” downward motion towards the center of the earth.  Natural motion is motion freed from all constraints.

[6] As a general comment, Galileo was at times frustrating to study since he didn’t always record what he did or the thought process behind his conclusions.