Weights make haste: Lighter linger

By P. Weiss

PISA, Italy, December 1612—In a new test of an old idea about motion, philosophers recently dropped objects from the cathedral bell tower, which tilts because of a construction flaw. The experimenters observed that large, heavy bodies fall faster than small, light ones of the same material—a behavior of matter described long ago by Aristotle but often disputed in recent decades.

Not only did the investigators witness a difference in speed, but they also noted that "in proportion as the weight increases, so does the velocity," says Giorgio Coresio, professor of Greek at the University of Pisa, who led the study.

"Thus was confirmed the statement of Aristotle, in the first book of De Caelo, that 'a mass of gold or lead, or of any other body endowed with weight is quicker in proportion to its size,'" Coresio concludes. He describes the experiment in a new book Operetta intorno al Galleggiare de Corpi Solidi.

Skeptics of Aristotle's statement say that they remain unconvinced, however. Such a test of the ancient assertion is so dramatic "that I meant to do it myself, but I don't recall if I ever got around to it," comments Galileo Galilei, philosopher and mathematician to the Grand Duke of Tuscany.

He contends that even casual observations negate Aristotle's law and Coresio's assertions. "If two stones were flung at the same moment from a high tower, one stone twice the size of the other, who would believe that when the smaller was half-way down the larger had already reached the ground?" he asks.

Galileo adds, "How ridiculous is this opinion of Aristotle is clearer than light" when one thinks it through, applying Archimedes' notions regarding the buoyancy of bodies in a medium.

He notes that other philosophers have carried out experiments similar to that of Coresio and his colleagues but have refuted Aristotle. Fifteen years ago, Jacopo Mazzoni, also of the University of Pisa, reported that he had observed objects falling at the same speed regardless of weight (SN: 5/15/1597, p. 310) and pieces of an object descending at the same rate as the whole.

On the other hand, it has proved difficult to demonstrate that bodies of different weights fall at exactly the same rate, Galileo concedes. For instance, experiments in which he has rolled balls down inclined planes have not yielded clear-cut evidence.

Regarding Mazzoni's experiment, Coresio replies, "Perhaps he made his experiment from his window, and because the window was low, all his heavy substances went down evenly. But we did it from the top of the cathedral tower." The 190-foot-tall tower provides an unusually well-placed perch from which to launch long descents.

A 30-foot drop would surely be enough to show the difference if Aristotle's proposition were true, contends Simon Stevin, engineer to Prince Maurice of Nassau. He and a fellow experimenter reported in 1586 that they had dropped two lead balls, one 10 times the other in weight, from such a height onto a plank. They heard "the sound of the two striking ... as one single report," he says (SN: 11/7/1605, p. 293). What's more, the same also held true for balls of different materials, he notes.


Author’s note:

In popular lore, Galileo Galilei (1564–1642) dramatically refuted Aristotle’s laws of motion by dropping unequal weights from the Leaning Tower of Pisa. In the scientist’s extensive writings, however, he never claimed to have conducted an experiment from that tower. Instead, his first biographer, Vincenzo Viviani, launched the story roughly a dozen years after the great man’s death, and other authors embellished it since, often with few facts to back up their tales.

Galileo hasn’t been noted to say, as quoted in the story, that he meant to conduct the same trial as Coresio. The fictitious quote merely highlights the ambiguity surrounding the Leaning Tower legend.

All the other quotations and descriptions of experiments are genuine, however, although they do not all date from the time of Coresio’s test. As for the arguments attributed to Galileo and to Stevin, those reflect their attitudes and experiences but not their actual words.

In the early 1980s, some researchers carried out experiments to test the veracity of the Leaning Tower legend. They focused on a curious and repeated assertion in Galilean texts that lighter objects start to fall faster than heavier ones. Doubting that Galileo would have made such an apparently false statement without good cause, the researchers filmed their experiments. They then examined the movie frame-by-frame and found that Galileo was right.

The effect results not from some wrinkle in the laws of physics, but apparently from muscle fatigue in the dropper’s hands affecting the ability to release a ball. This new evidence of Galileo’s superb powers of observation has made the Leaning Tower legend more plausible to some scholars. —P.W.

Aris, R., H.T. Davis, and R.H. Stuewer, eds. 1983. Springs of Scientific Creativity. Minneapolis: University of Minnesota Press.

Cooper, L. 1935. Aristotle, Galileo, and the Tower of Pisa. Ithaca, N.Y.: Cornell University Press.

Drake, S. 1978. Galileo at Work. Chicago: University of Chicago Press.

Reston Jr., J. 1994. Galileo. New York: Harper Collins Publishers.

Further Readings:

See a fascinating selection of Galileo’s original writings on motion, both in his own hand and translated into English, at the Web site http://www.mpiwg-berlin.mpg.de/Galileo_Prototype/INDEX.HTM.

From Science News, Vol. 156, No. 25 & 26, December 18 & 25, 1999, p. vii. Copyright © 1999, Science Service.