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Anatomy of a fall: Giovanni Battista Riccioli and the story of g

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The Asinelli Tower,which Giovanni Riccioli considered to be “as commodious as possible” to falling-body experiments, stands nearly 100 m above the heart of Bologna, Italy. (a) Riccioli’s sketch illustrates his experimental findings: A ball dropped from the tower’s summit, point O, reaches points C, Q, R, S, and T in times corresponding to 5, 10, 15, 20, and 25 pendulum strokes, respectively. (Image from ref. 14.) (b) A photo shows the tower as seen today.

The Asinelli Tower,which Giovanni Riccioli considered to be “as commodious as possible” to falling-body experiments, stands nearly 100 m above the heart of Bologna, Italy.
(a) Riccioli’s sketch illustrates his experimental findings: A ball dropped from the tower’s summit, point O, reaches points C, Q, R, S, and T in times corresponding to 5, 10, 15, 20, and 25 pendulum strokes, respectively.
(b) A photo shows the tower as seen today.

Christopher M. Graney
Every physics student learns about falling bodies and g, the acceleration due to Earth’s gravitational field. But few physicists learn the story of the first experiments—now more than three centuries old—to measure g.
That story begins in earnest with the famed Italian astronomer Galileo Galilei. In his 1632 tome, Dialogue Concerning the Two Chief World Systems, Galileo writes that the acceleration of straight motion in heavy [falling] bodies proceeds according to the odd numbers beginning from one.
That is, marking off whatever equal times you wish . . . if the moving body leaving a state of rest shall have passed during the first time such a space as, say, an ell, then in the second time it will go three ells; in the third, five; in the fourth, seven, and it will continue thus according to the successive odd numbers.
In sum, this is the same as to say that the spaces passed over by the body starting from rest have to each other the ratios of the squares of the times in which such spaces were traversed.
To Giovanni Battista Riccioli—an astronomer, Jesuit priest, and fellow Italian—Galileo’s claims were dubious, especially the assertion that an iron ball dropped from a height of 100 cubits took five seconds to reach the ground.
The ball seemed too heavy, and the time of fall too long, to be plausible. Plus, Galileo had provided few details about his experimental procedure.
So Riccioli conducted his own free-fall study. His experiments, which for the most part vindicated Galileo’s theory, have come to be regarded by historians as the first precise measurements of g.
Although historians of science have discussed the experiments in some detail, Riccioli’s own report has yet to be fully translated into a modern language. That remains the physics world’s loss, for Riccioli’s report on falling bodies tells the story of a remarkable experiment performed by a remarkable scientist….
Read more at www.physicstoday.org or scitation.aip.org

Written by physicsgg

May 5, 2013 at 8:54 am

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