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# Galileo's Leaning Tower of Pisa experiment

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### Galileo's Leaning Tower of Pisa experiment

Hammer and Feather Drop - Apollo 15 astronaut David Scott on the Moon recreating Galileo's famous experiment. (1.38 MB, ogg/Theora format).

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]:19–21 Via this method, he supposedly discovered that the objects fell with the same acceleration, proving his prediction true, while at the same time disproving Aristotle's theory of gravity (which states that objects fall at speed relative to their mass). 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.[1]:20 This was contrary to what Aristotle had taught: that heavy objects fall faster than lighter ones, in direct proportion to weight.[1]:9[2] 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.[3][4] An exception is Drake, who argues that it took place, more or less as Viviani described it, as a demonstration for students.[1]:19–21, 414–416

Galileo arrived at his hypothesis by a famous thought experiment outlined in his book On Motion.[5] 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.

In Dava Sobel's biographical book Galileo's Daughter she recounts that the balls did not land at the same time:

The larger ball, being less susceptible to the effects of what Galileo recognized as air resistance, fell faster, to the great relief of the Pisan philosophy department. The fact that it fell only fractionally faster gave Galileo scant advantage. "Aristotle says that a hundred pound ball falling from a height of a hundred braccia [arm lengths] hits the ground before a one pound ball has fallen one braccio. I say that they arrive at the same time," Galileo resummarised the dispute in its aftermath. "You find, on making the test, that the larger ball beats the smaller one by two inches. Now, behind those two inches you want to hide Aristotle's ninety-nine braccia and, speaking only of my tiny error, remain silent about his enormous mistake."
—Dava Sobel , Galileo's Daughter[6]

## Contents

• 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

## Notes

1. ^ a b c d Drake, Stillman (2003). Galileo at work : his scientific biography (Facsim. ed.). Mineola (N.Y.): Dover publ.
2. ^ Sharratt, M. (1994). Galileo: Decisive Innovator.
3. ^ Groleau, R. (July 2002). "Galileo's Battle for the Heavens".
4. ^ Ball, P. (30 June 2005). "Science history: Setting the record straight".
5. ^ Van Helden, Albert (1995). "On Motion". The Galileo Project. Retrieved 16 October 2013.
6. ^ Sobel, Dava (1999). Galileo's daughter a historical memoir of science, faith, and love. New York: Walker & Co. p. 20.