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Hammer and Feather Drop - Apollo 15 astronaut David Scott on the Moon demonstrates that Galileo was right. (1.38 MB, ogg/Theora format).

Vivani's early biography of Galileo informs us of the story that Galileo dropped two objects of different mass from the top of the Leaning Tower of Pisa. He did so as an experiment to disprove Aristotle's theory of gravity, which states that objects fall at a speed relative to their mass. It is generally accepted that this is not a true story, but rather a fictional tale passed down among other scientific folklore.

However Galileo did a simple bit of reasoning to rebut Aristotle, which is said to be one of the earliest and simplest thought experiments. It follows a common method:

• assume to be true what you think is false,
• find a contradiction,
• logically deduce that it is therefore in fact false (there are questions about the validity of the final step, considered originally by Aristotle).^{[citation needed]}

Galileo reasoned as follows:

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.

[edit] External links

• Falling body experiments
• Galileo and the Leaning Tower of Pisa
• Galileo's Experiment at the Leaning Tower of Pisa
• Newton's Modification to Kepler's Laws