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Galileo Galilei (1564-1642) lived in this place, Villa Arcetri (also called “Villa Il Gioiello”) in Florence, from 1631 until his death. While living here Galileo wrote his most important book, Two New Sciences, in which he presented his groundbreaking discoveries in the science of motion. In this book he describes experiments he performed with different objects falling in various liquids, e.g. balls made of wax with increasing quantities of sand grains (to make them increasingly denser) falling in water. Galileo took great pains to explore phenomena related to the behavior of various objects falling in different media. In this youtube video you can see a small homemade experiment that echoes Galileo’s effort: a plastic duck and a marble which fall more or less the same way in air, but conspicuously differently in canola oil.  



Galileo of course performed experiments much more sophisticated than the simple homemade experiment described here. One of his most important (and famous) experiments was to show that in thinner and thinner media the difference between the speeds of fall of different objects becomes increasingly smaller. Take for example the plastic duck and marble appearing in this video: in canola oil, the difference between their speeds of fall (actually, acceleration) is quite obvious; in air it is hard to notice any difference. Galileo showed (by such experiments) that if we used a very dense medium in relation to oil (such as mercury), this difference would have been significantly enhanced; on the other hand, if we used a medium thinner than air (or if we performed the experiment on a high mountain, where the air itself is thinner), we would get an even smaller difference in velocities (than noticeable in the video in the case of air). Galileo’s highly important (and correct) conclusion was that in a medium with zero density (i.e. vacuum) every object falls exactly the same, i.e. in the same acceleration.