|University of Missouri – Columbia
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Galileo Galilei (1564-1642) was the first to question the assertion of Aristotle that bodies fall with velocities proportional to their weights. He dramatically refuted this Aristotelian doctrine by dropping from the Tower of Pisa two iron balls of very unequal weight. This demonstration failed to convince a part of the faculty of the University of Pisa who tried to explain the phenomenon as an optical illusion or as caused by the intervention of the Devil. Galileo soon found untenable his first hypothesis that there were equal additions of velocity as the object fell equal distances and he held instead that there were equal additions to the velocity in equal intervals of time. He showed that the distance passed over would be proportional to the square of the time of falling and proved this conclusion experimentally by means of the inclined plane. To measure intervals of time he weighed the water which flowed from an orifice while the ball was rolling down the plane. Galileo observed also that as nearly as he could judge, using his pulse as a chronometer, a pendulum made oscillations in equal intervals of time regardless of the amplitude; he showed that the period of a pendulum was proportional to the square root of its length and suggested the pendulum as a means of measuring the passage of time.