which predicted that bodies of the same material falling through the same medium would fall at the same speed. This was contrary to what Aristotle had taught: that heavy objects fall faster than the lighter ones, and in direct proportion to their weight. While this story has been retold in popular accounts, there is no account by Galileo himself of such an experiment, and many historians believe that it was a thought experiment. An exception is Stillman Drake, who argues that it took place, more or less as Viviani described it, as a demonstration for students. Galileo set out his ideas about falling people, and about projectiles in general, in his book Two New Sciences (1638). The two sciences were the science of motion, which became the foundation-stone of physics, and the science of materials and construction, an important contribution to engineering. Galileo arrived at his hypothesis by a famous thought experiment outlined in his book On Motion. He writes: His argument is that 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. Later performances Astronaut David Scott performed a version of the experiment on the Moon during the Apollo 15 mission in 1971, dropping a feather and a hammer from his hands. Because of the negligible lunar atmosphere, there was no drag on the feather, which reached the lunar surface at the same time as the hammer. See also Delft tower experiment 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 Newton's second law Law of Inertia Notes Further reading External links Galileo experiment on the Moon Galileo and the Leaning Tower of Pisa The Hammer-Feather Drop in the world’s biggest vacuum chamber History of Pisa Physics experiments Leaning Tower of Pisa experiment Scientific folklore Articles containing video clips 1589 in the Grand Duchy of Tuscany 1589 in science Thought experiments in physics