Aristotelian theory of gravity
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The Aristotelian theory of gravity was a theory that stated that all bodies move towards their natural place. For some objects, Aristotle claimed the natural place to be the center of the earth, wherefore they fall towards it. For other objects, the natural place is the heavenly spheres, wherefore gases, steam for example, move away from the centre of the earth and towards heaven and to the moon. The speed of this motion was thought to be proportional to the mass of the object.
Aristotle's theory was superseded by the work of Galileo Galilei. According to legend, Galileo dropped balls of various densities from the Tower of Pisa and found that lighter and heavier ones fell at almost the same speed. In fact, he did quantitative experiments with balls rolling down an inclined plane, a form of falling that is slow enough to be measured without advanced instruments.
The confusion is due to the fact that a heavier body is less affected by the resistance of air than a lighter one of the same shape, and a heavier body has a proportionally larger mass, meaning that the greater force on it accelerates it no faster.
On the surface of the moon, David Scott famously repeated Galileo's supposed experiment by dropping a feather and a hammer from each hand at the same time. In the absence of a substantial atmosphere, the two objects fell and hit the moon's surface at the same time.
With his law of universal gravitation Isaac Newton was the first to mathematically codify the newer theory of gravity according to which any mass, not only the Earth, is attracted to other masses by a function of their mass and the inverse square of their distance.
In 1915, Newton's theory was advanced by Albert Einstein, who developed a new picture of gravitation, in the framework of his general theory of relativity. See gravity for a much more detailed complete discussion.
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