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.

Medieval criticisms

The Aristotelian theory of gravity was first criticized and/or modified by John Philoponus and later by Muslim physicists during the Middle Ages. Ja'far Muhammad ibn Mūsā ibn Shākir (800-873) of the Banū Mūsā wrote the Astral Motion and The Force of Attraction, where he discovered that there was a force of attraction between heavenly bodies,^[1] foreshadowing Newton's law of universal gravitation.^[2] Ibn al-Haytham (965-1039) also discussed the theory of attraction between masses, and it seems that he was aware of the magnitude of acceleration due to gravity and he discovered that the heavenly bodies "were accountable to the laws of physics".^[3] Abū Rayhān al-Bīrūnī (973-1048) was the first to realize that acceleration is connected with non-uniform motion, part of Newton's second law of motion.^[4] In 1121, al-Khazini, in The Book of the Balance of Wisdom, proposed that the gravity and gravitational potential energy of a body varies depending on its distance from the centre of the Earth.^[5] Hibat Allah Abu'l-Barakat al-Baghdaadi (1080-1165) wrote a critique of Aristotelian physics entitled al-Mu'tabar, where he negated Aristotle's idea that a constant force produces uniform motion, as he realized that a force applied continuously produces acceleration, a fundamental law of classical mechanics and an early foreshadowing of Newton's second law of motion.^[6] Like Newton, he described acceleration as the rate of change of velocity.^[7]

Modern criticisms

Aristotle's theory was superseded by the work of Galileo Galilei in Europe. 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 modified 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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