Aristotelian Physics - Medieval Commentary

Medieval Commentary

The Aristotelian theory of motion came under criticism and/or modification during the Middle Ages. The first such modification came from John Philoponus in the 6th century. He partly accepted Aristotle's theory that "continuation of motion depends on continued action of a force," but modified it to include his idea that the hurled body acquires a motive power or inclination for forced movement from the agent producing the initial motion and that this power secures the continuation of such motion. However, he argued that this impressed virtue was temporary; that it was a self-expending inclination, and thus the violent motion produced comes to an end, changing back into natural motion. In the 11th century, the Persian polymath Avicenna, in The Book of Healing (1027) was influenced by Philoponus' theory in its rough outline, but took it much further to present the first alternative to the Aristotelian theory. In the Avicennan theory of motion, the violent inclination he conceived was non-self-consuming, a permanent force whose effect was dissipated only as a result of external agents such as air resistance, making him "the first to conceive such a permanent type of impressed virtue for non-natural motion." Such a self-motion (mayl) is "almost the opposite of the Aristotelian conception of violent motion of the projectile type, and it is rather reminiscent of the principle of inertia, i.e., Newton's first law of motion."

The eldest Banū Mūsā brother, Ja'far Muhammad ibn Mūsā ibn Shākir (800-873), wrote the Astral Motion and The Force of Attraction. The Persian physicist, Ibn al-Haytham (965-1039), discussed the theory of attraction between bodies. 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". The Persian polymath 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. During his debate with Avicenna, al-Biruni also criticized the Aristotelian theory of gravity for denying the existence of levity or gravity in the celestial spheres and for its notion of circular motion being an innate property of the heavenly bodies.

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. 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. Like Newton, he described acceleration as the rate of change of speed.

In the 14th century, Jean Buridan developed the theory of impetus as an alternative to the Aristotelian theory of motion. The theory of impetus was a precursor to the concepts of inertia and momentum in classical mechanics. Buridan and Albert of Saxony also refer to Abu'l-Barakat in explaining that the acceleration of a falling body is a result of its increasing impetus. In the 16th century, Al-Birjandi discussed the possibility of the Earth's rotation. In his analysis of what might occur if the Earth were rotating, he developed a hypothesis similar to Galileo Galilei's notion of "circular inertia", which he described in the following observational test:

"The small or large rock will fall to the Earth along the path of a line that is perpendicular to the plane (sath) of the horizon; this is witnessed by experience (tajriba). And this perpendicular is away from the tangent point of the Earth’s sphere and the plane of the perceived (hissi) horizon. This point moves with the motion of the Earth and thus there will be no difference in place of fall of the two rocks."