Everything about Aristotelian Theory Of Gravity totally explained
The Greek
philosopher Aristotle (
384 BC –
322 BC) developed many
theories on the nature of physics that are completely different from what are now understood as the
laws of physics. These involved what Aristotle described as the
four elements, as well as a variety of principles, most of which modern science has now disproved, and which provide no significant roots to any area of modern physics.
He spoke intimately of the relation between these elements, of their dynamics, and how they impacted on the earth, and were, in many cases, attracted to each other by unspecified forces. He also taught miscellaneous other aspects of his physics model, including his widely believed theory relating to moving energy.
Aristotelian physics
Aristotle taught that the
elements that composed the
Earth were different from those that made up the heavens and
Outer space.
He also taught that
dynamics were mostly determined by the characteristics and nature of the
substances that the object that was moving was composed of.
He also held that all the heavens, and every particle of
matter in the
universe, was formed out of another, fifth element he called '
aether' (also transliterated as "ether")
Heavy substances such as
iron and metals were considered to be primarily consisting of the "element" of earth, with a limited amount of
matter from the other elements. Other, less heavy and/or dense objects were thought to be less earthy, and composed of a greater mixture of the other elements. Therefore, if all the applied forces on Earth were removed, such as the throwing of a rock, then nothing could move. This idea had flaws that were visible even at the time the concept was formulated. Many people questioned how an object such as an
arrow could continue to move forward after it had left the power released by the string, and continue to sail forward. Aristotle proposed an idea that arrows, etc., created a type of
vacuum behind them that forced them forward.
Gravity
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, foreshadowing
Newton's law of universal gravitation.
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".
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. He also criticized the Aristotelian theory of gravity for denying the existence of 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
velocity.
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:
Life and death of Aristotelian physics
The reign of Aristotelian notions of physics lasted for almost two millennia, and provide the earliest known speculative theories of physics. After the work of
Alhacen,
Avicenna,
Avempace,
al-Baghdadi,
Jean Buridan,
Galileo,
Descartes,
Isaac Newton, and many others, it became generally accepted that Aristotelian physics were not correct or viable.
Despite this, Aristotle's physics were able to live into the late
seventeenth century, and perhaps longer, as they were still taught in universities at the time. Aristotle's model of physics was the main academic impediment for the creation of the science of physics long after Aristotle's death.
In
Europe,
Aristotle's theory was first convincingly discredited by the work of
Galileo Galilei. Using a
telescope, Galileo observed that the moon wasn't entirely smooth, but had craters and mountains, contradicting the Aristotelian idea of an incorruptible perfectly smooth moon. Galileo also criticized this notion theoretically--- a perfectly smooth moon would reflect light unevenly like a shiny billiard ball, so that the edges of the moons disk would have a different brightness than the point where a tangent plane reflects sunlight directly to the eye. A rough moon reflects in all directions equally, leading to a disk of approximately equal brightness which is what is observed . Galileo also observed that
Jupiter has
moons, objects which revolve around a body other than the Earth. He noted the
phases of venus, convincingly demonstrating that Venus, and by implication Mercury, travels around the sun, not the Earth.
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's slow enough to be measured without advanced instruments.
Since Aristotle didn't believe that motion could be described without a surrounding medium, he couldn't treat
air resistance as a complicating factor. A heavier body falls faster than a lighter one of the same shape in a dense medium like water, and this led Aristotle to speculate that the rate of falling is proportional to the mass and inversely proportional to the density of the medium. From his experience with objects falling in water, he concluded that water is approximately ten times denser than air. By weighing a volume of compressed air, Galileo showed that this overestimates the density of air by a factor of forty. From his experiments with inclined planes, he concluded that all bodies fall at the same rate neglecting friction.
Galileo also advanced a theoretical argument to support his conclusion. He asked if two bodies of different masses and different rates of fall are tied by a string, does the combined system fall faster because it's now more massive, or does the lighter body in its slower fall hold back the heavier body? The only convincing answer is neither: all the systems fall at the same rate.
Followers of Aristotle were aware that the motion of falling bodies wasn't uniform, but picked up speed with time. Since time is an abstract quantity, the
peripatetics postulated that the speed was proportional to the distance. Galileo established experimentally that the speed is proportional to the time, but he also gave a theoretical argument that the speed couldn't possibly be proportional to the distance. In modern terms, if the rate of fall is proportional to the distance, the differential equation for the distance y travelled after time t is
»
with the condition that
. Galileo demonstrated that this system would stay at
for all time. If a perturbation set the system into motion somehow, the object would pick up speed exponentially in time, not quadratically .
On the surface of the
moon,
David Scott famously repeated Galileo's 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 a correct theory of gravity. In this theory, any mass is attracted to any other mass by a force which decreases as the inverse square of their distance. In 1915, Newton's theory was modified, but not invalidated, 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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