sign in sign up
<<
Home , John Philoponus

Intermediary Structural Analysis

Victor E. Saouma [email protected]

University of Colorado, Boulder

Fall 2017

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 1/12 Table of ContentsI

1 What is Force Aristotelian Medieval Newtonian

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 2/12 What is Force

What is Force?

Force is in general defined as the cause of acceleration. Since acceleration is observable, its cause must be something real. Thus, force is real. However, some physicists and philosophers of have, pointed out that force does not exist in reality. Broadly speaking, we can distinguish two movements: Aristotelian (pre 17th century) Newtonian (17th century onward) may possibly add one “Einstenian”. Excellent reference Concept of Force by M. Jammer.

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 3/12 What is Force Aristotelian

AristotelianI

Aristotle (384 BC-322 BC), pupil of , tutor of Alexander the Great, writings cover , biology, zoology, metaphysics, logic, ethics, aesthetics, poetry, theater, music, rhetoric, linguistics, politics and government and constitute the first comprehensive system of .

Rather than assigning a “divine” explanation of nature, through observation he sought to discover the principles and causes of (and not merely to describe) and then developed systematic set of ideas by categorizing things: most famously believed that earth was made up of four main elements: earth, air, water, and fire.

Likewise, he classified motion as either Natural associated with celestial motion (circular), or terrestial (rectilinear). Violent: Force responsible for motion had to be in constant physical contact with the moved body.

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 4/12 What is Force Aristotelian

AristotelianII

Motion of a projectile was a difficult one (as opposed to a cart being pulled by a horse). He thought that the medium somehow, provided the necessary force to push the projectile. Hence, the medium both sustains and resists the motion (V ∝ F/R). In a vacuum, there is no medium to sustain the motion, and in the absence of resistance the object will move at infinite speed! (thus Nature abhors vacuum).

His book Physics is one of the foundational books of Western science and philosophy. His physics has no present-day equivalent, as he really dwelled in the philosophy of nature.

From a scientific point of view, Aristotelian fusion of metaphysics and physics, was a disaster. For almost 19 centuries, his work was revered in a way that stifled further advances. For example, his assertion of the priority of rotating motion over rectilinear motion hindered discovery of mechanical laws (Bache, Kepler, Newton).

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 5/12 What is Force Medieval

MedievalI

Aristotelian theories, by virtue of his aurae, survived for many centuries. First major challenge came from John Philoponus, also known as John of (or John the Grammarian), (ca. 490-ca. 570 CE) Philoponus considered that the projectile acquires a motive force or impressed force that keeps it moving and thus cause of movement is internal and not external (as assumed by Aristotle). Hence, Philoponus is credited with establishing some of the foundations for modern, more critical thought and scientific analysis, breaking with some of the more flawed aspects of Aristotelian and Platonic logic by disproving their theory of gravity. Another critic of Aristotelian “theory of impetus” was Hibat Allah Abu’l-Barakat al-Baghdaadi (1080-1165) who wrote Kitab al-Mu’tabar, where he negated Aristotle’s idea that a constant force produces uniform motion. He made a distinction between velocity and acceleration and showed that force is proportional to acceleration rather than velocity, thus foreshadowing Newton’s second law of motion.

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 6/12 What is Force Medieval

MedievalII

In the 11th century, Abu-Ali-al-Husayn ibn AbdAllah ibn Sina (better known as ) revisited the work of Aristotle, and developed a more complex theory of motion. He suggested that a projectile receives from its “mover” mayla quantity related to the object’s heaviness (weight) which helps it carry motion. Hence, every object which accepts the “act of yielding” (al-mayl) from the outside has to have some natural inclination mayl tibaci which would be reduced by friction. It is not really clear if the mayl did also account for velocity, in which case this would have indeed been an early precursor to the concept of later developed by Newton. In the 14th century, (Johannes Buridanus; ca. 1295-1358) developed the theory of impetus, based on Avicenna’s theory of mayl and the work of John Philoponus, as an alternative to the Aristotelian theory of motion. He proposed that motion was maintained by some property of the body, imparted when it was set in motion. This motion-maintaining property was called impetus. He did also suggest that both the of air resistance and gravity will arrest the body. Buridan also suggested that the impetus imparted to a body increased with the speed with which it was set in motion, and with its quantity of

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 7/12 What is Force Medieval

Medieval III

matter. Clearly, we are now getting closer and closer to the modern concept of momentum. Contrarily to Newton, this was still a quantitative description: ...after leaving the arm of the thrower, the projectile would be moved by an impetus given to it by the thrower and would continue to be moved as long as the impetus remained stronger than the resistance, and would be of infinite duration were it not diminished and corrupted by a contrary force resisting it or by something inclining it to a contrary motion. A major pitfall of this impetus theory is that Buridan considered it to be cause and not the result of motion (a major difference).

Interestingly, here is Leonardo’s definition of force, “I say that force is a spiritual virtue, an invisible power, which, through accidental exterior violence, is caused by motion and placed and infused into bodies which are [thus] removed and deviated from their natural use, giving to such virtue an active life of marvelous power”.

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 8/12 What is Force Medieval

MedievalIV

Despite its revolutionary implications, Buridan did not use the concept of impetus to transform the science of . He was not, a forerunner of Galileo, and remained unapologetically Aristotelian in too many other respects.

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 9/12 What is Force Newtonian

Newtonian

Beginning in the 16th century, scientists began applying mathematics to the physical , and Aristotle’s work in this area was deemed hopelessly inadequate. Newton struggled to free himself of the idea of impetus. Through his discovery of gravity, Newton combined the celestial and terrestrial into a unified theory (the force inducing the fall of an apple is the same that keep the moon orbiting). It was Kepler, Galileo, Leibniz, Descartes, Newton, and many others who, collectively, completed the mechanical theories which we now know as Classical or Newtonian mechanics.

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 10/12 What is Force Newtonian

Einstein

Newton may have discovered gravity, but had no idea how it did work. He assumed that it acts instantaneously. Einstein determined that light does not travel instantaneously, and nothing can travel faster than light, not even gravity. He looked at space and time as intertwined together into a curved fabric, matter warps this space, and it is the warping or curving of that space-time that create what we feel as gravity. This is the essence of the general relativity theory. Special relativity is the one that gave e = mc2 and assumes an inertial frame of reference (no acceleration).

Victor E. Saouma; CVEN 4525/5525; Univ. of Colorado Introduction 11/12