CM [002] Cause of Motion in Ancient Greece

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MOTION IN ANCIENT GREECE CM [002] In search for the cause of motion


What makes a thing move?With things in motion, there comes the question of what makes an object move. Why do things fall? Who sets the heavenly bodies in motion? What is done on an object to make it achieve a velocity?


It Started with the Greeks

Normally the average person would not have concerned with such a question. Motion is natural. We walk from one location to another; we push and pull so as to make an object move; the sun rises and falls . . . But the ancient Greeks did not stop at that.


Aristotle Physics

The first comprehensive & systematic set of ideas about the physical world was developed by the great Greek philosopher Aristotle (384–322 BC) in the fourth century BC. It is often referred to as Aristotelian physics and the concept of a force is an integral part of Aristotelian cosmology.

Aristotle & his geocentric universe


Observations by Aristotle

Aristotle is a genius with very sharp observation and keen analytic power. However he was heavily handicapped because he did not have the notion of experimental instruments to carry them out. Neither did he has the correct concept of gravitation or the force of friction to start with. This is why he often made astounding but erroneous assumptions which led to complicated but also erroneous deductions. He can be considered as a genius with two heads like the god Janus but with one of them blind.

Blind


Celestial Motion

Aristotle did observe that:

❶ The stars and heavenly bodies circulate permanently around the earth as if moved by angel or god.

According to modern mechanics, celestial motion is the combined effect of linear motion and gravitation. Aristotle did not understand the interaction of gravitation and linear motion and came to the notion that motion is initiated by angels or god himself who decreed that the heavenly bodies should circulate permanently around the earth.

We leave celestial motion to the discussion on circular and orbital motions of celestial objects for later.


Gravitational Motion

❷ Things that have gone up will come done naturally towards the centre of the earth. Example is the fall of Icarus when his wings were melted by the heat of the sun as he went far too high and too close to the sun.

We know that this motion is rectilinear and is caused by the force of gravity. Aristotle did not understand these and so made such a classification.

This class of motion is to be studied in our discussions on gravitation.


Forced Motion

❸ Things on earth move because the application of force. So they are called forcedor unnatural motion by Aristotle.

This is the class of motions we are familiar with in our daily life. They are always interpreted as our bodily effort of a push or a pull.

However Aristotle also observed that things on earth will stop moving in the absence of continuous force application. He did not know that it is actually friction that is working against the motion.This is the misconception that acts as the prelude to our present discussion.

Force Friction

Forced motion

No motion


Motions with Movers

Aristotle concluded with the assumption that an object cannot move by itself because every movement depends on there being a mover – some body is needed to push or pull them.

Mover

Moved

Mover


Problem with Projectiles

But what about projectiles? They don’t obey Aristotle’s rule because they keep on moving after leaving the thrower or bow without the need of further force applied to them.


Air Behind Projectile Motions

Aristotle maintained that after leaving the shooter’s bow, the arrow still continues to move because a new force is still transmitted to the arrow through a medium such as air. It is the air closing in behind a moving arrow propelled it so that it will keep on moving until it drops.

❸Air turns around and pushes

the projectile forward.

❶ Air being pushed forward by stone in front and so turns back.

❷ Air flows to the side and then back of the stone.


Cannon Ball Trajectory

Thus according to Aristotle, a cannon ball will shoot up straight at first and then drops dead straight to the ground.

A medieval drawing showing a cannon ball’s trajectory.


Medieval Age

The Middle Ages in European history lasted from the fall of the western Roman Empire (circa A.D. 395) to the Renaissance in the 14th or 15th century. During this period, the civilisation of the Greeks and Romans were replaced by barbarism. Most of the ancient teachings and their records were lost or destroyed. Fortunately some of them were copied and preserved in the Muslim countries. The devastation of Greek and Roman civilizations


Revival of Greek Learnings

From the thirteenth century onward these writings came to see the light again in Europe. They were recovered as rare copies from forgotten corners in the attic or store rooms. Some of them were also brought back from the Muslim countries translated in Latin. After emerging from a thousand years of darkness, the European civilisation was in a badly retrograded shape. The ancient knowledge and philosophies appeared so far superior to theirs that they were treated with almost superstitious reverence. The teaching of Aristotle in particular became the guiding light of the time.


Authority of Aristotle’s Teachings

The weight of Aristotle's teachings were later further enhanced by St. Thomas Aquinas (1224-1274 B.C.) who brought them in line with the Bible, making them the answer books to all scientific enquiries.

In the centuries to come Aristotle's philosophy was regarded as the ultimate truth throughout Europe. When Aristotle said that a heavy rock falls faster than a lighter one, no one else would have thought otherwise.


PHILOPONUS’ IMPETUSTo be continued on CM [003]

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CM [002] Cause of Motion in Ancient Greece