PM [01] Circular Motion by Greeks

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CIRCULAR MOTIONHistoric Introduction to


Moving around

We see the stars and planets circulating above us. We see things turn, spin, or rotate.It is different from linear motion where things move in a straight line.


What makes a thing move in a circular way?

With things in circular motion, there comes the question of: What makes an object move in a circular way? Who sets the heavenly bodies in their circular motion?

Normally the average person would not have concerned with such a question. The sun and moon just rise and fall; the star move around . . . But the ancient Greeks did not stop at that.


Aristotelian 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 objects moving in a circle is an integral part of Aristotelian cosmology.

Aristotle & his circular 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


Forced Motion

Aristotle observed that:

❶ Things on earth move because the application of force. So they are called forced or 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.


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.


Celestial Motion

❸ 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.


Categories of Motions

To sum up, Aristotle divided motions into two categories:

1. One is ‘natural’ motion inherent in bodies such as that of the planets and stars; in general, all heavenly motions are circular.

2. The other is ‘unnatural’ or ‘forced’ motion - an emanation from substance such as a push or apull.

This way of dividing motion is wrong in our eyes. But they were held as the truth more than a thousand years into the dark ages.


Attention on Circular Motion

The phenomenon of heavenly bodies moving in circles led to the concept of geocentrism, putting earth at the centre of the circles and thus that of the universe. We have talked about geocentrism in our discussion on the universe in Cosmos [01] to [06]. Now we are turning our attention to circular motion which is a kind of motion distinct from linear motion.


CIRCULAR MOTIONTo be continued in:

ABCC

Science

PM [01] Circular Motion by Greeks