Concept of cause of motion in ancient Greece. Aristotle's idea on motion and his categories of motion. Concept of mover and moved. Demise and rise of Greek learning.

1. © ABCC Australia 2015 new-physics.com
MOTION IN ANCIENT GREECE
CM [002] In search for the cause of motion

2. © ABCC Australia 2015 new-physics.com
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?

3. © ABCC Australia 2015 new-physics.com
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.

4. © ABCC Australia 2015 new-physics.com
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

5. © ABCC Australia 2015 new-physics.com
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

6. © ABCC Australia 2015 new-physics.com
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.

7. © ABCC Australia 2015 new-physics.com
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.

8. © ABCC Australia 2015 new-physics.com
Forced Motion
❸ 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.
This is the misconception that acts as the
prelude to our present discussion.
Force Friction
Forced motion
No motion

9. © ABCC Australia 2015 new-physics.com
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

10. © ABCC Australia 2015 new-physics.com
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.

11. © ABCC Australia 2015 new-physics.com
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.
❸
Air 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.

12. © ABCC Australia 2015 new-physics.com
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.

13. © ABCC Australia 2015 new-physics.com
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

14. © ABCC Australia 2015 new-physics.com
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.

15. © ABCC Australia 2015 new-physics.com
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.

16. © ABCC Australia 2015 new-physics.com
PHILOPONUS’ IMPETUS
To be continued on CM [003]
ABCC