Aristotle's idea on circular motion

1. © ABCC Australia 2015 www.new-physics.com
CIRCULAR MOTION
Historic Introduction to

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

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

4. © ABCC Australia 2015 www.new-physics.com
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

5. © ABCC Australia 2015 www.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 www.new-physics.com
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.

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

9. © ABCC Australia 2015 www.new-physics.com
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 a
pull.
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.

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

11. © ABCC Australia 2015 www.new-physics.com
CIRCULAR MOTION
To be continued in:
ABCC