Online Astronomy eText: Background Physics: Motion and Forces
Notes on Motion Link for sharing this page on Facebook
(rough lecture notes comparing Hellenic and modern notions of motion)

How do things move?

On the Earth, there are lots of different kinds of things, which we can subdivide into groups and study, and make some effort to understand their basic nature.

We can define the ELEMENTS as fundamental (elementary) materials, out of which all others are made. Nowadays, there are more than 100 elements.In ancient (Hellenic) times, most philosophers presumed there were four elements (although there was great disagreement as to which they were).

Most familiar to those living nowadays (thanks in part to better press) is Aristotle and his elements of choice: fire, earth, air, water (defining in terms of the way they move, and their gross properties)

Things on the Earth (made of out earth) tend not to move around, unless they’re alive. You can make them move, by exerting forces (push/pull) on them, but they resist that with a sluggishness we call Inertia (related to their desire to not move).

There is one situation in which inertia does not seem to work. Pick something up, and let go.

(It falls, seems to violate the principle of inertia)


If something is in its natural place, inertia will make it want to not move; but if it is NOT in its natural place, it will do all it can, to get there.


(1) things fall (to get back to their natural place)

(2) things have weight (trying to get there)

Heavier things are more anxious to reach earth

SO, they presumably want to fall more, and fall faster (WRONG, but looked reasonable, because of poor experiments)

IMPORTANT NOTE: IN PHYSICS, A ‘THEORY’ IS A WELL-ESTABLISHED IDEA, with lots of support from other parts of physics, and a heck of a lot of support from direct observational evidence. BUT IN THE ABSENCE OF OBSERVATIONAL EVIDENCE, a theory may not be very well founded (and in fact, would normally be ignored, if other, better theories are available)

Ptolemaic Theory ― the Earth is fixed, and everything moves around us.

Copernican Theory ― the Sun and stars are fixed, and all the planets INCLUDING THE EARTH move around the Sun.

How to pick between these? Consider how things rotate, on the Earth, and in the sky:

Theory of rotation #1: The Earth moves to the East, and makes the stars appear to move to the West.

Theory of rotation #2: The Earth sits here, immovable, and the stars really do move to the West.

Suppose the stars really do move, over and over, along circular paths. WHY?


This material LIKES to move in uniform circular motion, without ever speeding up or slowing down, even though it isn’t getting anywhere.

The planetes (planets), or WANDERERS, have like the stars, uniform circular motion around the sky, but they have MORE THAN ONE MOTION.

Tycho’s choice:

He sees no parallax for stars, whatsoever.

For things that are close, like the Moon, you have parallax (even from the surface of Earth); for things further away, like comets, you have no parallax (using a small motion, like our rotation).

SO, if USING THE ORBITAL MOTION OF THE EARTH, YOU SEE NO PARALLAX, presumably the stars are very far away, OR we are not moving.

Copernican cosmos― the Sun is fixed, the Earth and other planets move around it.

Ptolemaic cosmos ― the Earth is fixed, and everything moves around it.

Tychonic cosmos ― the Earth is fixed, and the Sun, Moon and stars move around the Earth, but the planets move around th
e Sun.

BIG CHANGE (Revolution) is due to Galileo.

Experiments in physics (motion of things HERE)

Summary of results:

Things rolling down inclined planes accelerate.

Things rolling up inclined planes decelerate.

If neither rising nor falling, speed is constant.


INERTIA is a resistance to a CHANGE in motion