Newton’s First Law of Motion - Inertia
Ancient Greek Science
The ancient Greeks were first to propose comprehensive models of the
universe. A major player in one of the models was Aristotle. He presented
evidence that the Earth was the stationary, center of the Universe. His model is
called "Geocentric."
 The Earth is massive - easier to assume the things in the sky are moving.
 It looks like the sky is moving and we are standing still.
 If the Earth were spinning, objects would fly off of the surface.
 No observed stellar parallax (the shifting of a star position that must occur
if the Earth orbits the Sun)
Aristotle divided the universe into two realms. In the Terrestrial Realm motion
could be predicted only when the composition of the object was known.
Elements had natural tendencies of motion. Earth and water tended to sink,
while air and fire tended to rise. Objects that followed their natural tendencies
(natural motion) required no forces. Opposing the natural tendency (violent
motion) required force. There was an overall tendency in the Terrestrial Realm
to seek rest. Likewise in the Terrestrial Realm everything was corruptible
(changing).
Rules are different in the Celestial Realm. Here objects glowed but not by
fire. It was called Aether and had the property of glowing without consuming.
Motion of the celestial objects was constant and circular. Also celestial objects
were incorruptible. Objects like meteors or comets must belong to the Terrestrial
Realm in the upper Earth's atmosphere.
Aristarchus of Samos preferred the "Heliocentric" model with the Sun at the
center and the planets orbiting it. The lack of stellar parallax must now be
because the stars are too far away to have perceptible shifts. Thus the
Heliocentric model must be huge, an idea that ancient astronomers were not
prepared to accept. The model that comes down through history is the
Geocentric Model.
After the fall of Rome the knowledge of the ancient world moves into the new
world of Islam. These astronomers were not model builders, but they were very
good sky observers, who expanded the Greek models. In Europe illiteracy was
almost universal, communications were poor, and the Alexandrian library was
destroyed.
The Geocentric model is later tied to the teachings of the Christian religion
through the work of Thomas Aquinas.
The Renaissance
We can track the changes in scientific thought during the Renaissance
through the contributions of five men: Nicholas Copernicus, Tycho Brahe,
Johannes Kepler, Galileo Galilei, and Isaac Newton.
Copernicus taught Aristotelian philosophy at the University. He thinks the
old model of Ptolemy/Aristotle is too complex to be correct. Copernicus believes
the heliocentric model must be correct. The Earth is now placed as the third
planet moving around the Sun. The new heliocentric model was no more
accurate than the system of Ptolemy, in part because Copernicus still had
circular orbits.
Tycho Brahe was a late 16th century Danish nobleman who carried out an
extensive observing program of the planets. He believed that only through
observations could we discern one model from another. After being expelled
from Denmark and settling in Prague, he hired Johannes Kepler to show what
the orbits of the planets were.
Johannes Kepler devised the very first natural laws with his laws of
planetary motion. The first law showed that planets orbit the Sun in elliptical
paths, the Sun being at one focus of the orbit. The second law tells us how the
planets move on their orbits - faster closer to the Sun, slower farther away. The
third law relates the orbital period to the size of the orbit. The laws of planetary
motion were empirical and universal, although Kepler never correctly surmised
the cause of the orbits.
Galileo Galilei is given credit for being among the first to turn a telescope
onto the sky. In the course of three years of observing, he convincingly
overturned 1500 years of Aristotelian thought. Galileo also gave us the Law of
Inertia and the Scientific Method.
Isaac Newton closed the era by providing the three laws of general motion:
 Forces cause changes in motion.
 F = ma
 For every action force there is an equal and opposite reaction force.
These laws completely restructure physics. No longer is it necessary to know
the composition of an object in order to predict its motion. The artificial Celestial
and Terrestrial Realms have disappeared.
The First Law of Motion (the Law of Inertia) states that an object will continue
moving in exactly the way it had been moving, unless it is acted upon by external
forces. Galileo did much of the early experimental work in establishing this law.
In rolling a ball on an inclined plane, the ball speeds up going down the incline
and slows down going up the incline. Galileo determined that the speed of the
ball was given by the slope of the incline. The greater the slope the greater the
speeding up and slowing down of the ball.
The ball stopped on the second incline at the same vertical height from which
it had started. If the second incline were made shallower, then the ball would roll
farther before coming to rest. But if the second incline had zero slope (no incline
at all), then the ball would roll forever. The ball has a property which resists
changes in its motion. We call this property inertia and it is identical to mass.
In deciding whether an object will have its motion changed, it is important to
realize that force is a vector, and that the vector sum, or net force, must be
considered in the first law. Vector addition takes direction into account. Simply
lay the vectors off tip to tail to get the vector sum as in the example below.
B
A
A+B
If the net force is not zero, then the motion will be changed.
The special situation of the net force equal to zero (F = 0) is called
equilibrium. Equilibrium implies zero acceleration, but this does not necessarily
mean that there is no motion. We could have a constant velocity. An example of
equilibrium is standing on a bathroom scale. Your weight is a force directed
downward and the scale produces an equal force upward. Reaction forces of
this type that give objects support are called normal forces because they can
only act perpendicular (normal) to the surface.
Be able to distinguish between mass (the quantity of matter in an object) and
weight (the force of gravity acting on the object). The two concepts are related
through the acceleration due to gravity (W = mg). No matter where you go, you
will always have mass. Whether you have weight depends on the local gravity.