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 movingaround 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.

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.