-Newton
| The ancient Greeks believed in Aristotelian Mechanics. In particular,
they believed in a geocentric (earth-centred) universe and that the
planets move in circular orbits. To explain observed planetary motion,
astronomers created epicycles. If this still does not fully explain the
observations, epicycles on epicycles were added to make it work.
Ptolemy (200 AD) collected these ideas and published the book,
Almagest, which remained the principal astronomy text until
Copernicus' revolutionary theory. The geocentric model is also called the
Ptolemaic model.
Nicolaus Copernicus (1473-1543), a Polish astronomer, after studying Ptolemy's work realized that some of the difficulties of the Ptolemaic model can be overcome by heliocentric (sun-centred) model. To prevent prosecution from the church he delayed the publication of his work On the Revolutions of the Celestial Spheres until just before his death. But because it is a highly controversial work in those times, few people accepted it. |
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Major improvements on the astronomical tables did
not come until the Danish astronomer, Tycho Brahe (1546-1601),
built his great observatory near Copenhagen. He collected data which
improved the accuracy of the tables by 5 times. However, his observations
were all via the naked eye, because it is not until 1610 when the first
telescope was invented. Major breakthrough in astronomy had to wait for
Johannes Kepler (1571-1619), who fitted Tycho's wealth of data to
obtain his 3 laws of planetary motion. His first two laws was published in
Astronomia nove (The New Astronomy) and his third law in
Harmonices mundi (Harmony of the World). His work helped astronomy
became 100 times more accurate than before. The 3 laws are:
Kepler's First Law (K1L): The orbits of all planets are ellipses
with the sun as one focus. |
| Galileo Galilei (1564-1642) made major contributions to Newtonian Mechanics. Among his works is Il Saggiatore, which contained the Law of Pendulum, Law of Falling Bodies and his experiments with balls and inclined planes. His experiments made him the inventor of modern experimental science. He also discovered that an object in horizontal motion will keep moving horizontally at constant speed forever (Law of Constant Motion). His greatest work is Dialogue Concerning Two New Sciences (1638). In it, he combined the Law of Constant Motion and the Law of Falling Bodies to prove that projectiles move in a parabola. In addition, he improved on the first telescopes and pointed it to the sky, making many great observations. With all these works, Galileo has paved the way to Newton's 3 laws and the Law of Gravitation. Before that, Ren� Descartes (1596-1650) generalized Galileo's Law of Constant Motion to Newton's First Law of Motion. |  |
Isaac Newton (1642-1727) produced his famous work Principia in 1687. In it, he formulated what is now known as Newtonian or classical Mechanics. His 3 Laws of Motion are familiar to everybody:
| Newton's First Law (N1L): | Every body stays in its state of rest or constant motion in a straight line, unless acted upon |
| by external forces. | |
| Newton's Second Law (N2L): | Force exerted is proportional to the rate of change of momentum and the line of action is in |
| the direction of the change of velocity. | |
| Newton's Third Law (N3L): | Every action produces an equal and opposite reaction. |
And his Law of Gravitation is stated as
| Newton's Gravitational Law (NGL): | Gravitational Force between two bodies is proportional to the product of masses of |
| the bodies and inversely proportional to the square of the distance between them. |
In his line of reasoning, he assumed the truth of N1L, N2L, N3L and K3L. Then he proceeded to deduce K2L from N1L and N2L. He proved that centripetal force = mv2/r using N2L and the definition of velocity. With the centripetal force and K3L, he proved NGL. He did not stop here. He carried on to deduce K1L from NGL and his 3 Laws of Motion. He also showed that NGL and his 3 laws always produce a circular velocity diagram.
Subsequently, NGL was confirmed time and again, and became an indispensable tool for Celestial Mechanics. Newtonian Mechanics reigned for more than 2 centuries until the rise of Einsteinian Mechanics. The latter although far superior than Newton's Theory did not totally replace it, because of the simplicity and ease of use of Newton's Theory compared to Einstein's.
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