Complexity_ A Guided Tour - Melanie Mitchell [10]
Aristotle, 384–322 B.C.
(Ludovisi Collection)
Clearly Aristotle (like many theorists since) was not one to let experimental results get in the way of his theorizing. His scientific method was to let logic and common sense direct theory; the importance of testing the resulting theories by experiments is a more modern notion. The influence of Aristotle’s ideas was strong and continued to hold sway over most of Western science until the sixteenth century—the time of Galileo.
Galileo was a pioneer of experimental, empirical science, along with his predecessor Copernicus and his contemporary Kepler. Copernicus established that the motion of the planets is centered not about the Earth but about the sun. (Galileo got into big trouble with the Catholic Church for promoting this view and was eventually forced to publicly renounce it; only in 1992 did the Church officially admit that Galileo had been unfairly persecuted.) In the early 1600s, Kepler discovered that the motion of the planets is not circular but rather elliptical, and he discovered laws describing this elliptical motion.
Whereas Copernicus and Kepler focused their research on celestial motion, Galileo studied motion not only in the heavens but also here on Earth by experimenting with the objects one now finds in elementary physics courses: pendula, balls rolling down inclined planes, falling objects, light reflected by mirrors. Galileo did not have the sophisticated experimental devices we have today: he is said to have timed the swinging of a pendulum by counting his heartbeats and to have measured the effects of gravity by dropping objects off the leaning tower of Pisa. These now-classic experiments revolutionized ideas about motion. In particular, Galileo’s studies directly contradicted Aristotle’s long-held principles of motion. Against common sense, rest is not the natural state of objects; rather it takes force to stop a moving object. Heavy and light objects in a vacuum fall at the same rate. And perhaps most revolutionary of all, laws of motion on the Earth could explain some aspects of motions in the heavens. With Galileo, the scientific revolution, with experimental observations at its core, was definitively launched.
The most important person in the history of dynamics was Isaac Newton. Newton, who was born the year after Galileo died, can be said to have invented, on his own, the science of dynamics. Along the way he also had to invent calculus, the branch of mathematics that describes motion and change.
Galileo, 1564–1642 (AIP Emilio Segre Visual Archives, E. Scott Barr Collection)
Isaac Newton, 1643–1727 (Original engraving by unknown artist, courtesy AIP Emilio Segre Visual Archives)
Physicists call the general study of motion mechanics. This is a historical term dating from ancient Greece, reflecting the classical view that all motion could be explained in terms of the combined actions of simple “machines” (e.g., lever, pulley, wheel and axle). Newton’s work is known today as classical mechanics. Mechanics is divided into two areas: kinematics, which describes how things move, and dynamics, which explains why things obey the laws of kinematics. For example, Kepler’s laws are kinematic laws—they describe how the planets move (in ellipses with the sun at one focus)—but not why they move in this particular way. Newton’s laws are the foundations of dynamics: they explain the motion of the planets, and everything else, in terms of the basic notions of force and mass.
Newton’s famous three laws are as follows:
Constant motion: Any object not subject to a force moves with unchanging speed.
Inertial mass: When an object is subject to a