Physics in EnglishThe scientist does not study nature because it is useful;he studies it because he delights in it,and he delights in it because it is beautiful.If nature were not beautiful,it would not be worth knowing,and if nature were not worth knowing,life would not be worth living."------ Henri PoincareDepartment of PhysicsSchool of scienceXian Technological University 2012.9Chapter 1 What is Physics?Physics is natural science studying nature's LawsPhysics was called natural philosophy and fundamental science.The history of physicsThe history of physics encompasses two broad time periods in which classical physics and modern physics developed. Classical physics developed between 1600 and 1900. It embraces the general areas of physics known as mechanics, thermodynamics, and electromagnetism, culminating in relativity. Modern physics began developing between about 1890 and 1930, when it was realized that classical physics could not account for the newly discovered behavior of nature at the atomic and molecular level. Modern physics includes the theory of relativity as well as quantum mechanics and most of the subsequent new physics discovered and developed during the 20th century.Classical physicsClassical physics includes the traditional branches and topics that were recognized and well-developed before the beginning of the 20th century—classical mechanics, optics, thermodynamics, and electromagnetism. Classical mechanics is concerned with bodies acted on by forces and bodies in motion and may be divided into statics (study of the forces on a body or bodies at rest), kinematics (study of motion without regard to its causes), and dynamics (study of motion and the forces that affect it); Acoustics, the study of sound, is often considered a branch of mechanics because sound is due to the motions of the particles of air or other medium through which sound waves can travel and thus can be explained in terms of the laws of mechanics. Among the important modern branches of acoustics is ultrasonics, the study of sound waves of very high frequency beyond the range of human hearing. Optics, the study of light, is concerned not only with visible light but also with infrared and ultraviolet radiation, which exhibit all of the phenomena of visible light except visibility, e.g., reflection, refraction, interference, diffraction, dispersion, and polarization of light. Heat is a form of energy, the internal energy possessed by the particles of which a substance is composed; thermodynamics deals with the relationships between heat and other forms of energy. Electricity and magnetism have been studied as a single branch of physics since the intimate connection between them was discovered in the early 19th century; an electric current gives rise to a magnetic field and a changing magnetic field induces an electric current. Electrostatics deals with electric charges at rest, electrodynamics with moving charges, and magnetostatics with magnetic poles at restThe Emergence of Classical PhysicsClassical mechanics has its roots in the work of Johannes Kepler, who, between 1600 and 1619, became the first person to describe quantitatively and accurately the elliptic paths of the planets around the Sun.The problem of the shape of the planetary orbits was not easy to solve, because all historical observations of the planets were made from the Earth, itself a moving planet. Although a crude heliocentric(Sun-centered) model of the solar system using circular orbits was proposed by the Greek astronomer Aristarchus in the third century B.C. and resurrected by Copernicus in 1542,the planetary positions predicted by Copernicus were not as accurate as those found using Ptolemy's complicated geocentric (Earth-centered} model of the second century A.D.The pure Copernican model was inaccurate because Copernicus refused to abandon the flawed concept of early Greek thought that the natural path of a moving "perfect" celestial object, such as a planet, had to be in the shape of the "perfect" geometrical figure---a circle---with the planet moving at constant speed. a scientist living at the time of Copernicus would have been forced to reject the Copernican theory with its circular planetary orbits, since they did not account for the observed motions of the planets as accurately as the existing, but more complicated, geocentric theory of Ptolemy.The change from the accurate but complex Ptolemaic geocentric model of the solar system to the accurate and simple heliocentric model of Kepler illustrates the characteristics of a good theory. With Kepler's work, accuracy and simplicity blossomed simultaneously from the heliocentric hypothesis. Kepler's model and mathematical methods began the development of physics as we know it. His work was a watershed of quantitative thought and a new worldview.Observation of nature and experimental results typically precede the general theories that account for them. The extensive observations of the planets by Tycho Brahe just prior to 1600 provided Kepler with the data he needed to model the solar system accurately and precisely。
