Physics is the science of Nature in the broadest sense. Physicists study the behaviour and interactions of matter and energy, which are referred to as physical phenomena. Physics is the study of matter, energy, motion, and forces. Physics is a major branch of science, concerned with the fundamental components of the universe, the forces they exert on one another, and the results produced by these forces.

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Physicists study the properties and forms of matter and energy - heat, light, electricity and magnetism, and nuclear energy. They try to understand the forces that act in the universe, and the laws that these forces obey - e.g. matter and energy can't be destroyed, only changed from one to the other (a conservation law).

Theories of physics are generally expressed as mathematical relations. Well-established theories are often referred to as physical laws or laws of physics; however, like all scientific theories, they are ultimately provisional. Modern physics relates to the laws of symmetry and conservation, such as those pertaining to energy, momentum, charge, and parity.

The fundamental concepts of physics underlie all basic science -- astronomy, biology, chemistry, and geology. Physics is closely related to the other natural sciences and, in a sense, encompasses them. It is concerned with the most fundamental aspects of matter and energy and how they interact. Modern physics has discovered how atoms are made up of smaller particles and how these particles interact to build atoms into molecules and larger objects of matter.

Chemistry, for example, deals with the interaction of atoms to form molecules; Chemists use this knowledge to guide them in their work in studying all existing chemical compounds and in making new ones. Chemistry is the science of molecules and the chemical compounds that they form in bulk. Chemistry draws on many fields of physics, particularly quantum mechanics, thermodynamics and electromagnetism. However, chemical phenomena are sufficiently varied and complex that chemistry is usually regarded as a separate discipline.

Much of modern geology is largely a study of the physics of the earth and is known as geophysics; and astronomy deals with the physics of the stars and outer space. Even living systems are made up of fundamental particles and, as studied in biology, biophysics and biochemistry, they follow the same types of laws as the simpler particles traditionally studied by a physicist.

Physics also is essential to the applied science and engineering that has given us the supersonic jet, the laser, the fax, live satellite transmission, and the chips of a computer.

Physics may be loosely divided into classical physics and modern physics. Classical physics includes the traditional branches that were fairly well developed before the beginning of the 20th cent.

• Mechanics -- the study of motion and the forces that cause it
• Acoustics -- the study of sound
• Optics -- the study of light
• Thermodynamics -- the study of the relationships between heat and other forms of energy
• Electricity and Magnetism.

Physics recognises four fundamental forces of nature:

• gravitation, which was first adequately described by Isaac Newton;
• electromagnetism, codified in the 19th century by Maxwell's equations;
• the weak nuclear force, which is responsible for the decay of some subatomic particles;
• and the strong nuclear force, which binds together atomic
nuclei and is 10¹² times stronger than the weak nuclear force.

The laws of motion were codified in the 17th century by Isaac Newton, who provided a physical explanation of the motions of celestial bodies. Physics was extended in the 19th century, to study changes in physical form that take place, such as, for example, when a liquid freezes and becomes a solid. Changes of state due to heat are studied in the branch of physics called thermodynamics. Other changes in the form of matter, for example, those which occur when oxygen and hydrogen combine into water, are usually considered to be part of chemistry rather than physics.

The distinction between physics and chemistry is somewhat arbitrary since ideas from physics are routinely used in chemistry. Modern physics is concerned with the structure and behaviour of individual atoms and their components, while chemistry deals with the properties and reactions of molecules -- which depend on energy, especially heat, as well as on atoms; thus, there is a strong link between physics and chemistry. Chemists are more interested in the specific properties of different elements and compounds, whereas physicists are concerned with the general properties of all matter.

Astronomy is the science of the entire universe including the Earth's gross physical properties, such as its momentum and rotation, insofar as they interact with other bodies in the solar system. Until the 18th century, astronomers were concerned mainly with the Sun, Moon, planets, and comets. During the last two centuries, the study of stars, galaxies, nebulas, and the interstellar medium has become increasingly important. Celestial mechanics, the science of the motion of planets and other solid objects within the solar system, was the first proving ground for Newton's laws of motion, and thereby helped to establish the fundamental principles of classical (pre-20th-century) physics.

Astrophysics, the study of the physical properties of celestial bodies, developed during the 19th century and is closely connected with the determination of the chemical composition of those bodies. In the 20th century physics and astronomy have become more intimately linked through cosmological theories, especially those based on the theory of relativity.

Newton's mechanics dominated physics for two centuries, and can loosely be described as a 'clockwork' view of the universe - given the positions, masses, and velocities of all objects in the universe, then their future behaviour could, in principle, be predicted to arbitrary precision using Newtonian mechanics. This view has changed dramatically due to major developments in the early part of the 20th century. On the very small scale, and for rapidly moving objects, ordinary, commonsense notions of space, time, matter, and energy are no longer valid, and two major theories of modern physics present a different picture of these concepts from that of classical physics.

• Einstein's theories of relativity (special and general), which grew in part from Maxwell's work; the Theory of Relativity is concerned with the description of phenomena that take place in a frame of reference that is in motion with respect to an observer.
• quantum mechanics, which introduced the notion of uncertainty in the simultaneous observations of certain quantities, e.g. position and momentum. Quantum Mechanics is concerned with the discrete, rather than the continuous, nature of many phenomena at the atomic and subatomic level, and with the complementary aspects of particles and waves in the description of such phenomena.

Both of these areas involve concepts which are highly counter-intuitive - defying common sense - but which have been repeatedly confirmed by experiment.

Light We learn almost all of what we know about the world around us from the interaction of the objects in the world with electromagnetic radiation. Often, the word 'light' is used a little more broadly, to include electromagnetic radiation that is just outside the range we can see, in the ultraviolet and infrared.	Machines In physics, a simple machine is any device that only requires the application of a single force to work. It is a tool used to make work easier. Machines use a force (push or pull) to move a load.	Magnetism Magnets attract some metals, such as iron, nickel, cobalt, and some alloys. You can magnetise a piece of one of these metals by stroking it with a magnet.
Mechanics This document is a short introduction to basic concepts and definitions in classical physics. It assumes no mathematics beyond elementary algebra and geometry.	Nuclear One of the main quests of science throughout the ages has been to discover what matter is made of - and what holds it together. All matter is made out of many tiny particles called atoms. The study of how these atoms interact is called Chemistry. Modern physics has discovered how atoms are made up of smaller particles and how these particles interact to build atoms into molecules and larger objects of matter.	Optics The concept of mechanical waves leads to the concept of light, which is also a wave phenomenon. However, its nature is somewhat more devious than sound waves and other mechanical waves, and in fact has some maddeningly contradictory aspects that eventually broke through the boundaries of classical physics. This chapter provides a short description of light and optical technologies.
Relativity Albert Einstein's Theory of Relativity is a set of two theories in physics, special relativity and general relativity. The core idea of both theories is that two observers who move relative to each other will often measure different 'time' and 'space' intervals for the same events, but the content of physical law will be the same for both.	Thermodynamics One of the simplest, and most difficult to grasp, concepts in elementary physics is that of "heat", and that is the starting point for the study of "thermal physics" or "thermophysics". Thermal physics is a particular interesting aspect of physics as it underlies the operation of engines and refrigeration systems. This chapter provides an introduction to thermal physics, engines, and refrigeration systems.	Waves One of the properties of matter is that is can support transfers of mechanical energy, whether the matter is solid, liquid, or gas, without any net movement of the molecules involved. Such transfers are referred to as "wave motion", or a transitory displacement of atoms within the matter that passes on kinetic energy.