Mechanics (Greek Μηχανική) is the branch of physics concerned with the behaviour of physical bodies when subjected to forces or displacements, and the subsequent effect of the bodies on their environment.

The discipline has its roots in the old Greece where people like Aristotle studied the way bodies behaved when they were thrown through the air (e.g. a stone). However it was Galileo, Kepler and especially Newton who lay the foundation for much of the so called Newtonian mechanics we know today.

A person working in the discipline is known as a mechanician.


Mechanics is the original discipline of physics, dealing with the macroscopic world that humans perceive. It is therefore a huge body of knowledge about the natural world. Mechanics encompasses the movement of all matter in the universe under the four fundamental interactions (or forces)gravity, the strong and weak interactions, and the
electromagnetic interaction.

Mechanics also constitutes a central part of technology, the application of physical knowledge for humanly defined purposes. In this connection, the discipline is often known as engineering or applied mechanics. In this sense, mechanics is used to design and analyze the behavior of structures, mechanisms, and machines. Important aspects of
the fields of mechanical engineering, aerospace engineering, civil engineering, structural engineering, materials engineering, biomedical engineering and biomechanics were spawned from the study of mechanics.

Classical vs. Quantum

The major division of the mechanics discipline separates classical mechanics from quantum mechanics.

Historically, classical mechanics came first, while quantum mechanics is a comparatively recent invention. Classical mechanics is older than written history, while quantum mechanics didn’t appear until 1900. Both are commonly held to constitute the most

certain knowledge that exists about physical nature. Classical mechanics has especially often been viewed as a model for other so-called exact sciences. Essential in this respect is the relentless use of mathematics in theories, as well as the decisive role played by experiment in generating and testing them.

Quantum mechanics is, formally at least, of the widest scope, and can be seen as encompassing classical mechanics, as a sub-discipline which applies under certain restricted circumstances. According to the correspondence principle, there is no contradiction or conflict between the two subjects, each simply pertains to specific situations. While it is true that historically quantum mechanics has been seen as having superseded classical mechanics, this is only true on the hypothetical or foundational level. For practical problems, classical mechanics is able to solve problems which are unmanageably difficult in quantum mechanics and hence remains useful and well used.

Einsteinian vs. Newtonian
Analogous to the quantum vs. classical reformation, Einstein’s general and special theories of relativity have expanded the scope of mechanics beyond the mechanics of Newton and Galileo, and made small corrections to them. Relativistic corrections were also needed for quantum mechanics, although relativity is categorized as a classical

There are no contradictions or conflicts between the two, so long as the specific circumstances are carefully kept in mind. Just as one could, in the loosest possible sense, characterize classical mechanics as dealing with “large” bodies (such as engine parts), and quantum mechanics with “small” ones (such as particles), it could be said that relativistic mechanics deals with “fast” bodies, and non-relativistic mechanics with “slow” ones. However, “fast” and “slow” are relative concepts, depending on the state of motion of the observer. This means that all mechanics, whether classical or quantum, potentially needs to be described relativistically. On the other hand, as an observer, one may frequently arrange the situation in such a way that this is not really required.

Types of mechanical bodies
Thus the often-used term body needs to stand for a wide assortment of objects, including particles, projectiles, spacecraft, stars, parts of machinery, parts of solids, parts of fluids (gases and liquids), etc.

Other distinctions between the various sub-disciplines of mechanics, concern the nature of the bodies being described. Particles are bodies with little (known) internal structure, treated as mathematical points in classical mechanics. Rigid bodies have size and shape, but retain a simplicity close to that of the particle, adding just a few so-called degrees of freedom, such as orientation in space.

Otherwise, bodies may be semi-rigid, i.e. elastic, or non-rigid, i.e. fluid. These subjects have both classical and quantum divisions of study.

For instanceThe motion of a spacecraft, regarding its orbit and attitude (rotation), is described by the relativistic theory of classical mechanics. While analogous motions of an atomic nucleus are described by quantum mechanics.

Sub-disciplines in mechanics
The following are two lists of various subjects that are studied in mechanics.

Note that there is also the “theory of fields” which constitutes a separate discipline in physics, formally treated as distinct from mechanics, whether classical fields or quantum fields. But in actual practice, subjects belonging to mechanics and fields are closely interwoven. Thus, for instance, forces that act on particles are frequently derived from fields (electromagnetic or gravitational), and particles generate fields by acting as sources. In fact, in quantum mechanics, particles themselves are fields, as described theoretically by the wave function.

Classical mechanics
The following are described as forming Classical mechanics:

  • Newtonian mechanics, the original theory of motion (kinematics) and forces (dynamics)
  • Lagrangian mechanics, a theoretical formalism
  • Hamiltonian mechanics, another theoretical formalism
  • Celestial mechanics, the motion of stars, galaxies, etc.
  • Astrodynamics, spacecraft navigation, etc.
  • Solid mechanics, elasticity, the properties of (semi-)rigid bodies
  • Acoustics, sound in solids, fluids, etc.
  • Statics, semi-rigid bodies in mechanical equilibrium
  • Fluid mechanics, the motion of fluids
  • Continuum mechanics, mechanics of continua (both solid and fluid)
  • Hydraulics, fluids in equilibrium
  • Applied / Engineering mechanics
  • Biomechanics, solids, fluids, etc. in biology
  • Statistical mechanics, large assemblies of particles
  • Relativistic or Einsteinian mechanics, universal gravitation

Quantum mechanics
The following are categorized as being part of Quantum mechanics:

  • Particle physics, the motion, structure, and reactions of particles
  • Nuclear physics, the motion, structure, and reactions of nuclei
  • Condensed matter physics, quantum gases, solids, liquids, etc.
  • Quantum statistical mechanics, large assemblies of particles

Input is the term denoting either an entrance or changes which are inserted into a system and which activate/modify a process. It is an abstract concept, used in the modeling, system(s) design and system(s) exploitation. It is usually connected with other terms, e.g., input variable, input parameter, input value, input signal and input device.

From the most general systemics perspective, input is a subjective concept and depends on how the system is used. In such sense, the same system can have different inputs in different applications.

In the case of a process description/model, the concept input is closely connected with the concept output. Here, what enters is called input and what exits is called output.

ExampleFor an abstract system A(x,y,p), where x,y are variables and p is a parameter, x may denote input (variable) and y may denote the output for a processy = f(p,x), but, for another goal/(system application), the system A can be the carrier of a process x = g(p,y), where y is an input and x is an output.s

Usually, in the modeling of a problem/process, input are these variables which are known and output are those unknown to us yet.

In different contexts, input has several more concrete domain-dependent meanings.  Information processing
In information processing, input refers to either information received or the process of receiving it:

  • In human-computer interaction, input is the information produced by the user with the purpose of controlling the computer program. The user interface determines what kinds of input the program accepts (for example, control strings or text typed with keyboard and mouse clicks).
  • Input also comes from networks and storage devices such as disk drives.

Control theory
In control theory, the inputs of a system are the signals that can be observed or affected that feed into the system. Specifically, inputs are differentiated from states.

Equity theory
In equity theory, inputs are the skills, time, effort, expertise, experience or qualifications that an employee brings to his job.

Economic theory
In economics, inputs refer to priced or unpriced resources used in a production process, and outputs refer to the priced or unpriced results of that process. Normally, inputs are regarded as costs, and outputs as products. Outputs may be valued gross (before deduction of costs from sales) or net (after deduction of costs from sales). Input-output
economics was popularised by the economist Wassily Leontief who devised an ingenious system of input and output accounts and matrices to analyse the flows of goods and services between different sectors of a national economy. In national accounts an attempt is made to value national inputs and outputs according to consistent valuation principles, to measure the creation and distribution of wealth.

Signal processing is the processing, amplification and interpretation of signals, and deals with the analysis and manipulation of signals. Signals of interest include sound, images,

biological signals such as ECG, radar signals, and many others. Processing of such signals includes storage and reconstruction, separation of information from noise (e.g., aircraft identification by radar), compression (e.g., image compression), and feature extraction (e.g., speech-to-text conversion).

Signal classification
Signals can be either analog or digital, and may come from various sources.

There are various sorts of signal processing, depending on the nature of the signal, as in the following examples.

For analog signals, signal processing may involve the amplification and filtering of audio signals for audio equipment or the modulation and demodulation of signals for telecommunications. For digital signals, signal processing may involve the compression, error checking and error detection of digital signals.

  • Analog signal processing—for signals that have not been digitized, as in classical radio, telephone, radar, and television systems
  • Digital signal processing—for signals that have been digitized. Processing is done by digital circuits such as ASICs, FPGAs, general-purpose microprocessors or computers, or specialized digital signal processor chips.
  • Statistical signal processing—analyzing and extracting information from signals based on their statistical properties
  • Audio signal processing—for electrical signals representing sound, such as music
  • Speech signal processing—for processing and interpreting spoken words
  • Image processing—in digital cameras, computers, and various imaging systems
  • Video signal processing—for interpreting moving pictures
  • Array processing—for processing signals from arrays of sensors

‘Information processing’
In information processing, output is the process of transmitting information by an object (verb usage).
Output may also be used as a noun for information transmitted by a source (object).

Human-computer interaction
In human-computer interaction, output is information produced by the computer program and perceived by the user. The kinds of output the program produces, and the kinds of input the program accepts, define the user interface of the program. In this context, feedback and output are often used interchangeably. However, output tends to refer specifically to explicit output, something that is intentionally provided for the user, whereas feedback also encompasses byproducts of operation that happen to contain information (see low-key feedback).

In telecommunication, the term output can refer to:

  • Information retrieved from a functional unit or from a network, usually after some processing.
  • An output state, or sequence of states.
  • Pertaining to a device, process, or channel involved in the production of data by a computer or by any of its components.

Sourcefrom Federal Standard 1037C
Control theory
In control theory, the outputs of a system are what can be measured. Specifically, outputs are differentiated from states

In Macro-economics, output is the produced goods and services in an economy. A distinction is drawn between Gross Output and Net output.

Equity theory
In equity theory, output is the benefits that an employee receives, including money, perquisites, power, status or variety.

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