 Mechanics

This article is about an area of scientific study. For other uses, see Mechanic (disambiguation).
Mechanics (Greek Μηχανική) is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment. The discipline has its roots in several ancient civilizations (see History of classical mechanics and Timeline of classical mechanics). During the early modern period, scientists such as Galileo, Kepler, and especially Newton, laid the foundation for what is now known as classical mechanics. It is a branch of classical physics that deals with the particles that are moving either with less velocity or that are at rest. The system of study of mechanics is shown in the table below:
Contents
Classical versus quantum
Quantum mechanics
Introduction
Mathematical formulationsFundamental conceptsExperimentsDoubleslit · Davisson–Germer
Stern–Gerlach · Bell's inequality
Popper · Schrödinger's cat
Elitzur–Vaidman bomb tester
Quantum eraser
Delayed choice quantum eraser
Wheeler's delayed choiceFormulationsEquationsInterpretationsde Broglie–Bohm
Consciousnesscaused
Consistent histories · Copenhagen
Ensemble · Hidden variables
Manyworlds · Objective collapse
Pondicherry · Quantum logic
Relational · Stochastic
TransactionalScientistsBell · Bohm · Bohr · Born · Bose
de Broglie · Dirac · Ehrenfest
Everett · Feynman · Heisenberg
Jordan · Kramers · von Neumann
Pauli · Planck · Schrödinger
Sommerfeld · Wien · WignerThe 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 originated with Isaac Newton's Laws of motion in Principia Mathematica, 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 socalled 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 of a wider scope, as it encompasses classical mechanics as a subdiscipline 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. The correspondence principle states that the behavior of systems described by quantum theories reproduces classical physics in the limit of large quantum numbers. Quantum mechanics has superseded classical mechanics at the foundational level and is indispensable for the explanation and prediction of processes at molecular and (sub)atomic level. However, for macroscopic processes classical mechanics is able to solve problems which are unmanageably difficult in quantum mechanics and hence remains useful and well used. Modern descriptions of such behavior begin with a careful definition of such quantities as displacement (distance moved), time, velocity, acceleration, mass, and force. Until about 400 years ago, however, motion was explained from a very different point of view. For example, following the ideas of Greek philosopher and scientist Aristotle, scientists reasoned that a cannonball falls down because its natural position is in the earth; the sun, the moon, and the stars travel in circles around the earth because it is the nature of heavenly objects to travel in perfect circles.
The Italian physicist and astronomer Galileo brought together the ideas of other great thinkers of his time and began to analyze motion in terms of distance traveled from some starting position and the time that it took. He showed that the speed of falling objects increases steadily during the time of their fall. This acceleration is the same for heavy objects as for light ones, provided air friction (air resistance) is discounted. The English mathematician and physicist Sir Isaac Newton improved this analysis by defining force and mass and relating these to acceleration. For objects traveling at speeds close to the speed of light, Newton’s laws were superseded by Albert Einstein’s theory of relativity. For atomic and subatomic particles, Newton’s laws were superseded by quantum theory. For everyday phenomena, however, Newton’s three laws of motion remain the cornerstone of dynamics, which is the study of what causes motion.
Einsteinian versus Newtonian
Analogous to the quantum versus 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 fundamental corrections to them, that become significant and even dominant as speeds of material objects approach the speed of light, which cannot be exceeded.
For example,
In Newtonian Mechanics, Newton's laws of motion,
F=ma
whereas in Einsteinian Mechanics and Lorentz Transformations, which were first discovered by Hendrik Lorentz,
F=γma
where γ is the Lorentz Factor
Einsteinian vs. Quantum
Relativistic corrections are also needed for quantum mechanics, although General relativity has not been integrated; the two theories remain incompatible, a hurdle which must be overcome in developing the Theory of Everything.
History
Main articles: History of classical mechanics and History of quantum mechanicsAntiquity
Main article: Aristotelian mechanicsThe main theory of mechanics in antiquity was Aristotelian mechanics.^{[1]} A later developer in this tradition was Hipparchus.^{[2]}
Medieval age
Main article: Theory of impetusIn the Middle Ages, Aristotle's theories were criticized and modified by a number of figures, beginning with John Philoponus in the 6th century. A central problem was that of projectile motion, which was discussed by Hipparchus and Philoponus. This led to the development of the theory of impetus by 14th century French Jean Buridan, which developed into the modern theories of inertia, velocity, acceleration and momentum. This work and others was developed in 14th century England by the Oxford Calculators such as Thomas Bradwardine, who studied and formulated various laws regarding falling bodies.
On the question of a body subject to a constant (uniform) force, the 12th century JewishArab Nathanel (Iraqi, of Baghdad) stated that constant force imparts constant acceleration, while the main properties are uniformly accelerated motion (as of falling bodies) was worked out by the 14th century Oxford Calculators.
Early modern age
Two central figures in the early modern age are Galileo Galilei and Isaac Newton. Galileo's final statement of his mechanics, particularly of falling bodies, is his Two New Sciences (1638). Newton's 1687 Philosophiæ Naturalis Principia Mathematica provided a detailed mathematical account of mechanics, using the newly developed mathematics of calculus and providing the basis of Newtonian mechanics.^{[2]}
There is some dispute over priority of various ideas: Newton's Principia is certainly the seminal work and has been tremendously influential, and the systematic mathematics therein did not and could not have been stated earlier because calculus had not been developed. However, many of the ideas, particularly as pertain to inertia (impetus) and falling bodies had been developed and stated by earlier researchers, both the thenrecent Galileo and the lessknown medieval predecessors. Precise credit is at times difficult or contentious because scientific language and standards of proof changed, so whether medieval statements are equivalent to modern statements or sufficient proof, or instead similar to modern statements and hypotheses is often debatable.
Modern age
Two main modern developments in mechanics are general relativity of Einstein, and quantum mechanics, both developed in the 20th century based in part on earlier 19th century ideas.
Types of mechanical bodies
Thus the oftenused 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 subdisciplines 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 socalled degrees of freedom, such as orientation in space.
Otherwise, bodies may be semirigid, i.e. elastic, or nonrigid, i.e. fluid. These subjects have both classical and quantum divisions of study.
For instance, the motion of a spacecraft, regarding its orbit and attitude (rotation), is described by the relativistic theory of classical mechanics, while the analogous movements of an atomic nucleus are described by quantum mechanics.
Subdisciplines 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)
 Hamiltonian mechanics, a theoretical formalism, based on the principle of conservation of energy
 Lagrangian mechanics, another theoretical formalism, based on the principle of the least action
 Celestial mechanics, the motion of heavenly bodies: planets, comets, stars, galaxies, etc.
 Astrodynamics, spacecraft navigation, etc.
 Solid mechanics, elasticity, the properties of deformable bodies.
 Fracture mechanics
 Acoustics, sound ( = density variation propagation) in solids, fluids and gases.
 Statics, semirigid bodies in mechanical equilibrium
 Fluid mechanics, the motion of fluids
 Soil mechanics, mechanical behavior of soils
 Continuum mechanics, mechanics of continua (both solid and fluid)
 Hydraulics, mechanical properties of liquids
 Fluid statics, liquids in equilibrium
 Applied mechanics, or Engineering mechanics
 Biomechanics, solids, fluids, etc. in biology
 Biophysics, physical processes in living organisms
 Statistical mechanics, assemblies of particles too large to be described in a deterministic way
 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
Professional organizations
 Applied Mechanics Division, American Society of Mechanical Engineers
 Fluid Dynamics Division, American Physical Society
 Institution of Mechanical Engineers is the United Kingdom's qualifying body for Mechanical Engineers and has been the home of Mechanical Engineers for over 150 years.
 International Union of Theoretical and Applied Mechanics
See also
 Analytical mechanics
 Applied mechanics
 Dynamics
 Engineering
 Index of engineering science and mechanics articles
 Kinematics
 Kinetics
 Statics
 Wiesen Test of Mechanical Aptitude (WTMA)
References
 ^ "A history of mechanics". René Dugas (1988). p.19. ISBN 0486656322
 ^ ^{a} ^{b} "A Tiny Taste of the History of Mechanics". The University of Texas at Austin.
 ^ Walter Lewin (October 4, 1999) (in English) (ogg). Work, Energy, and Universal Gravitation. MIT Course 8.01: Classical Mechanics, Lecture 11. (videotape). Cambridge, MA USA: MIT OCW. Event occurs at 1:2110:10. http://ocw.mit.edu/courses/physics/801physicsiclassicalmechanicsfall1999/videolectures/lecture11/. Retrieved December 23, 2010.
Further reading
 Landau, L. D.; Lifshitz, E. M. (1972). Mechanics and Electrodynamics, Vol. 1. Franklin Book Company, Inc. ISBN 008016739X.
External links
 iMechanica: the web of mechanics and mechanicians
 Mechanics Blog by a Purdue University Professor
 The Mechanics program at Virginia Tech
 Physclips: Mechanics with animations and video clips from the University of New South Wales
 U.S. National Committee on Theoretical and Applied Mechanics
 Interactive learning resources for teaching Mechanics
 The Archimedes Project
Categories: Mechanics
 Greek loanwords
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