# Mechanics

**Mechanics** refers to:

- a craft relating to machinery (from the Latin
*mechanicus*, from the Greek*mechanikos*, meaning "one skilled in machines"), or - a range of disciplines in science and engineering.

## Mechanics in science and engineering

Mechanics can be seen as the prime, and even as the original, discipline of physics. It is a huge body of knowledge about the natural world. It also constitutes a central part of technology. That is, how to apply this knowledge for humanly defined purposes. Briefly stated, mechanics is concerned with the motion of physical bodies, and with the forces that cause, or limit, these motions, as well as with forces which such bodies may, in turn, give rise to. Due to the wide scope of the subject, one may well find topics that would not fit easily into even this general characterization. Thus the 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.

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 the year 1900. Both are commonly held to constitute the most certain knowledge that exists about physical nature. Especially classical mechanics has therefore 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. If properly interpreted, 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 While it is true that, historically, quantum mechanics has been seen as having superseded classical mechanics, this is only true on the abstract, or fundamental, level. In practice, classical mechanics remains as useful as ever.

In a somewhat analogous way, Einsteinian relativity has expanded the scope of mechanics. This is true for classical as well as quantum mechanics. Again, there are no contradictions, or conflicts, 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.

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 extension, 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 instance, the motion of a spacecraft is described by classical mechanics, regarding its orbit and attitude (i.e. by rotation with respect to the fixed stars). While an atomic nucleus is described by quantum mechanics in analogous situations.

### Sub-disciplines in mechanics

- 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 equilibrium
- Fluid mechanics, the motion of fluids
- Continuum mechanics, mechanics of continua (both solid and fluid)
- Hydraulics, fluids in equilibrium
- Biomechanics, solids, fluids, etc. in biology
- Statistical mechanics, large assemblies of particles
- Relativistic or Einsteinian mechanics, universal gravitation

- 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

Formally, "fields" constitute a separate discipline in physics, distinct from mechanics, whether classical fields or quantum fields. In actual practice, however, 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.

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