Mechanics

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Mechanics is the branch of physics concerned with the motion of physical bodies, the forces that cause or limit these motions, and the forces to which bodies may, in turn, give rise.

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

Significance

Mechanics is 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. In this connection, the discipline is often known as engineering mechanics or applied 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. 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 While it is true that, historically, quantum mechanics has been seen as having superseded classical mechanics, this is only true on the hypothetical or foundational level. For practical problems, classical mechanics is able to solve problems which are unmanagably difficult in quantum mechanics and hence remains useful and well used.

Einstein vs. Newton

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 Gallileo, and made small corrections to them. Relativistic corrections were also needed for quantum mechanics, although relativity is categorized as a classical theory.

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 instance: The 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 mechanics
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

Professional Organizations

Applied Mechanics Division, American Society of Mechanical Engineers
Fluid Dynamics Division, American Physical Society

External links

Blogs:

Mechanics | Physics | Engineering

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