Mechanics
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.
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.
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 While it is true that, historically, quantum mechanics has been seen as having superseded classical mechanics, this is only true on the hypothetical or foundational level.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.
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.
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.
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
gravitationQuantum 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
*
Physics*
Engineering*
Applied Mechanics*
Applied Mechanics Division,
American Society of Mechanical Engineers*Fluid Dynamics Division,
American Physical SocietyBlogs:
*
Mechanics Blog by a Purdue University Professor*
Mechanics Blog by a Group of University Professors
