mechanics (794282)
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CLASICAL AND MODERN MECHANICS
Mechanics is closely identified with physics and engineering. There exist many extensive branches of General or Theoretical Mechanics which have their own principles and independent significance. All the fields and subjects of Mechanics apply the methods and equations of Theoretical Mechanics. The primitive elements of Mechanics are Bodies, Forces, Motion. These basic elements arc governed by assumptions, principles or Laws which describe Mechanics as a whole. Mechanical Laws abstract, codify and record the common features of all mechanical phenomena. Scientists are seeking for Laws because Laws are theory and experimental knowledge combined. Extracting and deducing Laws is one of the great activities of a scientist. Most Laws in science state the relationships between measurements of two (three) quantities. Almost all scientific Laws can be reworded with the word "constant" as their essential characteristic. Most mechanical laws were derived inductively from experiments. In some cases, however, scientists deduce Laws or rules from some theoretical scheme. The general principles in Mechanics arc illustrated by constitutive equations which abstract the differences among Bodies. The fundamental entity in Mechanics is the material particle and Bodies arc considered as aggregates of such particles. The general Laws of Mechanics apply to all Bodies and all Motions. The elements in terms of which Motion is described are Position in Space and Time. Space that is used in Mechanics is Euclidean three-dimensional; Time is absolute. The assumption of absolute time did not lead to contradictions when applied to facts known up to the nineteenth century. With the advent of Einstein's theory of relativity Mechanics came to be regarded as a geometry of four-dimensions and space became finite but unbounded. The concept of time was changed: there is no absolute time according to the relativistic viewpoint. It was in the concept of time that the classical and relativistic developments diverged. Mechanics is seeking the simplest possible description of how bodies actually move, it makes no pretence of explaining why bodies move. The description of motion involves general Mecprinciples stated in mathematical terms embracing all particular motions as special cases. The natural philosophers of ancient Greece liked to do experiments in their heads. Centuries later Galileo developed the "thought experiment", intended for the imagination only, into a fruitful method of inquiry of free fall. In our time "thought experiments" appealed strongly to such scientists as Einstein and Fermi. Of all the Forces playing part in Mechanics (contact, i. e., pushes and pulls, frictional, electrical, nuclear, etc.) the most important is the force of gravity. The force of gravity acts on any object vertically downward and it is proportional to its mass or inertia. Under the influence of gravity all objects fall with the same acceleration. Throughout recorded history the speculations of civilized men on the nature of gravitation ranged from the naive (Aristotle: objects fall to the earth, because that is their natural place) to the sophisticated (Einstein). Newton was the first to recognize that the force of gravity is only a special case of a general attraction between any two masses. This genera] attraction is responsible for keeping the earth and other planets on their courses around the sun. Every scientific theory, though speculative in its character, is meaningful only if it can be tested and verified by Experiment. It dies if it fails such tests. A genuine understanding of theory and its relationship with experiment is essential if one wishes to know science. Einstein presented a new concept of gravitation. There is, he claimed, no absolute force of gravity pulling objects down. On the contrary, every mass has within it a force in proportion to its mass which attracts objects to it. This attraction force of masses is also responsible for the curvature of the universe and for variations in orbits of celestial bodies. Today many scientist firmly believe that Einstein's genera! theory of relativity which explains gravity as a curvature or warping of space and time, is the correct theory of gravitation. They praise its beauty and agreement with observation and experiment. Other workers, however, are openly dubious of the general relativity and suggest that alternative theories provide a better description of gravity. The general theory of relativity nowadays has many competing successors. For such new theories to be viable they have to meet observational and theoretical criteria that are steadily becoming more rigorous. The general theory of relativity predicts that accelerated masses radiate gravitational waves, i. e., gravitational fields propagating with the speed of light. Such waves resemble electromagnetic waves as they carry energy, momentum and information. Whereas electromagnetic waves interact only with electric charges and currents, however, gravitational waves interact with all forms of matter energy. Experiments designed to detect gravitational waves record evidence that they are being emitted in bursts from the direction of the centre of all galaxy. The origin of the observed gravitational radiation is not determined, only the direction of its arrival. These findings are stimulating much theorizing (conjecturing) and a good deal of disagreement among astrophysicists and gravitationalists. It is conjectured that the source might be an unusual object such as a pulsating-neutron star very much closer than the galactic centre. It is conceivable that the mass at the galactic centre is acting as a giant lens, focussing gravitational radiation from an earlier epoch of the universe. Since gravitational radiation is not appreciably absorbed by matter, the authors of this hypothesis maintain, it should have been accumulating since, perhaps, the beginning of Time. The relatively large radiation intensity apparently observed may be telling us when the Time began.
MAIN BRANCHES OF MECHANICS
1.Theoretical Mechanics. It is as old as Mechanics itself. It makes research in various' trends. The theories of stable motion, of automatic and optimal operations, the dynamics of flight, etc. are treated. The problems concerning the behaviour of mass-points, motion, density, liquids, gases, plasma and their states are dealt with. Ideal physical and mechanical models arc introduced to make formulation of Mechanics possible with the aid of differential and integral calculus.
3. Applied Mechanics. The theory of machines and mechanical devices, the theories of vibrations, regulations, gyroscope, automatic control, etc. are developed. Most problems are of engineering and manufacturing aspect.
4. Electromechanics commonly implies the interaction of currents with fluids and the construction of practical electromechanical energy-converting devices. The theory covers topics regarding the nature of the mechanical and eiectrical properties of the interacting medium. It makes a great difference whether the fluid is a gas, a liquid, or a plasma to say nothing of the diversity of properties associated with each of these media.
6. Celestial Mechanics. Stellar Astronomy. Astrophysics. The investigations of the gravitational fields, celestial bodies with various configurations, the evolution of planetary and satellite systems and their stability, the motion of cosmic dust, topological peculiarities of the rotation of celestial bodies, stellar atmosphere, cosmic gas dynamics, the structure and evolution of stars, etc. are being performed in all these fields.
7. Continuum Mechanics. The theories of elasticity, plasticity, creep and relaxation in solids, the theory of motion of plasma constitute the subject matter of the field. The problems of the elastic limit, the origin of a residual plastic deformation, the motion of highly compressed liquids and gases, or conversely, rarefied gases, mechanical models for polymeric plastic materials, etc. are dealt with.
Wave mechanics or quantum mechanics was developed about 1925 by the theoretical physicists De Broglie, Schrodinger, and others. Electrons and other particles are shown to have wave properties, and the wave-particle theory applies to matter as well as to photons,
The heart of the theory can be stated in highly simplified terms as follows: both the units of matter (atoms, and the electrons, protons, and other particles which make up atoms) and the units of radiant or electromagnetic energy (quanta, or photons) behave like particles which move through space as though controlled by wavelike patterns.
The theories of wave mechanics differ, however, from the example of the swallows in many ways. One difference is that the 'control' which led the swallows into wavelike motion existed in space outside the birds; that is, in the distribution of insects in the air. On the other hand, the wave and particle natures of matter and of radiant energy do not have an independent existence. The waves originate where the particles are and they travel outward as the particles move.
Under this two-part theory, the wavelike pattens show where energy will go when it is travelling across space and when it encounters matter, as light does when it strikes mirrors, lenses, and other optical devices. But when energy penetrates atoms and makes them react, the quantized 'energy chunks' produce the reactions. Similarly, in electrical devices electrons act as particies. a beam of electrons is sent through thin gold foil, however, the wave properties make trons show diffraction effects just like those shown by light and other kinds of wave motion in suitable experiments.
THE QANTUM THEORY AND REALITY
One expects that any successful theory in the physical sciences makes accurate predictions. Given some well-defined experiment, this theory should correctly specify the outcome or should at least assign the correct probabilities to all possible outcomes. From this point of view Quantum Mechanics must be judged highly successful. As the fundamental modern theory of atoms, of molecules, of elementary particles, of electromagnetic radiation and of the solid state, it supplies methods for calculating the results of experiments in all these realms.
Apart from experimental confirmation, however, something more is generally demanded of a theory. It is expected that it not only determines the results of an experiment but also provides some understanding of the physical events that underlie the observed results. In other words, the theory should not only give the position of a pointer on a dial but also explains why the pointer takes up that position. When one seeks information of this kind in the Quantum theory, certain conceptual difficulties arise. For example, in Quantum Mechanics an elementary particle such as an electron is represented by the mathematical expression called a wave function, which often describes the electron as if it were smeared out over a large region of space.
It now turns out that even this renumeration is not entirely satisfactory. Even if Quantum Mechanics is no more than a set of rules, it is still in conflict with a view of the world that many people consider obvious or natural. This world view is based on three assumptions, or premises that must be accepted without proof. One is realism, the doctrine that regularities in observed phenomena are caused by some physical reality whose existence is independent of human observers. The second premise holds that inductive inference is a valid mode of reasoning and can be applied freely, so that legitimate conclusions can be drawn from consistent observations. The third premise is called Einstein separability or Einstein locality, and it states that no influence of any kind can proparate faster than the speed of light. Of the three premises realism is the most fundamental. The three premises, which are often have the status of well-established truths, form the basis of what is called local realistic theories of nature.
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