Online Encyclopedia

Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.

MECHANICS

Online Encyclopedia
Originally appearing in Volume V17, Page 956 of the 1911 Encyclopedia Britannica.
» Make a correction to this article.
» Add information or comments to this article.
Spread the word: del.icio.us del.icio.us it!

MECHANICS . The subject of mechanics may be dividedGuided by experience, we are able to See also:

frame rules which enable us to say with more or less accuracy what will be the consequences, or what were the antecedents, of a given See also:state of things. These rules are sometimes dignified by the name of " See also:laws of nature," but they have relation to our See also:present state of know-ledge and to the degree of skill with which we have succeeded in giving more or less compact expression to it. They are therefore liable to be modified from See also:time to time, for to be superseded by more convenient or more comprehensive modes of statement. Again, we do not aim at anything so hopeless, or indeed so useless, as a See also:complete description of any phenomenon. Some features are naturally more important or more interesting to us than others; by their relative simplicity and evident constancy they have the first hold on our See also:attention, whilst those which are apparently accidental and vary from one occasion to another are ignored, or postponed for later examination. It follows that for the p•-'rposes of such description as is possible some See also:process of See also:abstraction is inevitable if our statements are to be See also:simple and definite. Thus in studying the See also:flight of a See also:stone through the See also:air we replace the See also:body in See also:imagination by a mathematical point endowed with a See also:mass-coefficient. The See also:size and shape, the complicated See also:spinning See also:motion which it is seen to execute, the See also:internal strains and vibrations which doubtless take See also:place, are all sacrificed in the See also:mental picture in See also:order that attention may be concentrated on those features of the phenomenon which are in the first place most interesting to us. At a later See also:stage in our subject the conception of the ideal rigid body is introduced; this enables us to fill in some details which were previously wanting, but others are still omitted. Again, the conception of a force as concentrated in a mathematical See also:line is as unreal as that of a mass concentrated in a point, but it is a convenient fiction for our purpose, .owing to the simplicity which it lends to our statements. The laws which are to be imposed on these ideal representations are in the first instance largely at our choice.

Any See also:

scheme of abstract See also:dynamics constructed in this way, provided it be self-consistent, is mathematically legitimate; but from the See also:physical point of view we require that it should help us to picture the sequence of phenomena as they actually occur. Its success or failure in this respect can only be judged a posteriori. by comparison of the results to which it leads with the facts. It is to be noticed, moreover, that all available tests apply only to the scheme as a whole; owing to the complexity of phenomena we cannot submit any one of its postulates to verification apart from the See also:rest. It is from this point of view that the question of relativity of motion, which is often See also:felt to be a stumbling-See also:block on the very See also:threshold of the subject, is to be judged. By " motion" we mean of See also:necessity motion relative to some frame of reference which is conventionally spoken of as " fixed." In the earlier stages of our subject this may be any rigid, or apparently rigid, structure fixed relatively to the See also:earth. If we meet with phenomena which do not See also:fit easily into this view, we have the alternatives either to modify our assumed laws of motion, or to See also:call to our aid See also:adventitious forces, or to examine whether the discrepancy can be reconciled by the simpler expedient of a new basis of reference. It is hardly necessary to say that the latter See also:procedure has hitherto been found to be adequate. As a first step we adopt a See also:system of rectangular axes whose origin is fixed in the earth, but whose directions are fixed by relation to the stars; in the planetary theory the origin is transferred to the See also:sun, and afterwards to the mass-centre of the See also:solar system; and so on. At each step there is a gain in accuracy and comprehensiveness; and the conviction is cherished that some system of rectangular axes exists with respect to which the Newtonian scheme holds with all imaginable accuracy. A similar See also:account might be given of the conception of time as a measurable quantity, but the remarks which it is necessary to make under this See also:head will find a place later. into two parts: (1) theoretical or abstract mechanics, and (2) applied mechanics. I.

THEORETICAL MECHANICS Historically theoretical mechanics began with the study of See also:

practical contrivances such as the See also:lever, and the name mechanics Gr. -rd µn7Xavuca), which might more properly be restricted to the theory of mechanisms, and which was indeed used in this narrower sense by See also:Newton, has clung to it, although the subject has See also:long attained a far wider See also:scope. In See also:recent times it has been proposed to adopt the See also:term dynamics (from Gr. 5bpa1.0 force,) as including the whole See also:science of the See also:action of force on bodies, whether at rest or in motion. The subject is usually expounded under the two divisions of See also:statics and See also:kinetics, the former dealing with the conditions of rest or See also:equilibrium and the latter with the phenomena of motion as affected by force. To this latter See also:division the old name of dynamics (in a restricted sense) is still often applied. The See also:mere geometrical description and See also:analysis of various types of motion, apart from the See also:consideration of the forces concerned, belongs to See also:kinematics. This is sometimes discussed as a See also:separate theory, but for our present purposes it is more convenient to introduce kinematical motions as they are required. We follow also the traditional practice of dealing first with statics and then with kinetics. This is, in the See also:main, the See also:historical order of development, and for purposes of exposition it has many advantages. The laws of equilibrium are, it is true, necessarily included as a particular See also:case under those of motion; but there is no real inconvenience in formulating as the basis of statics a few provisional postulates which are afterwards seen to be comprehended in a more See also:general scheme. The whole subject rests ultimately on the Newtonian laws of motion and on some natural extensions of them.

As these laws are discussed under a separate heading (MOTION, LAWS OF), it is here only necessary to indicate the standpoint from which the present See also:

article is written. It is a purely empirical one. The following synopsis shows the scheme on which the treatment is based See also:Part I. Statics. 1. Statics of a particle. 2. Statics of a system of particles. 3. See also:Plane kinematics of a rigid body. 4. Plane statics.

5. Graphical statics. 6. Theory of frames. 7. Three-dimensional kinematics of a rigid body. 8. Three-dimensional statics. 9. See also:

Work. to. Statics of inextensible chains.

II. Theory of mass-systems. Part 2.—Kinetics. 12. Rectilinear motion. 13. General motion of a particle. 14. Central forces. See also:

Hodograph. 15. Kinetics of a system of discrete particles.

16. Kinetics of a rigid body. Fundamental principles. 17. Two-dimensional problems. 18. Equations of motion in three dimensions. 19. See also:

Free motion of a solid. 20. Motion of a solid of revolution. 21.

Moving axes of reference. 22. Equations of motion in generalized co-ordinates. 23. Stability of equilibrium. Theory of vibrations.

End of Article: MECHANICS

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.
Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
MECHANICAL 		[next]
MECHANICVILLE

Site © 2006 - Net Industries