Tài liệu Vector_mechanics_for_engineers

THIRD SI J\l\ETRIC EDITION. Vector Mechanics for Engineers FERDINAND P. BEER Lehigh University E. RUSSELL JOHNSTON, JR. University of Connecticut With the collaboration of Elliot R. Eisenberg Pennsylvania State University SI Metric adaptation by Theodore Wildl Sperika Enterprises McGraw-Hili Ryerson Toronto New York Burr Ridge Bangkok Bogota Caracas Lisbon London Madrid Mexico City Milan New Delhi Seoul Singapore Sydney Taipei McGraw-Hill Ryerson Limited A Subsidiary of The McGraw-Hill Companies Vector Mechanics for Engineers: Statics Third SI Metric Edition Copyright © 1998, 1988, 1984, 1977, 1972, 1962 McGraw-Hili Ryerson Limited, a Subsidiary of The McGraw-Hili Companies. Copyright © 1996, 1988, 1984, 1977, 1972, 1962 McGraw-Hili, Inc. All rights reserved. No part of this pubiication may be reproduced or transmitted in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of McGraw-Hili Ryerson Limited, or in the case of photocopying or other reprographic copying, a licence from CANCOPY (the Canadian Copyright Licensing Agency), 6 Adelaide Street East, Suite 900, Toronto, Ontario, Canada M5C 1H6. Any request for photocopying, recording, directed in writing to CANCOPY. or taping of any part of this publication shall be ISBN: 0-07-560076-5 2 3 4 5 6 7 8 9 10 VH 7 6 5 4 3 2 1 0 9 Care has been taken to trace ownership of copyright material contained in this text; however, the pubiishers wili welcome any information that enables them to rectify any reference or credit for subsequent editions. Sponsoring Editor: Dave Ward Supervising Editor: Margaret Henderson Developmental Editor: Laurie Graham Proofreader: Matthew Kudelka Production Coordinator: Nicla Dattolico Designer: Merrill Haber Illustrations: FineLine Illustrations, Inc. Cover Photo: Derek Croucher/First Light Typesetter: York Graphic Services, Inc. Typeface: New Caledonia Printer: Van Hoffman Press, Inc. The cover photograph is of the pyramid designed by the American architect I. M. Pei to serve as the principle entrance to the Grand Louvre museum in Paris, France. It is 21 metres high with a 33-metre square base and consists of four sides made of glass that are supported by a truss system composed of thin stainless-steel tubes and cables located inside the pyramid, close to its surface. This design technique and the materials used combine to give to the pyramid its remarkably graceful and translucent appearance. PHOTO CREDITS: Cover: Derek Croucher/First Light Authors' photograph: B. J. Clark, 1995 Chapter 1: Bill Sanderson/Science Photo Library/Photo Researchers; Chapter 2: d'Arazien/lmage Bank; Chapter 3: John Coletti/Stock, Boston; Chapter 4: T. Zimmermann/FPG; Chapter 5: Bruce Hands/Stock, Boston; Chapter 6: Jeff Gnass/Stock Market; Chapter 7: Brian Yarvin/Photo Researchers; Chapter 8: Wayne Hoy/Picture Cube; Chapter 9: Paul Steel/Stock Market; Chapter 10: Wolf Von Dem Bussche/lmage Block Canadian Cataloguing in Publication Beer, Ferdinand P., (date)Vector mechanics for engineers: statics 3rd SI metric ed. Includes index. ISBN 0-07-560076-5 1. Mechanics, Applied. 2. Statics. 3. Vector analysis. 4. Mechanics, Applied-Problems, exercises, etc. I. Johnston, E. Russell (Eiwood Russell), (date)-. II. Eisenberg, Eliiot R. III. Wildi, Theodore, (date)-. IV. Title. TA351.B441998 620.1'053'0151563 C98-930601-1 About the Authors "How did you happen to write your books together, with one of you at Lehigh and the other at UConn, and how do you manage to keep collaborating on their successive revisions?" These are the two questions most often asked of our two authors. The answer to the first question is simple. Russ Johnston's first teaching appointment was in the Department of Civil Engineering and Mechanics at Lehigh University. There he met Ferd Beer, who had joined that department two years earlier and was in charge of the courses in mechanics. Born in France and educated in France and Switzerland (he holds an M.S. degree from the Sorbonne and an Sc.D. degree in the field of theoretical mechanics from the University of Geneva), Ferd had come to the United States after serving in the French army during the early part of World War II and had taught for four years at Williams College in The Williams-MIT joint arts and engineering program. Born in Philadelphia, Russ had obtained a B.S. degree in civil engineering from the University of Delaware and an Sc.D. degree in the field of structural engineering from MIT. Ferd was delighted to discover that the young man who had been hired chiefly to teach graduate structural engineering courses was not only willing but eager to help him reorganize the mechanics courses. Both believed that these courses should be taught from a few basic principles and that the various concepts involved would be best understood and remembered by the students if they were presented to them in a graphic way. Together they wrote lecture notes in statics and dynamics, to which they later added problems they felt would appeal to future engineers, and soon they produced the manuscript of the first edition of Mechanics for Engineers. The second edition of Mechanics for Engineers and the first edition of Vector Mechanics for Engineers found Russ Johnston at Worcester Polytechnic Institute and the next editions at the University of Connecticut. In the meantime, both Ferd and Russ had assumed administrative responsibilities in their departments, and both were involved in research, consulting, and supervising graduate students-Ferd in the area of stochastic processes and random vibrations, and Russ in the area of elastic v vi About the Authors stability and structural analysis and design. Howe\"er. their interest in improving the teaching of the basic mechanics courses had not subsided, and they both taught sections of these courses as they kept re\ising their texts and began writing the manuscript of the first edition of Mechanics of Materials. This brings us to the second question: How did the authors manage to work together so effectively after Russ Johnston had left Lehigh? Part of the answer is provided by their phone bills and the money they have spent on postage. As the publication date of a new edition approaches, they call each other daily and rush to the post office with express-mail packages. There are also visits between the two families. At one time there were even joint camping trips, with both families pitching their tents next to each other. Now, with the advent of the fax machine, they do not need to meet so frequently. Their collaboration has spanned the years of the revolution in computing. The first editions of Mechanics for Engineers and of Vector Mechanics for Engineers included notes on the proper use of the slide rule. To guarantee the accuracy of the answers given in the back of the book, the authors themselves used oversize 20-inch slide rules, then mechanical desk calculators complemented by tables of trigonometric functions, and later four-function electronic calculators. With the advent of the pocket multifunction calculators, all these were relegated to their respective attics, and the notes in the text on the use of the slide rule were replaced by notes on the use of calculators. Now problems requiring the use of a computer are included in each chapter of their texts, and Ferd and Russ program on their own computers the solutions of most of the problems they create. Ferd and Russ's contributions to engineering education have earned them a number of honors and awards. They were presented with the Western Electric Fund Award for excellence in the instruction of engineering students by their respective regional sections of the American Society for Engineering Education, and they both received the Distinguished Educator Award from the Mechanics Division of the same society. In 1991 Russ received the Outstanding Civil Engineer Award from the Connecticut Section of the American Society of Civil Engineers, and in 1995 Ferd was awarded an honorary Doctor of Engineering degree by Lehigh University. A new collaborator, Elliot Eisenberg, Professor of Engineering at the Pennsylvania State University, has joined the Beer and Johnston team for this new edition. Elliot holds a B.S. degree in engineering and an M.E. degree, both from Cornell University. He has focused his scholarly activities on professional service and teaching, and he was recognized for this work in 1992 when the American Society of Mechanical Engineers awarded him the Ben C. Sparks Medal for his contributions to mechanical engineering and mechanical engineering technology education and for service to that society and to the American Society for Engineering Education. And finally, there are the contributions of Theodore Wildi to the integrated conversion of this Third SI Metric Edition. He is Chair of the CSA Technical Committee on the International System of Units and author of Metric Units and Conversion Charts, a widely used handbook for professional engineers. Contents Preface xiii List of Symbols xvii 1 INTRODUCTION 1 1.1 1.2 1.3 1.4 1.5 What Is Mechanics? 2 Fundamental Concepts and Principles Systems of Units 5 Method of Problem Solution 9 Numerical Accuracy 9 2 2 STATICS OF PARTICLES 11 2.1 Introduction 12 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 Forces in a Plane 12 Force on a Particle. Resultant of Two Forces 12 Vectors 13 Addition of Vectors 14 Resultant of Several Concurrent Forces 16 Resolution of a Force into Components 17 Rectangular Components of a Force. Unit Vectors 23 Addition of Forces by Summing x and y Components 26 Equilibrium of a Particle 31 Newton's First Law of Motion 32 Problems Involving the Equilibrium of a Particle. Free-Body Diagrams 32 Forces in Space 41 2.12 Rectangular Components of a Force in Space 2.13 Force Defined by Its Magnitude and Two Points on Its Line of Action 44 2.14 Addition of Concurrent Forces in Space 45 41 vii viii Contents 2.15 Equilibrium of a Particle in Space Review and Summary for Chapter 2 Review Problems 63 53 60 3 RIGID BODIES: EQUIVALENT SYSTEMS OF FORCES 67 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 *3.21 Introduction 68 External and Internal Forces 68 Principle of Transmissibility. Equivalent Forces 69 Vector Product of Two Vectors 71 Vector Products Expressed in Terms of Rectangular Components 73 Moment of a Force about a Point 75 Varignon's Theorem 77 Rectangular Components of the Moment of a Force 77 Scalar Product of Two Vectors 87 Mixed Triple Product of Three Vectors 89 Moment of a Force about a Given Axis 91 Moment of a Couple 101 Equivalent Couples 102 Addition of Couples 104 Couples Can Be Represented by Vectors 104 Resolution of a Given Force Into a Force at 0 and a Couple 105 Reduction of a System of Forces to One Force and One Couple 116 Equivalent Systems of Forces 118 Equipollent Systems of Vectors 118 Further Reduction of a System of Forces 119 Reduction of a System of Forces to a Wrench 121 Review and Summary for Chapter 3 Review Problems 145 140 4 EQUILIBRIUM 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 Introduction 150 Free-Body Diagram OF RIGID BODIES 149 151 Equilibrium in Two Dimensions 152 Reactions at Supports and Connections for a Two-Dimensional Structure 152 Equilibrium of a Rigid Body in Two Dimensions 154 Statically Indeterminate Reactions. Partial Constraints Equilibrium of a Two-Force Body 173 Equilibrium of a Three-Force Body 174 Equilibrium in Three Dimensions 181 Equilibrium of a Rigid Body in Three Dimensions Reactions at Supports and Connections for a Three-Dimensional Structure 181 Review and Summary for Chapter 4 Review Problems 200 198 185 156 Contents 5 DISTRIBUTED FORCES: CENTROIDS AND CENTERS OF GRAVITY 204 5.1 5.2 5.3 5.4 5.5 5.6 5.7 *5.8 *5.9 Introduction 206 Areas and Lines 206 Center of Gravity of a Two-Dimensional Body 206 Centroids of Areas and Lines 208 First Moments of Areas and Lines 209 Composite Plates and Wires 212 Determination of Centroids by Integration 223 Theorems of Pappus-Guldinus 225 Distributed Loads on Beams 236 Forces on Submerged Surfaces 237 Volumes 247 5.10 Center of Gravity of a Three-Dimensional Body. Centroid of a Volume 247 5.11 Composite Bodies 250 5.12 Determination of Centroids of Volumes by Integration Review and Summary for Chapter 5 Review Problems 266 262 6 ANALYSIS OF STRUCTURES 270 6.1 6.2 6.3 6.4 *6.5 *6.6 6.7 *6.8 Introduction 271 Trusses 272 Definition of a Truss 272 Simple Trusses 274 Analysis of Trusses by the Method of Joints 275 Joints under Special Loading Conditions 277 Space Trusses 279 Analysis of Trusses by the Method of Sections 289 Trusses Made of Several Simple Trusses 290 Frames and Machines 301 6.9 Structures Containing Multiforce Members 301 6.10 Analysis of a Frame 301 6.11 Frames Which Cease to Be Rigid When Detached from Their Supports 302 6.12 Machines 317 Review and Summary for Chapter 6 Review Problems 332 329 7 FORCES IN BEAMS AND CABLES 337 *7.1 *7.2 Introduction 338 Internal Forces in Members *7.3 Beams 345 Various Types of Loading and Support 338 345 250 ix X Contents *7.4 *7.5 *7.6 Shear and Bending Moment in a Beam 346 Shear and Bending-Moment Diagrams 348 Relations among Load, Shear, and Bending Moment *7.7 *7.8 *7.9 *7.10 Cables 367 Cables with Concentrated Loads 367 Cables with Distributed Loads 368 Parabolic Cable 369 Catenary 378 Review and Summary for Chapter 7 Review Problems 389 356 386 8 FRICTION 392 8.1 8.2 8.3 8.4 8.5 8.6 *8.7 *8.8 *8.9 *8.10 Introduction 393 The Laws of Dry Friction. Coefficients of Friction Angles of Friction 396 Problems Involving Dry Friction 397 Wedges 413 Square-Threaded Screws 413 Journal Bearings. Axle Friction 422 Thrust Bearings. Disk Friction 424 Wheel Friction. Rolling Resistance 425 Belt Friction 432 Review and Summary for Chapter 8 Review Problems 446 323 443 9 DISTRIBUTED 9.1 Introduction FORCES: MOMENTS OF INERTIA 451 452 Moments of Inertia of Areas 453 Second Moment, or Moment of Inertia, of an Area 453 Determination of the Moment of Inertia of an Area by Integration 454 9.4 Polar Moment of Inertia 455 9.5 Radius of Gyration of an Area 456 9.6 Parallel-Axis Theorem 463 9.7 Moments of Inertia of Composite Areas 464 *9.8 Product of Inertia 476 *9.9 Principal Axes and Principal Moments of Inertia 477 *9.10 Mohr's Circle for Moments and Products of Inertia 485 9.2 9.3 Moments of Inertia of Masses 491 Moment of Inertia of a Mass 491 Parallel-Axis Theorem 493 Moments of Inertia of Thin Plates 494 Determination of the Moment of Inertia of a Three-Dimensional Body by Integration 495 9.15 Moments of Inertia of Composite Bodies 495 *9.16 Moment of Inertia of a Body with Respect to an Arbitrary Axis through O. Mass Products of Inertia 510 9.11 9.12 9.13 9.14 *9.17 *9.18 Ellipsoid of Inertia. Principal Axes of Inertia 511 Determination of the Principal Axes and Principal Moments of Inertia of a Body of Arbitrary Shape 513 Review and Summary for Chapter 9 Review Problems 530 524 10 METHOD OF VIRTUAL WORK 535 *10.1 *10.2 *10.3 *10.4 *10.5 *10.6 *10.7 *10.8 *10.9 Introduction 536 Work of a Force 536 Principle of Virtual Work 539 Applications of the Principle of Virtual Work 540 Real Machines. Mechanical Efficiency 542 Work of a Force during a Finite Displacement 556 Potential Energy 558 Potential Energy and Equilibrium 559 Stability of Equilibrium 560 Review and Summary for Chapter 10 Review Problems 573 u.s. 570 Appendix CUSTOMARY UNITS AND CONVERSIONS TO SI 577 A.1 A.2 Index U.S. Customary Units 577 Conversion from One System of Units to Another 583 Answers to Problems 589 578 Contents xi Preface The main objective of a first course in mechanics should be to develop in the engineering student the ability to analyze any problem in a simple and logical manner and apply to its solution a few, well-understood basic principles. It is hoped that this text, designed for the first course in statics offered in the sophomore year, and the volume that follows, Vector Mechanics for Engineers: Dynamics, will help the instructor achieve this goal. t Vector algebra is introduced early in the text and is used in the presentation and the discussion of the fundamental principles of mechanics. Vector methods are also used to solve many problems, particularly three-dimensional problems where these techniques result in a simpler and more concise solution. The emphasis in this text, however, remains on the correct understanding of the principles of mechanics and on their application to the solution of engineering problems, and vector algebra is presented chiefly as a convenient tool.! One of the characteristics of the approach used in these volumes is that the mechanics of particles has been clearly separated from the mechanics of rigid bodies. This approach makes it possible to consider simple practical applications at an early stage and to postpone the introduction of more difficult concepts. In this volume, for example, the statics of particles is treated first (Chap. 2); after the rules of addition and subtraction of vectors have been introduced, the principle of equilibrium of a particle is immediately applied to practical situations involving only concurrent forces. The statics of rigid bodies is considered in Chaps. 3 and 4. In Chap. 3, the vector and scalar products of two vectors are introduced and used to define the moment of a force about a point and about an axis. The presentation of these new concepts is followed by a thorough and rigorous discussion of equivalent systems of forces leading, in Chap. 4, to many practical applications involving the equilibrium of rigid bodies tBoth texts are also available in a single volume, Vector Mechanics for Engineers: Statics and Dynamics, sixth edition. JIn a parallel text, Mechanics for Engineers: Statics, fourth edition, the use of vector algebra is limited to the addition and subtraction of vectors. xiii xiv Preface under general force systems. In the volume on dnlamics, the same division is observed. The basic concepts of force, m~s, and acceleration, of work and energy, and of impulse and momentum are introduced and first applied to problems involving only particles. Thus students can familiarize themselves with the three basic methods used in dmamics and learn their respective advantages before facing the difficulti~s associated with the motion of rigid bodies. Since this text is designed for a first course in statics, new concepts are presented in simple terms and every step is explained in detail. On the other hand, by discussing the broader aspects of the problems considered, a definite maturity of approach is achieved. For example, the concepts of partial constraints and of static indeterminacy are introduced early in the text and then are used throughout. The fact that mechanics is essentially a deductive science based on a few fundamental principles is stressed. Derivations are presented in their logical sequence and with all the rigor warranted at this level. However, the learning process being largely inductive, simple applications are considered first. Thus, the statics of particles precedes the statics of rigid bodies, and problems involving internal forces are postponed until Chap. 6. Also, in Chap. 4, equilibrium problems involving only coplanar forces are considered first and are solved by ordinary algebra, while problems involving three-dimensional forces, which require the full use of vector algebra, are discussed in the second part of the chapter. Free-body diagrams are introduced early, and their importance is emphasized throughout the text. Color has been used to distinguish forces from other elements of the free-body diagrams. This makes it easier for the students to identify the forces acting on a given particle or rigid body and to follow the discussion of sample problems and other examples given in the text. Free-body diagrams are used not only to solve equilibrium problems but also to express the equivalence of two systems of forces or, more generally, of two systems of vectors. This approach is particularly useful as a preparation for the study of the dynamics of rigid bodies. As will be shown in the volume on dynamics, by placing the emphasis on "free-body-diagram equations" rather than on the standard algebraic equations of motion, a more intuitive and more complete understanding of the fundamental principles of dynamics can be achieved. Because of the current trend among engineers to adopt the international system of units (SI units), the SI units most frequently used in mechanics are introduced in Chap. 1 and are used throughout the text. A large number of optional sections are included. These sections are indicated by asterisks and thus are easily distinguished from those which form the core of the basic statics course. They may be omitted without prejudice to the understanding of the rest of the text. Among the topics covered in these additional sections are the reduction of a system of forces to a wrench, applications to hydrostatics, shear and bendingmoment diagrams for beams, equilibrium of cables, products of inertia and Mohr's circle, mass products of inertia and principal axes of inertia for three-dimensional bodies, and the method of virtual work. An optional section on the determination of the principal axes and moments of inertia of a body of arbitrary shape has also been included in this new edition (Sec. 9.18). The sections on beams are especially useful when the course in statics is immediately followed by a course in mechanics of materials, while the sections on the inertia properties of three-dimensional bodies are primarily intended for the students who will later study in dynamics the three-dimensional motion of rigid bodies. The material presented in the text and most of the problems require no previous mathematical knowledge beyond algebra, trigonometry, and elementary calculus, and all the elements of vector algebra necessary to the understanding of the text are carefully presented in Chaps. 2 and 3. In general, a greater emphasis is placed on the correct understanding of the basic mathematical concepts involved than on the nimble manipulation of mathematical formulas. In this connection, it should be mentioned that the determination of the centroids of composite areas precedes the calculation of centroids by integration, thus making it possible to establish the concept of moment of area firmly before introducing the use of integration. The presentation of numerical solutions takes into account the universal use of calculators by engineering students, and instructions on the proper use of calculators for the solution of typical statics problems have been included in Chap. 2. Each chapter begins with an introductory section setting the purpose and goals of the chapter and describing in simple terms the material to be covered and its application to the solution of engineering problems. The body of the text is divided into units, each consisting of one or several theory sections, one or several sample problems, and a large number of homework problems. Each unit corresponds to a well-defined topic and generally can be covered in one lesson. In a number of cases, however, the instructor will find it desirable to devote more than one lesson to a given topic. Each chapter ends with a review and summary of the material covered in that chapter. Marginal notes are included in these sections to help students organize their review work, and cross-references are used to help them find the portions of material requiring their special attention. The sample problems are set up in much the same form that students will use when solving the assigned problems. They thus serve the double purpose of amplifying the text and demonstrating the type of neat and orderly work that students should cultivate in their own solutions. A section entitled Solving Problems on Your Own has been added to each lesson, between the sample problems and the problems to be assigned. The purpose of these new sections is to help students organize in their own minds the preceding theory of the text and the solution methods of the sample problems so that they may more successfully solve the homework problems. Also included in these sections are specific suggestions and strategies which will enable the students to more efficiently attack any assigned problems. Most of the problems are of a practical nature and should appeal to engineering students. They are primarily designed, however, to illustrate the material presented in the text and to help students understand the basic principles of mechanics. The problems have been grouped according to the portions of material they illustrate and have been arranged in order of increasing difficulty. Problems requiring special attention have been indicated by asterisks. Answers to 70% of the problems are given at the end of the book. Problems for which no answer is given are indicated by a number set in italic. Preface XV xvi Preface The inclusion in the engineering cmrimIum of instruction in computer programming and the widespread avaiJability of personal computers or mainframe terminals on most campuses make it possible for engineering students to solve a number of challenging mechanics problems. At one time these problems would have been considered inappropriate for an undergraduate course because of the large number of computations their solutions require. In this new edition of Vector Mechanics for Engineers: Statics, a group of problems designed to be solved with a computer follow the review problems at the end of each chapter. Many of these problems are relevant to the design process; they may involve the analysis of a structure for various configurations and loadings of the structure, or the determination of the equilibrium positions of a given mechanism which may require an iterative method of solution. Developing the algorithm required to solve a given mechanics problem will benefit the students in two different ways: (1) it will help them gain a better understanding of the mechanics principles involved; (2) it will provide them with an opportunity to apply the skills acquired in their computer programming course to the solution of a meaningful engineering problem. The authors wish to acknowledge the helpful collaboration of Professor Elliot Eisenberg to this sixth edition of Vector Mechanics for Engineers and thank him especially for contributing many new and challenging problems. The authors also gratefully acknowledge the many helpful comments and suggestions offered by the users of the previous editions of Mechanics for Engineers and of Vector Mechanics for Engineers. Ferdinand P. Beer E. Russell Johnston, Jr. List of Symbols Constant; radius; distance Reactions at supports and connections Points Area Width; distance Constant Centroid Distance Base of natural logarithms Force; friction force Acceleration of gravity Center of gravity; constant of gravitation Height; sag of cable U nit vectors along coordinate axes Moment of inertia Centroidal moment of inertia product of inertia Polar moment of inertia Spring constant Radius of gyration Centroidal radius of gyration Length Length; span Mass Couple; moment Moment about point 0 Moment resultant about point 0 Magnitude of couple or moment; mass of earth Moment about axis OL Normal component of reaction Origin of coordinates Pressure Force; vector Force; vector xvii xviii List of Symbols Position vector Radius; distance; polar coordinate Resultant force; resultant vector; reaction Radius of earth Position vector Length of arc; length of cable Force; vector Thickness Force Tension Work Vector product; shearing force Volume; potential energy; shear Load per unit length Weight; load Rectangular coordinates; distances Rectangular coordinates of centroid or center of gravity Angles Elongation Virtual displacement Virtual work Unit vector along a line Efficiency Angular coordinate; angle; polar coordinate Coefficient of friction Density Angle of friction; angle THIRD 51 METRIC EDITION Vector Mechanics for Engineers 2 Introduction 1.1. WHAT IS MECHANICS? Mechanics can be defined as that science which describes and predicts the conditions of rest or motion of bodies under the action of forces. It is divided into three parts: mechanics of rigid bodies, mechanics of deformable bodies, and mechanics of fluids. The mechanics of rigid bodies is subdivided into statics and dynamics, the former dealing with bodies at rest, the latter with bodies in motion. In this part of the study of mechanics, bodies are assumed to be perfectly rigid. Actual structures and machines, however, are never absolutely rigid and deform under the loads to which they are subjected. But these deformations are usually small and do not appreciably affect the conditions of equilibrium or motion of the structure under consideration. They are important, though, as far as the resistance of the structure to failure is concerned and are studied in mechanics of materials, which is a part of the mechanics of deformable bodies. The third division of mechanics, the mechanics of fluids, is subdivided into the study of incompressible fluids and of compressible fluids. An important subdivision of the study of incompressible fluids is hydraulics, which deals with problems involving water. Mechanics is a physical science, since it deals with the study of physical phenomena. However, some associate mechanics with mathematics, while many consider it as an engineering subject. Both these views are justified in part. Mechanics is the foundation of most engineering sciences and is an indispensable prerequisite to their study. However, it does not have the empiricism found in some engineering sciences, i.e., it does not rely on experience or observation alone; by its rigor and the emphasis it places on deductive reasoning it resembles mathematics. But, again, it is not an abstract or even a pure science; mechanics is an applied science. The purpose of mechanics is to explain and predict physical phenomena and thus to lay the foundations for engineering applications. 1.2. FUNDAMENTAL CONCEPTS AND PRINCIPLES Although the study of mechanics goes back to the time of Aristotle (384-322 B.C.) and Archimedes (287-212 B.C.), one has to wait until Newton (1642-1727) to find a satisfactory formulation of its fundamental principles. These principles were later expressed in a modified form by d'Alembert, Lagrange, and Hamilton. Their validity remained unchallenged, however, until Einstein formulated his theory of relativity (1905). While its limitations have now been recognized, newtonian mechanics still remains the basis of today's engineering sciences. The basic concepts used in mechanics are space, time, mass, and force. These concepts cannot be truly defined; they should be accepted on the basis of our intuition and experience and used as a mental frame of reference for our study of mechanics. The concept of space is associated with the notion of the position of a point P. The position of P can be defined by three lengths measured from a certain reference point, or origin, in three given directions. These lengths are known as the coordinates of P. To define an event, it is not sufficient to indicate its position in space. The time of the event should also be given. The concept of mass is used to characterize and compare bodies on the basis of certain fundamental mechanical experiments. Two bodies of the same mass, for example, will be attracted by the earth in the same manner; they will also offer the same resistance to a change in translational motion. A force represents the action of one body on another. It can be exerted by actual contact or at a distance, as in the case of gravitational forces and magnetic forces. A force is characterized by its point of application, its magnitude, and its direction; a force is represented by a vector (Sec. 2.3). In newtonian mechanics, space, time, and mass are absolute concepts, independent of each other. (This is not true in relativistic mechanics, where the time of an event depends upon its position, and where the mass of a body varies with its velocity.) On the other hand, the concept of force is not independent of the other three. Indeed, one of the fundamental principles of newtonian mechanics listed below indicates that the resultant force acting on a body is related to the mass of the body and to the manner in which its velocity varies with time. You will study the conditions of rest or motion of particles and rigid bodies in terms of the four basic concepts we have introduced. By particle we mean a very small amount of matter which may be assumed to occupy a single point in space. A rigid body is a combination of a large number of particles occupying fixed positions with respect to each other. The study of the mechanics of particles is obviously a prerequisite to that of rigid bodies. Besides, the results obtained for a particle can be used directly in a large number of problems dealing with the conditions of rest or motion of actual bodies. The study of elementary mechanics rests on six fundamental principles based on experimental evidence. The Parallelogram Law for the Addition of Forces. This states that two forces acting on a particle may be replaced by a single force, called their resultant, obtained by drawing the diagonal of the parallelogram which has sides equal to the given forces (Sec. 2.2). The Principle of Transmissibility. This states that the conditions of equilibrium or of motion of a rigid body will remain unchanged if a force acting at a given point of the rigid body is replaced by a force of the same magnitude and same direction, but acting at a different point, provided that the two forces have the same line of action (Sec. 3.3). Newton's Three Fundamental Laws. Formulated by Sir Isaac Newton in the latter part of the seventeenth century, these laws can be stated as follows: FIRST LAW. If the resultant force acting on a particle is zero, the particle will remain at rest (if originally at rest) or will move with constant speed in a straight line (if originally in motion) (Sec. 2.10). 1.2. Fundamental Concepts and Principles 3