Online electronic version May not be emailed or posted ANYWHERE May not be copied, or printed without express written permission of the authors. Introduction to S TATICS and D YNAMICS Filename:Saskyalaunch3517 Andy Ruina and Rudra Pratap OxfordUniversityPress(Preprint) MostrecentmodificationsonAugust21,2013. Reference Tables: The front and back tables concisely summarize much of the text material. Summary of Mechanics 0) Thelawsofmechanicsapplytoanycollectionofmaterialor‘body.’ Thisbodycouldbetheoverallsystemofstudy oranypartofit. Intheequationsbelow,theforcesandmomentsarethosethatshowonafreebodydiagram. Interacting bodiescauseequalandoppositeforcesandmomentsoneachother. I) LinearMomentumBalance(LMB)/ForceBalance EquationofMotion Fi L The total force on a body is equal (I) to its rate of change of linear momentum. Impulse-momentum t2 (integratingintime) Fi·dt L Netimpulseisequaltothechangein (Ia) t1 momentum. Conservationofmomentum L=0 When there is no net force the linear (Ib) (if Fi 0) L=L2 L1 0 momentum does not change. Statics Fi 0 If the inertial terms are zero the (Ic) (ifLisnegligible) net force on system is zero. II) AngularMomentumBalance(AMB)/MomentBalance Equationofmotion M H The sum of moments is equal to the (II) C C rateofchangeofangularmomentum. Impulse-momentum(angular) t2 M dt H The net angular impulse is equal to (IIa) (integratingintime) C C t1 the change in angular momentum. Conservationofangularmomentum H 0 C If there is no net moment about point (IIb) (if M 0) H H H 0 C C C2 C1 C then the angular momentum about point C does not change. Statics M 0 If the inertial terms are zero then the (IIc) (ifH isnegligible) C C total moment on the system is zero. III) PowerBalance(1stlawofthermodynamics) Equationofmotion Q P E E E Heat flow plus mechanical power (III) K P int into a system is equal to its change E in energy (kinetic + potential + internal). t2 t2 forfinitetime Qdt Pdt E Thenetenergyflowgoinginisequal (IIIa) t1 t1 tothenetchangeinenergy. ConservationofEnergy E 0 If no energyflows into a system, (IIIb) (ifQ P 0) E E E 0 2 1 then its energydoesnotchange. Statics Q P E E If there is no change of kinetic energy (IIIc) (ifE isnegligible) P int K then the change of potential and internal energy is due to mechanical work and heat flow. PureMechanics (ifheatflowanddissipation P E E In a system well modeled as purely (IIId) K P arenegligible) mechanical the change of kinetic and potential energy is due to mechanical work on the system. Filename:Summaryofmechanics Some definitions (Alsoseetheindexandbacktables) *r or *x Position e.g.,*r *r is the position of a point i i (cid:17) i=O relativetotheorigin,O. d*r *v Velocity e.g.,*v *v is the velocity of a point i (cid:17) dt i (cid:17) i=O relativetoO,measuredinanon-rotatingref- erenceframe. d*v d2*r *a Acceleration e.g.,*a *a istheaccelerationofapointi (cid:17) dt D dt2 i (cid:17) i=O relativetoO,measuredinaNewtonianframe. * F Force e.g.,theforceonAfromBisF . AfromB * * * M or M M MomentorTorque e.g., the moment of a collection of forces CD =C aboutpointC. *! Angularvelocity Ameasureofrotationalvelocityofarigidob- ject.*! =angularvelocityofrigidobjectB. B *(cid:11) *! Angularacceleration Ameasureofrotationalaccelerationofarigid (cid:17) P object. m*v discrete * i i L Linearmomentum Ameasureofasystem’snettranslationalrate (cid:17) 8 P*vdm continuous (weightedbymass). < m R*v D :tot cm m*a discrete * i i LP Rate of change of linear momen- The aspect of motion that balances the net (cid:17) 8 P*adm continuous tum forceonasystem. < m R*a D :tot cm *r m*v discrete H* i=C(cid:2) i i AngularmomentumaboutpointC Ameasureoftherotationalrateofasystem =C (cid:17) 8< P*r=C (cid:2)*vdm continuous about a point C (weighted by mass and dis- tancefromC). R : *r m*a discrete H*P=C (cid:17) 8< P*r=iC=C(cid:2)(cid:2)*admi i continuous RtuamteaobfocuhtapnogientoCfangularmomen- TtohrequaesopnecatsoyfstmemotiaobnouthtaatpboainlatnCc.es the net R : 1 m v2 discrete E 2 i i Kineticenergy Ascalarmeasureofnetsystemmotion. K (cid:17) 8 1Pv2dm continuous < 2 R : E (heat-liketerms) Internalenergy The non-kinetic non-potential part of a sys- int D tem’stotalenergy. P F**v M* *! Powerofforcesandtorques The mechanical energy flow into a system. (cid:17) i(cid:1) i C i(cid:1) i Also,P W,rateofwork. P P (cid:17) P Icm Icm Icm xx xy xz (cid:140)Icm(cid:141) 2 Icm Icm Icm 3 Moment of inertia matrix about Ameasureofthemassdistributioninarigid (cid:17) xy yy yz centerofmass(cm) object. 66 Ixczm Iyczm Izczm 77 6 7 4 5 iv Chapter0. (cid:13)c Rudra Pratap and Andy Ruina, 1994-2013. All rights reserved. No part of this book may be reproduced, stored in a retrievalsystem,ortransmitted,inanyformorbyanymeans,electronic,mechanical,photocopying,orotherwise,without priorwrittenpermissionoftheauthors. Thisbookisapre-releaseversionofabookinprogressforOxfordUniversityPress. Acknowledgements. Thefollowingareamongstthosewhohavehelpedwiththisbookaseditors,artists,texprogrammers, advisors,criticsorsuggestersandcreatorsofcontent:WilliamAdams,AlexaBarnes,PranavBhounsule,JosephBurns,Jason Cortell,GaborDomokos,MaxDonelan,ThuDong,GailFish,MikeFox,JohnGibson,RobertGhrist,SaptarsiHaldar,Dave Heimstra, Theresa Howley, Herbert Hui, Michael Marder, Elaina McCartney, Horst Nowacki, Jim Papadopoulos, Kalpana Pratap, DaneQuinn, RichardRand, C.V.Radakrishnan, NidhiRathi, PhoebusRosakis, LesSchaffer, IshanSharma, David Shipman, JillStartzell, SaskyavanNouhuys, TianTang, KimTurnerandBillZobrist. Ouron-againoff-againeditorPeter Gordonhasbeensupportivethroughout. Manyotherfriends,colleagues,relatives,students,andanonymousreviewershave alsomadehelpfulsuggestions. WecertifyArthurOgawa,IvanDobrianov,andStephenHicksasTeXgeniuses. MikeColemanworkedextensivelyonthetext,wrotemanyoftheexamplesandhomeworkproblemsandmademanyfigures. DavidHo,R.Manjula,AbhayandMiekeRuinadreworimprovedmostofthedrawings. Creditforsomeofthehomework problems retrieved from Cornell archives is due to various Theoretical and Applied Mechanics faculty. Harry Soodak and MartinTierstenprovidedsomeproblemsfromtheirincompletebook. SoftwarewehaveusedtopreparethisbookincludesTEXshop(forLATEX)withmanycustomfeaturesimplementedbyStephen Hicks,AdobeIllustrator,GraphicsConverterandMATLAB. IntroductiontoStaticsandDynamics,(cid:13)c AndyRuinaandRudraPratap1994-2013. IntroductiontoStaticsandDynamics,(cid:13)c AndyRuinaandRudraPratap1994-2013. 1 2 Chapter0. Brief Contents Fronttables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i BriefContents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 DetailedContents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Tothestudent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Part I: Basics for Mechanics 24 1 Whatismechanics? . . . . . . . . . . . . . . . . . . . . . . . . . 24 2 Vectors: position,forceandmoment . . . . . . . . . . . . . . . . 38 3 FBDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 Part II: Statics 186 4 Staticsofoneobject. . . . . . . . . . . . . . . . . . . . . . . . . 186 5 Trussesandframes . . . . . . . . . . . . . . . . . . . . . . . . . 258 6 Transmissionsandmechanisms. . . . . . . . . . . . . . . . . . . 324 7 Tension,shearandbendingmoment . . . . . . . . . . . . . . . . 378 8 Hydrostatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402 Part III: Dynamics 418 9 Dynamicsin1D . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 10 Vibrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498 11 Particlesinspace . . . . . . . . . . . . . . . . . . . . . . . . . . 544 12 Manyparticlesinspace . . . . . . . . . . . . . . . . . . . . . . . 592 13 Straightlinemotion . . . . . . . . . . . . . . . . . . . . . . . . . 618 14 Circularmotionofaparticle . . . . . . . . . . . . . . . . . . . . 658 15 Circularmotionofarigidobject . . . . . . . . . . . . . . . . . . 690 16 Planarmotionofanobject . . . . . . . . . . . . . . . . . . . . . 770 17 Time-varyingbasisvectors . . . . . . . . . . . . . . . . . . . . . 856 18 Constrainedparticlesandrigidobjects . . . . . . . . . . . . . . . 926 Appendices 994 A Unitsanddimensions . . . . . . . . . . . . . . . . . . . . . . . . 994 B Friction: perspectivesonfrictionlaws . . . . . . . . . . . . . . .1006 C ThesimplestODEsandtheirsolutions . . . . . . . . . . . . . . .1016 D TheoremsforSystems . . . . . . . . . . . . . . . . . . . . . . .1020 Answerstosomehomeworkproblems . . . . . . . . . . . . . . . . .1030 Backtables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1039 IntroductiontoStaticsandDynamics,(cid:13)c AndyRuinaandRudraPratap1994-2013. Detailed Contents Fronttables i Summaryofmechanics . . . . . . . . . . . . . . . . . . . i Somebasicdefinitions . . . . . . . . . . . . . . . . . . . . ii BriefContents 2 DetailedContents 3 Preface 12 General issues about content, level, organization, style and motivation. Studyadvicestartsonpage ??. Tothestudent 16 Howtostudy. Theuseofcomputers. 0.1 Anoteoncomputation . . . . . . . . . . . . . . . . . . . . . 20 Box: Informalcomputercommands . . . . . . . . . . . . . 23 Part I: Basics for Mechanics 24 1 Whatismechanics? 24 Mechanicscanpredictforcesandmotionsbyusingthethreepillarsofthe subject: I. models of physical behavior, II. geometry, and III. the basic mechanicsbalancelaws. Thelawsofmechanicsareinformallysumma- rized in this introductory chapter. The extreme accuracy of Newtonian mechanicsisemphasized,despiterelativityandquantummechanicssup- posedly having ‘overthrown’ seventeenth century physics. Various uses oftheword‘model’aredescribed. 1.1 Thethreepillars . . . . . . . . . . . . . . . . . . . . . . . . 25 1.2 Mechanicsiswrong,whystudyit? . . . . . . . . . . . . . . 31 1.3 Thehierarchyofmodels . . . . . . . . . . . . . . . . . . . . 33 2 Vectors: position,forceandmoment 38 The key vectors for statics, namely relative position, force, and mo- ment, are used to develop vector skills. Notational clarity is empha- sized because good vector calculation demands distinguishing vectors fromscalars. Vectoradditionismotivatedbytheneedtoaddforcesand relativepositions. Dotproductsaremotivatedasthetoolwhichreduces vectorequationstoscalarequations.Andcrossproductsaremotivatedas IntroductiontoStaticsandDynamics,(cid:13)c AndyRuinaandRudraPratap1994-2013. 3 4 Chapter0.DetailedContents DetailedContents theformulawhichcorrectlycalculatestheheuristicallymotivatedquan- titiesofmomentandmomentaboutanaxis. 2.1 Notationandaddition . . . . . . . . . . . . . . . . . . . . . 40 Box2.1Thescalarsinmechanics . . . . . . . . . . . . . . 41 Box2.2TheVectorsinMechanics . . . . . . . . . . . . . 42 2.2 Thedotproductoftwovectors . . . . . . . . . . . . . . . . 58 Box2.3Basicfeaturesofthevectordotproduct. . . . . . . 58 Box2.4abcos(cid:18) a b a b a b . . . . . . . 63 x x y y z z ) C C 2.3 Vectorcrossproduct . . . . . . . . . . . . . . . . . . . . . . 67 Box2.5Usesofthecrossproduct . . . . . . . . . . . . . . 68 Box2.6Crossproductasamatrixmultiply . . . . . . . . . 74 Box2.7Thecrossproduct: fromgeometrytocomponents . 75 2.4 Moment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 2.5 Solvingvectorequations . . . . . . . . . . . . . . . . . . . . 93 Box2.8Therulesofvectoralgebra. . . . . . . . . . . . . 94 Box2.9Vectortrianglesandthelawsofsinesandcosines . 96 Box2.10Existence,uniqueness,andgeometry . . . . . . . 108 ProblemsforChapter2 . . . . . . . . . . . . . . . . . . . . . . . 113 3 FBDs 122 Afree-bodydiagramisasketchofthesystemtowhichyouwillapplythe lawsofmechanics. Thediagramshowsallofthenon-negligibleexternal forcesandcoupleswhichactonthesystem. Thediagramtellswhatma- terial is in the system and also what is known, and what is not known, about the forces. Mechanics reasoning depends on free body diagrams so we give tips about how to avoid common mistakes. On a free body diagram systems of forces are often replaced with ‘equivalent’ forces, a specialcaseofwhichisaweightforceatthecenterofgravity. 3.1 Equivalentforcesystems . . . . . . . . . . . . . . . . . . . . 125 Box3.1 meansadd . . . . . . . . . . . . . . . . . . . . 127 Box3.2Equivalentatonepoint equivalentatallpoints 128 P ) Box3.3A“wrench”canrepresentanyforcesystem . . . . 129 3.2 Centerofmassandgravity. . . . . . . . . . . . . . . . . . . 134 Box3.4Like ,thesymbol alsomeansadd . . . . . . . 135 Box3.5Eachsubsystemislikeaparticle . . . . . . . . . . 139 P R Box3.6TheCOMofatriangleisath=3 . . . . . . . . . . 143 3.3 Interactions,forces&partialFBDs . . . . . . . . . . . . . . 149 VectornotationforFBDs . . . . . . . . . . . . . . . . . . 151 Box3.7Freebodydiagramfirst,mechanicsreasoningafter 159 Box3.8ActionandreactiononpartialFBD’s . . . . . . . 161 3.4 Contact: Sliding,friction,androlling . . . . . . . . . . . . . 164 ProblemsforChapter3 . . . . . . . . . . . . . . . . . . . . . . . 178 IntroductiontoStaticsandDynamics,(cid:13)c AndyRuinaandRudraPratap1994-2013. Chapter0.DetailedContents DetailedContents 5 Part II: Statics 186 4 Staticsofoneobject 186 One object is in equilibrium if the forces and moments balance. For a particle,forcebalancetellsall. Butforanextendedobject,momentbal- ance is also essential. There are special shortcuts for an objects that hasexactlytwoorexactlythreeforcesactingonit. Iffrictionforcesare relevant the possibility of motion needs to be taken into account. Many real-world problems are not statically determinate and thus yield either only partial solutions, or yield full solutions after you have made extra assumptions. 4.1 Staticequilibriumofaparticle . . . . . . . . . . . . . . . . . 188 Box4.1Existenceanduniqueness . . . . . . . . . . . . . 192 Box4.2Thesimplificationofdynamicstostatics . . . . . . 195 4.2 Equilibriumofoneobject . . . . . . . . . . . . . . . . . . . 202 Box4.3Two-forcebodies . . . . . . . . . . . . . . . . . . 207 Box4.4Three-forcebodies . . . . . . . . . . . . . . . . . 208 Box4.5Momentbalanceabout3pointsissufficientin2D . 209 4.3 Equilibriumwithfrictionalcontact . . . . . . . . . . . . . . 214 Box4.6Undrivenwheelsandtwoforcebodies . . . . . . 218 4.4 Internalforces . . . . . . . . . . . . . . . . . . . . . . . . . 228 4.5 3Dstaticsofonepart . . . . . . . . . . . . . . . . . . . . . 234 Box4.7Staticallydeterminatewaystoholdanobjectin3D 239 ProblemsforChapter4 . . . . . . . . . . . . . . . . . . . . . . . 243 5 Trussesandframes 258 Hereweconsidercollectionsofpartsassembledsoastoholdsomething up or hold something in place. Emphasis is on trusses, assemblies of bars connected by pins at their ends. Trusses are analyzed by drawing free body diagrams of the pins or of bigger parts of the truss (method ofsections). Frameworksbuiltwithotherthantwo-forcebodiesarealso analyzedbydrawingfreebodydiagramsofparts. Structurescanberigid or not and redundant or not, as can be determined by the collection of equilibriumequations. 5.1 Methodofjoints . . . . . . . . . . . . . . . . . . . . . . . . 260 5.2 Themethodofsections . . . . . . . . . . . . . . . . . . . . 277 5.3 Solvingtrussesonacomputer . . . . . . . . . . . . . . . . . 284 5.4 Framesandstructures . . . . . . . . . . . . . . . . . . . . . 295 Box5.1The‘methodofbarsandpins’fortrusses . . . . . 298 5.5 Advancedtrussconcepts: determinacy . . . . . . . . . . . . 305 Box5.2Structuralrigidityandgeometriccongruence . . . 310 Box5.3Rigidity,redundancy,linearalgebraandmaps . . 311 ProblemsforChapter5 . . . . . . . . . . . . . . . . . . . . . . . 316 6 Transmissionsandmechanisms 324 Some collections of solid parts are assembled so as to cause force or torque in one place given a different force or torque in another. These include levers, gear boxes, presses, pliers, clippers, chain drives, and crank-drives. Besides solid parts connected by pins, a few special- IntroductiontoStaticsandDynamics,(cid:13)c AndyRuinaandRudraPratap1994-2013. 6 Chapter0.DetailedContents DetailedContents purpose parts are commonly used, including springs and gears. Tricks for amplifying force are usually based on principals idealized by pul- leys, levers, wedges and toggles. Force-analysis of transmissions and mechanisms is done by drawing free body diagrams of the parts, writ- ingequilibriumequationsforthese,andsolvingtheequationsfordesired unknowns. 6.1 Springs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 Box6.1‘Zero-length’springs . . . . . . . . . . . . . . . . 327 Box6.2Howstiffaspringisasolidrod . . . . . . . . . . 334 Box6.3Stifferbutweaker . . . . . . . . . . . . . . . . . . 334 Box6.4Apuzzlewithtwospringsandthreeropes. . . . . . 335 Box6.52Dgeometryofspringstretch . . . . . . . . . . . 338 6.2 Forceamplification . . . . . . . . . . . . . . . . . . . . . . 347 6.3 Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . 358 Box6.6Shearswithgears . . . . . . . . . . . . . . . . . . 362 ProblemsforChapter6 . . . . . . . . . . . . . . . . . . . . . . . 369 7 Tension,shearandbendingmoment 378 The ‘internal forces’ tension, shear and bending moment can vary from point to point in long narrow objects. Here we introduce the notion of graphingthisvariationandnotingthefeaturesofthesegraphs. 7.1 Arbitrarycuts . . . . . . . . . . . . . . . . . . . . . . . . . 379 7.2 Singularityfunctions . . . . . . . . . . . . . . . . . . . . . . 395 ProblemsforChapter7 . . . . . . . . . . . . . . . . . . . . . . . 400 8 Hydrostatics 402 Hydrostaticsconcernstheequivalentforceandmomentduetodistributed pressure on a surface from a still fluid. Pressure increases with depth. With constant pressure the equivalent force has magnitude = pressure times area, acting at the centroid. For linearly-varying pressure on a rectangular plate the equivalent force is the average pressure times the area acting 2/3 of the way down. The net force acting on a totally sub- merged object in a constant density fluid is the displaced fluid’s weight actingatthecentroid. 8.1 Fluidpressure . . . . . . . . . . . . . . . . . . . . . . . . . 403 Box8.1AddingforcestoderiveArchimedes’principle . . . 406 Box8.2Pressuredependsonpositionbutnotonorientation 407 ProblemsforChapter8 . . . . . . . . . . . . . . . . . . . . . . . 415 Part III: Dynamics 418 9 Dynamicsin1D 418 ThescalarequationF Dmaintroducestheconceptsofmotionandtime derivatives to mechanics. In particular the equations of dynamics are seen to reduce to ordinary differential equations, the simplest of which have memorable analytic solutions. The harder differential equations needbesolvedonacomputer. Weexplorevariousconceptsandapplica- IntroductiontoStaticsandDynamics,(cid:13)c AndyRuinaandRudraPratap1994-2013.
Description: