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VARIATIONALPRINCIPLES IN CLASSICALMECHANICS

DouglasCline

UniversityofRochester

9August2017

c °2017DouglasCline

ISBN:978-0-9988372-4-6e-book(AdobePDFcolor)

ISBN:978-0-9988372-5-3print(Paperbackgrayscale)

VariationalPrinciplesinClassicalMechanics

Contributors

Author:DouglasCline

Illustrator:MeghanSarkis

PublishedbyUniversityofRochesterRiverCampusLibraries UniversityofRochester Rochester,NY14627

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Version1.0

1Abriefhistoryofclassicalmechanics1

1.1Introduction..............................................1

1.2Prehistoricastronomy........................................1

1.3Greekantiquity............................................1

1.4MiddleAges..............................................2

1.5AgeofEnlightenment........................................3

1.6 19 century..............................................5

1.7The 20 centuryrevolutioninphysics...............................7

2ReviewofNewtonianmechanics9

2.1Introduction..............................................9

2.2Newton’sLawsofmotion......................................9

2.3Inertialframesofreference......................................10

2.4First-orderintegralsinNewtonianmechanics...........................11

2.4.1LinearMomentum......................................11

2.4.2Angularmomentum.....................................11

2.4.3Kineticenergy........................................12

2.5Conservationlawsinclassicalmechanics..............................12

2.6Motionof finite-sizedandmany-bodysystems...........................12

2.7Centerofmassofamany-bodysystem...............................13

2.8Totallinearmomentumofamany-bodysystem..........................14

2.8.1Center-of-massdecomposition................................14

2.8.2Equationsofmotion.....................................14

2.9Angularmomentumofamany-bodysystem............................16

2.9.1Center-of-massdecomposition................................16

2.9.2Equationsofmotion.....................................16

2.10Workandkineticenergyforamany-bodysystem.........................18

2.10.1Center-of-masskineticenergy................................18

2.10.2Conservativeforcesandpotentialenergy..........................18

2.10.3Totalmechanicalenergy...................................19

2.10.4Totalmechanicalenergyforconservativesystems.....................20

2.11VirialTheorem............................................22

2.12ApplicationsofNewton’sequationsofmotion...........................24

2.12.1Constantforceproblems...................................24

2.12.2LinearRestoringForce....................................25

2.12.3Position-dependentconservativeforces...........................25

2.12.4Constrainedmotion.....................................27

2.12.5VelocityDependentForces..................................28

2.12.6SystemswithVariableMass.................................29

2.12.7Rigid-bodyrotationaboutabody-fixedrotationaxis...................31

2.12.8Timedependentforces....................................34

2.13Solutionofmany-bodyequationsofmotion............................37

2.13.1Analyticsolution.......................................37

2.13.2Successiveapproximation..................................37

2.13.3Perturbationmethod.....................................37

2.14Newton’sLawofGravitation....................................38

2.14.1Gravitationalandinertialmass...............................38

2.14.2Gravitationalpotentialenergy

2.14.3Gravitationalpotential

2.14.4Potentialtheory.......................................41

2.14.5Curlofthegravitational

2.14.6Gauss’sLawforGravitation.................................43

2.14.7CondensedformsofNewton’sLawofGravitation.....................44 2.15Summary...............................................46

3.4.4Planependulum.......................................57

3.5Linearly-dampedfreelinearoscillator

3.5.1Generalsolution.......................................58 3.5.2Energydissipation......................................61

3.6Sinusoidally-drive,linearly-damped,linearoscillator...

3.6.1Transientresponseofadrivenoscillator

3.6.2Steadystateresponseofadrivenoscillator ........................63

3.6.3Completesolutionofthedrivenoscillator.........................64

3.6.4Resonance...........................................65

3.6.5Energyabsorption......................................65

3.7Waveequation............................................68

3.8Travellingandstandingwavesolutionsofthewaveequation...................69

3.9Waveformanalysis..........................................70

3.9.1Harmonicdecomposition...................................70

3.9.2Thefreelinearly-dampedlinearoscillator .........................70

3.9.3Dampedlinearoscillatorsubjecttoanarbitraryperiodicforce.............71

3.10Signalprocessing...........................................72

3.11Wavepropagation ..........................................73

3.11.1Phase,group,andsignalvelocitiesofwavepackets....................74

3.11.2Fouriertransformofwavepackets.............................79

3.11.3Wave-packetUncertaintyPrinciple.............................80

3.12Summary...............................................82

4Nonlinearsystemsandchaos89

4.1Introduction..............................................89

4.2Weaknonlinearity..........................................90

4.3Bifurcation,andpointattractors..................................92

4.4Limitcycles..............................................93

4.4.1Poincaré-Bendixsontheorem................................93

4.4.2vanderPoldampedharmonicoscillator:..........................94

4.5Harmonically-driven,linearly-damped,planependulum......................97

4.5.1Closetolinearity.......................................97

4.5.2Weaknonlinearity......................................99

4.5.3Onsetofcomplication....................................100

4.5.4Perioddoublingandbifurcation...............................100

4.5.5Rollingmotion ........................................100

4.5.6Onsetofchaos........................................101

4.6Differentiationbetweenorderedandchaoticmotion........................102

4.6.1Lyapunovexponent.....................................102

4.6.2Bifurcationdiagram.....................................103

4.6.3PoincaréSection.......................................104

4.7Wavepropagationfornon-linearsystems... ...........................105

4.7.1Phase,group,andsignalvelocities.............................105

4.7.2Solitonwavepropagation ..................................107

4.8Summary...............................................108 Workshopexercises.............................................110 Problems..................................................110

5Calculusofvariations

5.1Introduction..............................................111

5.2Euler’sdifferentialequation.....................................112

5.3ApplicationsofEuler’sequation...................................114

5.4Selectionoftheindependentvariable................................117

5.5Functionswithseveralindependentvariables  () ........................119

5.6Euler’sintegralequation.......................................121

5.7Constrainedvariationalsystems...................................122

5.7.1Holonomicconstraints....................................122

5.7.2Geometric(algebraic)equationsofconstraint.......................122

5.7.3Kinematic(differential)equationsofconstraint......................122

5.7.4Isoperimetric(integral)equationsofconstraint......................123

5.7.5Propertiesoftheconstraintequations...........................123

5.7.6Treatmentofconstraintforcesinvariationalcalculus...................124

5.8Generalizedcoordinatesinvariationalcalculus..........................125

5.9Lagrangemultipliersforholonomicconstraints..........................126

5.9.1Algebraicequationsofconstraint..............................126

5.9.2Integralequationsofconstraint...............................128

5.10Geodesic................................................130

5.11Variationalapproachtoclassicalmechanics............................131

5.12Summary...............................................132 Workshopexercises.............................................133

6Lagrangiandynamics 135

6.1Introduction..............................................135

6.2NewtonianplausibilityargumentforLagrangianmechanics ...................136

6.3Lagrangeequationsfromd’Alembert’sPrinciple..........................138

6.3.1d’Alembert’sPrincipleofvirtualwork...........................138

6.3.2Transformationtogeneralizedcoordinates.........................139

6.3.3Lagrangian..........................................140

6.4LagrangeequationsfromHamilton’sPrinciple...........................141

6.5Constrainedsystems.........................................142

6.5.1Choiceofgeneralizedcoordinates..............................142

6.5.2Minimalsetofgeneralizedcoordinates...........................142

6.5.3Lagrangemultipliersapproach...............................142

6.5.4Generalizedforcesapproach.................................144

6.6ApplyingtheEuler-Lagrangeequationstoclassicalmechanics..................144

6.7Applicationstounconstrainedsystems...............................146

6.8Applicationstosystemsinvolvingholonomicconstraints.....................148

6.9Applicationsinvolvingnon-holonomicconstraints.........................161

6.10Velocity-dependentLorentzforce..................................168

6.11Time-dependentforces........................................169

6.12Impulsiveforces............................................170

6.13TheLagrangianversustheNewtonianapproachtoclassicalmechanics.............172

6.14Summary...............................................173 Workshopexercises.............................................176 Problems..................................................178

7Symmetries,InvarianceandtheHamiltonian179

7.1Introduction..............................................179

7.2Generalizedmomentum.......................................179

7.3InvarianttransformationsandNoether’sTheorem.........................181

7.4Rotationalinvarianceandconservationofangularmomentum..................183

7.5Cycliccoordinates..........................................184

7.6Kineticenergyingeneralizedcoordinates.............................185

7.7GeneralizedenergyandtheHamiltonianfunction.........................186

7.8Generalizedenergytheorem.....................................187

7.9Generalizedenergyandtotalenergy................................187

7.10Hamiltonianinvariance........................................188

7.11Hamiltonianforcycliccoordinates.................................193

7.12Symmetriesandinvariance.....................................193

7.13Hamiltonianinclassicalmechanics.................................193

7.14Summary...............................................194

Workshopexercises.............................................196 Problems..................................................197

8Hamiltonianmechanics199

8.1Introduction..............................................199

8.2LegendreTransformationbetweenLagrangianandHamiltonianmechanics...........200

8.3Hamilton’sequationsofmotion...................................201

8.3.1Canonicalequationsofmotion...............................202

8.4Hamiltonianindifferentcoordinatesystems............................203

8.4.1Cylindricalcoordinates  ................................203

8.4.2Sphericalcoordinates,  .................................204

8.5ApplicationsofHamiltonianDynamics...............................205

8.6Routhianreduction..........................................210

8.6.1R -RouthianisaHamiltonianforthecyclicvariables................211

8.6.2R -RouthianisaHamiltonianforthenon-cyclicvariables...........212

8.7Dissipativedynamicalsystems....................................216

8.7.1Generalizeddragforce....................................216

8.7.2Rayleigh’sdissipationfunction...............................217

8.8Summary...............................................221

Workshopexercises.............................................223 Problems..................................................224

9Conservativetwo-bodycentralforces227

9.1Introduction..............................................227

9.2Equivalentone-bodyrepresentationfortwo-bodymotion.....................228

9.3Angularmomentum L ........................................230

9.4Equationsofmotion.........................................231

9.5Differentialorbitequation:......................................232

9.6Hamiltonian..............................................233

9.7Generalfeaturesoftheorbitsolutions...............................234

9.8Inverse-square,two-body,centralforce...............................235

9.8.1Boundorbits.........................................236

9.8.2Kepler’slawsforboundplanetarymotion.........................237

9.8.3Unboundorbits........................................238

9.8.4Eccentricityvector... ...................................239

9.9Isotropic,linear,two-body,centralforce..............................241

9.9.1Polarcoordinates.......................................242

9.9.2Cartesiancoordinates....................................243

9.9.3Symmetrytensor A0 .....................................244

9.10Closed-orbitstability.........................................245

9.11Thethree-bodyproblem.......................................250

9.12Two-bodyscattering.........................................251

9.12.1Totaltwo-bodyscatteringcrosssection..........................251

9.12.2Differentialtwo-bodyscatteringcrosssection.......................252

9.12.3Impactparameterdependenceonscatteringangle....................252

9.12.4Rutherfordscattering....................................254

9.13Two-bodykinematics.........................................256 9.14Summary...............................................262

10Non-inertialreferenceframes267

10.1Introduction..............................................267

10.2Translationalaccelerationofareferenceframe ...........................267

10.3Rotatingreferenceframe.......................................268

10.3.1Spatialtimederivativesinarotating,non-translating,referenceframe.........268

10.3.2Generalvectorinarotating,non-translating,referenceframe..............269

10.4Referenceframeundergoingrotationplustranslation... ....................270

10.5Newton’slawofmotioninanon-inertialframe..........................270

10.6Lagrangianmechanicsinanon-inertialframe...........................271

10.7Centrifugalforce...........................................272

10.8Coriolisforce.............................................273

10.9Routhianreductionforrotatingsystems..............................277

10.10EffectivegravitationalforcenearthesurfaceoftheEarth....................280

10.11Freemotionontheearth.......................................282

10.12Weathersystems...........................................284

10.12.1Low-pressuresystems:....................................284

10.12.2High-pressuresystems:....................................286

10.13Foucaultpendulum..........................................286

10.14Summary...............................................288

Workshopexercises.............................................289 Problems..................................................290

11Rigid-bodyrotation 291

11.1Introduction..............................................291

11.2Rigid-bodycoordinates........................................292

11.3Rigid-bodyrotationaboutabody-fixedpoint...........................292

11.4Inertiatensor.............................................294

11.5Matrixandtensorformulationsofrigid-bodyrotation......................295

11.6Principalaxissystem.........................................295

11.7Diagonalizetheinertiatensor. ...................................296

11.8Parallel-axistheorem.........................................297 11.9Perpendicular-axistheoremforplanelaminae...........................300

11.10Generalpropertiesoftheinertiatensor...............................301 11.10.1Inertialequivalence......................................301

11.10.2Orthogonalityofprincipalaxes ...............................302

11.11Angularmomentum L andangularvelocity ω vectors......................303

11.12Kineticenergyofrotatingrigidbody................................305

11.13Eulerangles..............................................307

11.14Angularvelocity ω ..........................................309

11.15KineticenergyintermsofEulerangularvelocities........................310 11.16Rotationalinvariants.........................................311

11.17Euler’sequationsofmotionforrigid-bodyrotation........................312

11.18Lagrangeequationsofmotionforrigid-bodyrotation.......................313

11.19Hamiltonianequationsofmotionforrigid-bodyrotation.....................315

11.20Torque-freerotationofaninertially-symmetricrigidrotor....................315 11.20.1Euler’sequationsofmotion:.................................315 11.20.2Lagrangeequationsofmotion:...............................319

11.21Torque-freerotationofanasymmetricrigidrotor.........................321

11.22Stabilityoftorque-freerotationofanasymmetricbody.. ....................322

12.7Two-bodycoupledoscillatorsystems

12.11Dampedcoupledlinearoscillators

13Hamilton’sprincipleofleastaction381 13.1Introduction..............................................381

13.2PrincipleofLeastAction......................................382

13.2.1Hamilton’sPrinciple.....................................382 13.2.2Least-actionprincipleinHamiltonianmechanics.....................383 13.2.3Abbreviatedaction......................................384

13.3StandardLagrangian.........................................385

13.4GaugeinvarianceoftheLagrangian.................................385

13.5Non-standardLagrangians......................................387

13.6Inversevariationalcalculus.....................................387

13.7DissipativeLagrangians.......................................388

13.8Linearvelocity-dependentdissipation................................389 13.9Summary...............................................392

14AdvancedHamiltonianmechanics393 14.1Introduction..............................................393

14.2PoissonbracketrepresentationofHamiltonianmechanics....................395 14.2.1PoissonBrackets.......................................395 14.2.2FundamentalPoissonbrackets:...............................395 14.2.3Poissonbracketinvariancetocanonicaltransformations.................396 14.2.4CorrespondenceofthecommutatorandthePoissonBracket...............397 14.2.5ObservablesinHamiltonianmechanics...........................398 14.2.6Hamilton’sequationsofmotion...............................401

14.2.7Liouville’sTheorem.. ...................................405 14.3CanonicaltransformationsinHamiltonianmechanics.......................407 14.3.1Generatingfunctions.....................................408 14.3.2Applicationsofcanonicaltransformations.........................410 14.4Hamilton-Jacobitheory.......................................412 14.4.1Time-dependentHamiltonian................................412 14.4.2Time-independentHamiltonian...............................414 14.4.3Separationofvariables....................................415 14.4.4Visualrepresentationoftheactionfunction  .......................422 14.4.5AdvantagesofHamilton-Jacobitheory...........................422 14.5Action-anglevariables........................................423 14.5.1Canonicaltransformation..................................423

14.5.2Adiabaticinvarianceoftheactionvariables........................426

14.6Canonicalperturbationtheory...................................428 14.7Symplecticrepresentation......................................430 14.8ComparisonoftheLagrangianandHamiltonianformulations..................430 14.9Summary...............................................432 Workshopexercises.............................................435 Problems..................................................436

15Analyticalformulationsforcontinuoussystems437 15.1Introduction..............................................437

15.2Thecontinuousuniformlinearchain................................437

15.3TheLagrangiandensityformulationforcontinuoussystems...................438 15.3.1Onespatialdimension....................................438 15.3.2Threespatialdimensions..................................439

15.4TheHamiltoniandensityformulationforcontinuoussystems..................440

15.5Linearelasticsolids..........................................441

15.5.1Stresstensor.........................................442

15.5.2Straintensor.........................................442

15.5.3Moduliofelasticity......................................443

15.5.4Equationsofmotioninauniformelasticmedia......................444

15.6Electromagnetic fieldtheory.....................................445

15.6.1Maxwellstresstensor....................................445

15.6.2Momentumintheelectromagnetic field..........................446

15.7Ideal fluiddynamics.........................................447

15.7.1Continuityequation.....................................447

15.7.2Euler’shydrodynamicequation...............................447

15.7.3Irrotational flowandBernoulli’sequation

15.7.4Gas flow............................................448

15.8Viscous fluiddynamics........................................450

15.8.1Navier-Stokesequation....................................450

15.8.2Reynoldsnumber.......................................451

15.8.3Laminarandturbulent fluid flow..............................451

15.9Summaryandimplications.....................................453

16Relativisticmechanics

16.3.3TimeDilation:........................................458

16.3.4LengthContraction.....................................459

16.3.5Simultaneity.........................................459

16.4Relativistickinematics........................................462

16.4.1Velocitytransformations...................................462

16.4.2Momentum..........................................462

16.4.3Centerofmomentumcoordinatesystem..........................463 16.4.4Force.............................................463

16.4.5Energy.............................................463

16.5Geometryofspace-time.......................................465

16.5.1Four-dimensionalspace-time................................465

16.5.2Four-vectorscalarproducts.................................466

16.5.3Minkowskispace-time....................................467

16.5.4Momentum-energyfourvector...............................468

16.6Lorentz-invariantformulationofLagrangianmechanics......................469

16.6.1Parametricformulation...................................469

16.6.2ExtendedLagrangian....................................469

16.6.3Extendedgeneralizedmomenta...............................471

16.6.4ExtendedLagrangeequationsofmotion..........................471

16.7Lorentz-invariantformulationsofHamiltonianmechanics.....................474

16.7.1Extendedcanonicalformalism................................474

16.7.2ExtendedPoissonBracketrepresentation.........................476

16.7.3ExtendedcanonicaltransformationandHamilton-Jacobitheory.............476

16.7.4ValidityoftheextendedHamilton-Lagrangeformalism..................476

16.8TheGeneralTheoryofRelativity..................................478

16.8.1Fundamentalconcepts....................................478

16.8.2Einstein’spostulatesoftheGeneralTheoryofRelativity.................479

16.8.3Experimentalevidence....................................479

16.9Implicationsofrelativistictheorytoclassicalmechanics.....................480

16.10Summary...............................................481

Workshopexercises.............................................482 Problems..................................................482

17Thetransitiontoquantumphysics483

17.1Introduction..............................................483

17.2Briefsummaryoftheoriginsofquantumtheory.........................483

17.2.1Bohrmodeloftheatom...................................485

17.2.2Quantization.........................................485

17.2.3Wave-particleduality....................................486

17.3Hamiltonianinquantumtheory...................................487

17.3.1Heisenberg’smatrix-mechanicsrepresentation.......................487

17.3.2Schrödinger’swave-mechanicsrepresentation.......................489

17.4Lagrangianrepresentationinquantumtheory...........................490

17.5CorrespondencePrinciple......................................491

17.6Summary...............................................492

18Epilogue 493

Appendices

AMatrixalgebra 495

A.1Mathematicalmethodsformechanics................................495

A.2Matrices................................................495

A.3Determinants.............................................499

A.4Reductionofamatrixtodiagonalform... ...........................501

BVectoralgebra 505

B.1Linearoperations...........................................505 B.2Scalarproduct............................................505

B.3Vectorproduct............................................506

B.4Tripleproducts............................................507

COrthogonalcoordinatesystems509

C.1Cartesiancoordinates ( ) ....................................509

C.2Curvilinearcoordinatesystems ...................................509

C.2.1Two-dimensionalpolarcoordinates ( ) ..........................510

C.2.2CylindricalCoordinates ( ) ..............................512

C.2.3SphericalCoordinates () ................................512

C.3Frenet-Serretcoordinates......................................513

DCoordinatetransformations515

D.1Translationaltransformations....................................515

D.2Rotationaltransformations.....................................515

D.2.1Rotationmatrix.......................................515

D.2.2Finiterotations........................................518

D.2.3Infinitessimalrotations....................................519

D.2.4Properandimproperrotations...............................519

D.3Spatialinversiontransformation...................................520

D.4Timereversaltransformation....................................521

ETensoralgebra 523

E.1Tensors................................................523

E.2Tensorproducts............................................524

E.2.1Tensorouterproduct.....................................524

E.2.2Tensorinnerproduct.....................................524

E.3Tensorproperties...........................................525

E.4Contravariantandcovarianttensors................................526

E.5Generalizedinnerproduct......................................527

E.6Transformationpropertiesofobservables..............................528

FAspectsofmultivariatecalculus529

F.1Partialdifferentiation........................................529

F.2Linearoperators...........................................529

F.3TransformationJacobian.......................................531

F.3.1Transformationofintegrals:.................................531

F.3.2Transformationofdifferentialequations:..........................531

F.3.3PropertiesoftheJacobian:.................................531

F.4Legendretransformation.......................................532

GVectordifferentialcalculus533

G.1Scalardifferentialoperators.....................................533

G.1.1Scalar field..........................................533

G.1.2Vector field..........................................533

G.2Vectordifferentialoperatorsincartesiancoordinates.......................533

G.2.1Scalar field..........................................533

G.2.2Vector field..........................................534

G.3Vectordifferentialoperatorsincurvilinearcoordinates.. ....................535

G.3.1Gradient:...........................................535

G.3.2Divergence:..........................................536

G.3.3Curl:..............................................536

G.3.4Laplacian:...........................................536

HVectorintegralcalculus537

H.1Lineintegralofthegradientofascalar field............................537

H.2Divergencetheorem.........................................537

H.2.1Fluxofavector fieldforGaussiansurface.........................537

H.2.2Divergenceincartesiancoordinates.............................538

H.3StokesTheorem............................................540

H.3.1Thecurl............................................540

H.3.2Curlincartesiancoordinates................................541

H.4Potentialformulationsofcurl-freeanddivergence-free fields...................543

IWaveformanalysis 545

I.1Harmonicwaveformdecomposition.................................545

I.1.1PeriodicsystemsandtheFourierseries...........................545

I.1.2AperiodicsystemsandtheFourierTransform.......................547

I.2Time-sampledwaveformanalysis..................................548

I.2.1Delta-functionimpulseresponse..............................549

I.2.2Green’sfunctionwaveformdecomposition.........................550

Example:Bolasthrownbygaucho

2.5 Example:Theidealgaslaw

2.6 Example:Themassofgalaxies

2.7 Example:Diatomicmolecule

2.8 Example:Rollercoaster

Example:Merry-go-round

2.14 Example:Centerofpercussionofabaseballbat

2.15 Example:Energytransferincharged-particlescattering

3.3 Example:Waterwavesbreakingonabeach

3.6 Example:FouriertransformofaGaussianwavepacket:

3.7 Example:Fouriertransformofarectangularwavepacket:

3.8 Example:Acousticwavepacket

3.9 Example:Gravitationalredshift ....................................82

3.10 Example:Quantumbaseball

4.1 Example:Non-linearoscillator

5.1 Example:Shortestdistancebetweentwopoints

5.2 Example:Brachistochroneproblem ...................................114

5.3 Example:Minimaltravelcost ......................................116

5.4 Example:Surfaceareaofacylindrically-symmetricsoapbubble

5.5 Example:Fermat’sPrinciple ......................................119

5.6 Example:Minimumof (∇)2 inavolume

5.7 Example:Twodependentvariablescoupledbyoneholonomicconstraint

5.8 Example:Catenary

5.9 Example:TheQueenDidoproblem

6.1 Example:Motionofafreeparticle,U=0 ...............................146

6.2 Example:Motioninauniformgravitational field

6.3 Example:Centralforces .........................................147

6.4 Example:Diskrollingonaninclinedplane

6.5 Example:Twoconnectedmassesonfrictionlessinclinedplanes

6.6 Example:Twoblocksconnectedbyafrictionlessbar

6.7 Example:Blockslidingonamovablefrictionlessinclinedplane

6.8 Example:Sphererollingwithoutslippingdownaninclinedplaneonafrictionless floor.

6.9 Example:Massslidingonarotatingstraightfrictionlessrod.

6.10

Example:Sphericalpendulum ......................................155

6.11 Example:Springplanependulum ....................................156

6.12 Example:Theyo-yo

6.13 Example:Massconstrainedtomoveontheinsideofafrictionlessparaboloid

6.14 Example:Massonafrictionlessplaneconnectedtoaplanependulum

6.15 Example:Twoconnectedmassesconstrainedtoslidealongamovingrod

6.16 Example:Massslidingonafrictionlesssphericalshell

6.17 Example:Rollingsolidsphereonasphericalshell

6.18 Example:Solidsphererollingplusslippingonasphericalshell

6.19 Example:Smallbodyheldbyfrictionontheperipheryofarollingwheel

6.20 Example:Planependulumhangingfromavertically-oscillatingsupport

6.21 Example:Series-coupleddoublependulumsubjecttoimpulsiveforce

7.1 Example:Feynman’sangular-momentumparadox

7.2 Example:Atwoodsmachine .......................................182

7.3 Example:Conservationofangularmomentumforrotationalinvariance: ..............183

7.4 Example:Diatomicmoleculesandaxially-symmetricnuclei .....................184

7.5 Example:Linearharmonicoscillatoronacartmovingatconstantvelocity

7.6 Example:Isotropiccentralforceinarotatingframe

7.7 Example:Theplanependulum .....................................191

7.8 Example:Oscillatingcylinderinacylindricalbowl

8.1 Example:Motioninauniformgravitational field ...........................205

8.2 Example:One-dimensionalharmonicoscillator ............................205

8.3 Example:Planependulum ........................................206

8.4 Example:Hooke’slawforceconstrainedtothesurfaceofacylinder

8.5 Example:Electronmotioninacylindricalmagnetron ........................208

8.6 Example:SphericalpendulumusingHamiltonianmechanics

8.7 Example:Sphericalpendulumusing 

8.8 Example:Sphericalpendulumusing

8.9 Example:Singleparticlemovinginaverticalplaneundertheinfluenceofaninverse-square centralforce ...............................................216

8.10 Example:Driven,linearly-damped,coupledlinearoscillators

8.11 Example:Kirchhoff ’srulesforelectricalcircuits

9.1 Example:Centralforceleadingtoacircularorbit  =2

9.2 Example:Orbitequationofmotionforafreebody

9.3 Example:Lineartwo-bodyrestoringforce

9.4 Example:Inversesquarelawattractiveforce

9.5 Example:Attractiveinversecubiccentralforce

9.6 Example:Spirallingmassattachedbyastringtoahangingmass

9.7 Example:Two-bodyscatteringbyaninversecubicforce

10.1 Example:Acceleratingspringplanependulum

10.2 Example:Surfaceofrotatingliquid

10.3 Example:Thepirouette

10.4 Example:Crankedplanependulum

10.5 Example:Nucleonorbitsindeformednuclei

10.6 Example:Freefallfromrest .......................................283

10.7 Example:Projectile firedverticallyupwards

10.8 Example:MotionparalleltoEarth’ssurface

11.1 Example:Inertiatensorofasolidcuberotatingaboutthecenterofmass.

11.2 Example:Inertiatensorofaboutacornerofasolidcube.

11.3 Example:Inertiatensorofahulahoop ................................301 11.4 Example:Inertiatensorofathinbook .................................301

11.5 Example:Rotationaboutthecenterofmassofasolidcube

11.6 Example:Rotationaboutthecornerofthecube

11.7 Example:Eulerangletransformation

11.8 Example:Rotationofadumbbell

11.9

Example:Precessionratefortorque-freerotatingsymmetricrigidrotor

11.10 Example:Tennisracquetdynamics ...................................323

11.11 Example:Rotationofasymmetrically-deformednuclei

11.12 Example:TheSpinning"Jack"

11.13 Example:TheTippeTop

11.14

Example:Tippingstabilityofarollingwheel

11.15 Example:Pivoting

11.16 Example:Rolling

11.17 Example:Forcesonthebearingsofarotatingcirculardisk

12.1 Example:TheGrandPiano

12.2 Example:Twocoupledlinearoscillators

12.3 Example:Twoequalmassesseries-coupledbytwoequalsprings

12.4 Example:Twoparallel-coupledplanependula

12.5 Example:Theseries-coupleddoubleplanependula

12.6 Example:Threeplanependula;mean-fieldlinearcoupling

12.7 Example:Threeplanependula;nearest-neighborcoupling

12.8 Example:Systemofthreebodiescoupledbysixsprings

12.9 Example:LineartriatomicmolecularCO 2

12.10 Example:Benzenering .........................................365

12.11 Example:Twolinearly-dampedcoupledlinearoscillators

12.12 Example:Collectivemotioninnuclei .................................375

13.1 Example:Gaugeinvarianceinelectromagnetism

13.2 Example:Thelinearly-damped,linearoscillator:

14.1 Example:Checkthatatransformationiscanonical

14.2 Example:Angularmomentum:

14.3 Example:Lorentzforceinelectromagnetism ..............................402

14.4 Example:Wavemotion: .........................................402

14.5 Example:Two-dimensional,anisotropic,linearoscillator

14.6 Example:Theeccentricityvector ....................................404 14.7 Example:Theidentitycanonicaltransformation

14.8 Example:Thepointcanonicaltransformation

14.9 Example:Theexchangecanonicaltransformation

14.10 Example:Infinitessimalpointcanonicaltransformation

14.11 Example:1-Dharmonicoscillatorviaacanonicaltransformation

14.12 Example:Freeparticle ..........................................415

14.13 Example:Pointparticleinauniformgravitational field

14.14 Example:One-dimensionalharmonicoscillator

14.15 Example:Thecentralforceproblem

14.16 Example:Linearly-damped,one-dimensional,harmonicoscillator

14.17 Example:Adiabaticinvarianceforthesimplependulum

14.18 Example:Harmonicoscillatorperturbation ..............................428

14.19 Example:Lindbladresonanceinplanetaryandgalacticmotion

15.1 Example:Acousticwavesinagas ...................................449 16.1 Example:Muonlifetime .........................................460

16.2 Example:RelativisticDopplerEffect ..................................461

16.3 Example:Twinparadox .........................................461

16.4 Example:Rocketpropulsion .......................................464

16.5 Example:Lagrangianforarelativisticfreeparticle ..........................472

16.6 Example:Relativisticparticleinanexternalelectromagnetic field

16.7 Example:TheBohr-Sommerfeldhydrogenatom ............................477

A.1 Example:Eigenvaluesandeigenvectorsofarealsymmetricmatrix

A.2 Example:Degenerateeigenvaluesofrealsymmetricmatrix

D.1 Example:Rotationmatrix: .......................................517

D.2 Example:Proofthatarotationmatrixisorthogonal

E.1 Example:Displacementgradienttensor

F.1 Example:Jacobianfortransformfromcartesiantosphericalcoordinates ..............531

H.1 Example:Maxwell’sFluxEquations ..................................539

H.2 Example:Buoyancyforcesin fluids ..................................540

H.3 Example:Maxwell’scirculationequations ...............................542

H.4 Example:Electromagnetic fields: ....................................543

I.1

Example:Fouriertransformofasingleisolatedsquarepulse: ....................548

I.2 Example:FouriertransformoftheDiracdeltafunction: .......................548

Preface

Thegoalofthisbookistointroducethereadertotheintellectualbeauty,andphilosophicalimplications, ofthefactthatnatureobeysvariationalprinciplesthatunderlietheLagrangianandHamiltoniananalytical formulationsofclassicalmechanics.Thesevariationalmethods,whichweredevelopedforclassicalmechanics duringthe 18 19 century,havebecomethepreeminentformalismsforclassicaldynamics,aswellasfor manyotherbranchesofmodernscienceandengineering.Theambitiousgoalofthisbookistoleadthestudent fromtheintuitiveNewtonianvectorialformulation,tointroductionofthemoreabstractvariationalprinciples thatunderlietheLagrangianandHamiltoniananalyticalformulations.Thisculminatesindiscussionofthe contributionsofvariationalprinciplestothedevelopmentofrelativisticandquantummechanics.Thebroad scopeofthisbookattemptstounifytheundergraduatephysicscurriculumbybridgingthechasmthat dividestheNewtonianvector-differentialformulationandtheintegralvariationalformulationofclassical mechanics,andthecorrespondingchasmthatexistsbetweenclassicalandquantummechanics.Powerful variationaltechniquesinmathematics,thatunderliemuchofmodernphysics,areintroducedandproblem solvingskillsaredevelopedinordertochallengestudentsatthecrucialstagewhenthey firstencounterthis sophisticatedandchallengingmaterial.Theunderlyingfundamentalconceptsofclassicalmechanics,and theirapplicationstomodernphysics,areemphasizedthroughoutthecourse.

Afullunderstandingofthepowerandbeautyofvariationalprinciplesinclassicalmechanics,isbest acquiredby firstlearningtheconceptsofthevariational approach,andthenapplyingtheseconceptsto manyexamplesinclassicalmechanics.Classicalmechanicsistheidealtopicforlearningtheprinciplesand thepowerofusingthevariationalapproachpriortoapplyingthesetechniquestootherbranchesofscience andengineering.TheunderlyingphilosophicalapproachadoptedbythisbookwasespousedbyGalileo Galilei"Youcannotteachamananything;youcanonlyhelphim finditwithinhimself."

Thedevelopmentofthistextbookwasinfluencedbythreetextbooks:"TheVariationalPrinciplesof Mechanics "byCorneliusLanczos(1949)[La49],"ClassicalMechanics" (1950)byHerbertGoldstein[Go50], and"ClassicalDynamicsofParticlesandSystems "(1965)byJerryB.Marion[Ma65].Marion’sexcellent textbookwasunusualinpartiallybridgingthechasmbetweentheoutstandinggraduatetextsbyGoldstein andLanczos,andabevyofintroductorytextsbasedonNewtonianmechanicsthatwereavailableatthat time.Thepresenttextbookwasdevelopedtocoverthetechniquesandphilosophicalimplicationsofthe variationalapproachestoclassicalmechanics,withabreadthanddepthclosetothatprovidedbyGoldstein andLanczos,butinaformatthatbettermatchestheneedsoftheundergraduatestudent.Anadditional goalistobridgethegapbetweenclassicalandmodernphysicsintheundergraduatecurriculum.

Thisbookwaswritteninsupportofthephysicsjunior/seniorundergraduatecourseP235Wentitled "VariationalPrinciplesinClassicalMechanics"thattheauthortaughtattheUniversityofRochesterbetween 1993 2015.Theselecturenotesweredistributedtostudents toallowpre-lecturestudy,facilitatedaccurate transmissionofthecomplicatedformulae,andminimizednotetakingduringlectures.Theselecturenotes evolvedintothepresenttextbookthatwasusedforthiscourse.Thetargetaudienceofthecourse,upon whichthistextbookisbased,typicallycomprised ≈ 70% junior/seniorundergraduates, ≈ 25% sophomores, ≤ 5% graduatestudents,andtheoccasionalwell-prep aredfreshman.Thetargetaudiencewasphysics andastrophysicsmajors,butitattractedasignificantfractionofmajorsfromotherdisciplinessuchas mathematics,chemistry,optics,engineering,music,andthehumanities.Asaconsequence,thebookincludes appreciableintroductorylevelphysics,plusmathematicalreviewmaterial,toaccommodatethediverse rangeofpriorpreparationofthestudents.Thistextbookincludesmaterialthatextendsbeyondwhat reasonablycanbecoveredduringaone-termcourse.Thissupplementalmaterialispresentedtoshowthe importanceandbroadapplicabilityofvariationalconceptstoclassicalmechanics.Thebookincludes 162 workedexamplestoillustratetheconceptspresented. Advancedgroup-theoreticconceptsareminimizedto

betteraccommodatethemathematicalskillsofthetypicalundergraduatephysicsmajor.Forcompatibility withmodernliteratureinthis field,thisbookfollowsthewidely-adoptednomenclatureusedin"Classical Mechanics"byGoldstein[Go50],withrecentadditionsbyJohns[Jo05].

Thebookisbrokenintofourmajorsections.This firstreviewsectionsetsthestagebyincludinga briefhistoricalintroduction(chapter 1),reviewoftheNewtonianformulationofmechanicsplusgravitation (chapter 2),linearoscillatorsandwavemotion(chapter 3),andanintroductiontonon-lineardynamics andchaos(chapter 4).Extensivereadingassignmentsareassignedtominimizethetimespentonthis reviewofNewtonianvectorialmechanics.Buildingontheintroductorysection,thesecondsectionofthe bookintroducesthevariationalprinciplesofanalyticalmechanicsthatunderliethisbook.Itincludesan introductiontothecalculusofvariations(chapter 5),theLagrangianformulationofmechanicswithapplicationstoholonomicandnon-holonomicsystems(chapter 6),adiscussionofsymmetries,invariance,plus Noether’stheorem(chapter 7)andanintroductiontotheHamiltonianandtheHamiltonianformulation ofmechanicsplustheRouthianreductiontechnique(Chapter 8).Thethirdsectionofthebook,applies LagrangianandHamiltonianformulationsofclassicaldynamicstocentralforceproblems(chapter 9),motioninnon-inertialframes(chapter 10),rigid-bodyrotation(chapter 11),andcoupledoscillators(chapter 12).The finalsectionofthebookdiscussesHamilton’sPrincipleplusadvancedapplicationsofLagrangian mechanics(chapter 13),HamiltonianmechanicsincludingPoisson brackets,Liouville’stheorem,canonical transformations,Hamilton-Jacobitheory,theaction-angletechnique(chapter 14),andclassicalmechanics inthecontinua(chapter 15).ThisisfollowedbyabriefreviewoftherevolutioninclassicalmechanicsintroducedbyEinstein’stheoryofrelativisticmechanics.TheextendedtheoryofLagrangianandHamiltonian mechanicsisusedtoapplyvariationaltechniquesto theSpecialTheoryofRelativityfollowedbyasuperficial introductiontotheconceptsofGeneralTheoryofRelativity(chapter 16).Thebook finisheswithabrief reviewoftheroleofvariationalprinciplesinbridgingthegapbetweenclassicalmechanicsandquantum mechanics,(chapter 17).Theseadvancedtopicsextendbeyondthetypicalsyllabusforanundergraduate classicalmechanicscourse.Thereasonforintroducingtheseadvancedtopicsistostimulatestudentinterest inphysicsbygivingthemaglimpseofthephysicsatthesummitthattheyhavestruggledtoclimb.This glimpseillustratesthebreadthofclassicalmechanics, andtherolethatvariationalprincipleshaveplayed inthedevelopmentofclassical,relativistic, quantal,andstatisticalmechanics.These finalsupplemental lecturesillustratethebeautyandunityofclassicalmechanics,andthefoundationthatclassicalmechanics hasprovidedtothedevelopmentofmodernphysics.Theappendicessummarizeaspectsofthemathematical methodsthatareexploitedinclassicalmechanics.

Thepresenttextbookcontainsmorematerialthanrequiredforajunior/seniorundergraduateclassical mechanicscourse,andthus,itcouldserveasthetextforagraduatecoursebyfocussingthecourseonthe variationalprinciplescoveredbychapters 5 17.Thepartitioningandorderingofthetopicsinthebook aretheresultofmanypermutationstriedwhileteachingclassicalmechanicsformanyyears.Chapters 1 through 3 plusthemathematicalappendices,areusedasreadingassignmentsduringthe firstthreeweeks ofclasstominimizethetimespentreviewingNewtonianmechanics.Thismaximizestheclasstimeavailable tocoverthevariationalapproach,thatis,chapters 5 through 14.Thebriefreviewsofthemechanicsinthe continua,andthetransitiontoquantummechanics,providethestudentwithaglimpseoftheimplications ofanalyticalmechanicstothesemoreadvancedtopics.

InformationregardingtheassociatedP235undergraduatecourseattheUniversityofRochesterisavailableonthewebsiteathttp://www.pas.rochester.edu/~cline/P235/index.shtml.Informationaboutthe authorisavailableattheClinehomewebsite:http://www.pas.rochester.edu/~cline/index.html.

TheauthorthanksMeghanSarkiswhopreparedmanyof theillustrations,JoeEasterlywhodesignedthe bookcoverplusthewebpage,andMorianaGarciawhoorganizedpublication.AndrewSifaindevelopedthe diagnosticworkshopquestions.Theauthorappreciates thepermission,grantedbyProfessorStruckmeier,to quotehispublishedarticleontheextendedHamilton-Lagrangianformalism.Theauthoracknowledgesthe feedbackandsuggestionsmadebymanystudentswhohavetakenthiscourse,aswellashelpfulsuggestions byhiscolleagues;AndrewAbrams,AdamHayes,ConnieJones,AndrewMelchionna,DavidMunson,Alice Quillen,RichardSarkis,JamesSchneeloch,Steven Torrisi,DanWatson,andFrankWolfs.Theselecture notesweretypedinLATEXusingScientificWorkPlace(MacKichanSoftware,Inc.),whileAdobeIllustrator, Photoshop,Origin,Mathematica,andMUPAD,wereusedtopreparetheillustrations.

DouglasCline, UniversityofRochester,2017

Prologue

Twodramaticallydifferentphilosophicalapproachestoscienceweredevelopedinthe fieldofclassicalmechanicsduringthe 17 - 18 centuries.ThistimeperiodcoincidedwiththeAgeofEnlightenmentinEurope duringwhichremarkableintellectualandphilosophicaldevelopmentsoccurred.Thiswasatimewhenboth philosophicalandcausalargumentswereequallyacceptableinscience,incontrastwithcurrentconvention wherethereappearstobetacitagreementtodiscourageuseofphilosophicalargumentsinscience.

Snell’sLaw: Thegenesisoftwocontrastingphilosophicalapproachestosciencerelatesbacktoearlystudiesofthereflection andrefractionoflight.Thevelocityoflightinamediumofrefractiveindex  equals  =   .Thusalightbeamincidentatan angle  1  tothenormalofaplaneinterfacebetweenmedium 1 andmedium 2 isrefractedatanangle  2 inmedium 2 wherethe anglesarerelatedbySnell’sLaw.

IbnSahlofBagdad (984) firstdescribedtherefractionoflight, whileSnell (1621) derivedhislawmathematically.Bothofthese scientistsusedthe"vectorialapproach"wherethelightvelocity  isconsideredtobeavectorpointinginthedirectionofpropagation.

Fermat’sPrinciple: Fermat’sprincipleofleasttime (1657), whichisbasedontheworkofHeroofAlexandria (∼ 60) andIbn al-Haytham (1021),statesthat"lighttravelsbetweentwogiven pointsalongthepathofshortesttime ",wherethetransittime  ofalightbeambetweentwolocations  and  inamediumwith position-dependentrefractiveindex () isgivenby

(Fermat’sPrinciple)

Fermat’sPrincipleleadstothederivationofSnell’sLaw. Philosophicallythephysicsunderlyingthecontrastingvectorial andFermat’sPrinciplederivationsofSnell’sLawaredramatically different.Thevectorialapproachisbasedondifferentialrelations betweenthevelocityvectorsinthetwomedia,whereasFermat’s variationalapproachisbasedonthefactthatthelightpreferentiallyselectsapathforwhichtheintegralofthetransittime betweentheinitiallocation  andthe finallocation  isminimized.Thatis,the firstapproachisbasedon"vectorialmechanics"whereasFermat’sapproachisbasedon variationalprinciplesinthatthepathbetweentheinitialand finallocationsisvariedto findthepaththat minimizesthetransittime.Fermat’senunciationofvariationalprinciplesinphysicsplayedakeyroleinthe historicaldevelopment,andsubsequentexploitation,of theprincipleofleastactioninanalyticalformulations ofclassicalmechanicsasdiscussedbelow.

Figure1:VectorialandvariationalrepresentationofSnell’sLawforrefractionoflight.

Newtonianmechanics: MomentumandforcearevectorsthatunderlietheNewtonianformulationof classicalmechanics.Newton’smonumentaltreatise,entitled"PhilosophiaeNaturalisPrincipiaMathematica" ,publishedin 1687,establishedhisthreeuniversallawsofmotion,theuniversaltheoryofgravitation, thederivationofKepler’sthreelawsofplanetarymotion,andthedevelopmentofcalculus.Newton’sthree universallawsofmotionprovidethemostintuitiveapproachtoclassicalmechanicsinthattheyarebasedon vectorquantitieslikemomentum,andtherateofchangeofmomentum,whicharerelatedtoforce.Newton’s equationofmotion

(Newton’sequationofmotion)

isavectordifferentialrelationbetweentheinstantaneousforcesandrateofchangeofmomentum,orequivalentinstantaneousaccelerations,allofwhicharevector quantities.Momentumandforceareeasytovisualize, andbothcauseandeffectareembeddedinNewtonianmechanics.Thus,ifalloftheforces,includingthe constraintforces,actingonthesystemareknown,thenthemotionissolvablefortwobodysystems.The mathematicsforhandlingNewton’s"vectorialmechanics"approachtoclassicalmechanicsiswellestablished.

Analyticalmechanics: Variationalprinciplesunderlietheanalyticalformulationofmechanics.Leibniz, whowasacontemporaryofNewton,introducedmethodsbasedonaquantitycalled"visviva", whichis Latinfor "livingforce" andequalstwicethekineticenergy.LeibnizbelievedinthephilosophythatGod createdaperfectworldwherenaturewould bethriftyinallitsmanifestations.In 1707,Leibnizproposed thattheoptimumpathisbasedonminimizingthetimeintegralofthe visviva,whichisequivalentto theactionintegralofLagrangian/Hamiltonianmechanics.In 1744 EulerderivedtheLeibnizresultusing variationalconceptswhileMaupertuisrestatedtheLeibnizresultbasedonteleologicalarguments.The developmentofLagrangianmechanicsculminatedinthe 1788 publicationofLagrange’smonumentaltreatise entitled "MécaniqueAnalytique".Lagrangianmechanicsderivesthemagnitudeanddirectionoftheoptimum trajectoriesandforcesbasedontheconceptofleastaction,whichisdefinedtobethetimeintegralofthe differencebetweenthekineticandpotentialenergies.Hamilton’sPrinciple (1834),whichunderliesLagrange’s leastactionprinciple,minimizestheactionintegral  givenby

= Z 

(q ˙ q) (Hamilton’sPrinciple)

wheretheLagrangian (q ˙ q) equalsthedifferencebetweenthekineticenergy  andthepotentialenergy  .ThisLagrangianisafunctionof  generalizedcoordinates  plustheircorrespondingvelocities 

Theculminationofthedevelopmentofanalyticalmechanicsoccurredin 1834 whenHamiltondeveloped thepremiervariationalapproach,calledHamiltonianmechanics,thatisbasedontheHamiltonian  (q p) whichisafunctionofthe  fundamentalconjugateposition  plusthemomentum  variables.In 1843 Jacobiprovidedthemathematicalframeworkrequired tofullyexploitthepowerofHamiltonianmechanics. NotethattheLagrangian,Hamiltonian,andtheactionintegral,allarescalarquantitieswhichsimplifies derivationoftheequationsofmotioncomparedwiththevectorcalculususedbyNewtonianmechanics.

Philosophicaldevelopments: Variationalprinciplesapplytoallaspectsofourdailylife.Typicalexamplesinclude;selectingtheoptimumcompromiseinqualityandcostwhenshopping,selectingthefastest routetotravelfromhometowork,orselectingtheoptimumcompromisetosatisfythedisparatedesiresof theindividualscomprisingafamily.Itisastonishingthatthelawsofnatureareconsistentwithvariational principlesinvolvingtheprincipleofleastaction.Minimizingtheactionintegralledtothedevelopmentofthe mathematical fieldofvariationalcalculusplustheanalyticalvariationalapproachestoclassicalmechanics byEuler,Lagrange,Hamilton,andJacobi.

TheanalyticalapproachtoclassicalmechanicsappearedcontradictorytoNewton’sintuitivevectorialtreatmentofforceandmomentum.ThereisadramaticdifferenceinphilosophybetweenthevectordifferentialequationsofmotionderivedbyNewtonianmechanics,whichrelatetheinstantaneousforceto thecorrespondinginstantaneousacceleration,andanalyticalmechanics,whereminimizingthescalaraction integralinvolvesintegralsoverspaceandtimebetweenspecifiedinitialand finalstates.Analyticalmechanics usesvariationalprinciplestodeterminetheoptimumtrajectory,fromacontinuumoftentativepossibilities byrequiringthattheoptimumtrajectoryminimizestheactionintegralbetweenspecifiedinitialand final conditions.

Figure2:ChronologicalroadmapoftheparalleldevelopmentoftheNewtonianandthevariationalapproaches toclassicalmechanics.

Initiallytherewasconsiderableprejudiceandphilosophicaloppositiontouseofthevariationalapproach whichisbasedontheassumptionthatnaturefollowstheprinciplesofeconomy.Thevariationalapproach isnotintuitive,andthusitwasconsideredtobespeculativeand"metaphysical",butitwastoleratedasan efficienttoolforexploitingclassicalmechanics.Thisoppositiontothevariationalprinciples,thatunderlie analyticalmechanics,delayedfullappreciationofthevariationalapproachuntilthestartofthe 20 century. Asaconsequence,theintuitiveNewtonianformulation reignedsupremeinclassicalmechanicsforovertwo centuries,eventhoughtheremarkableproblem-solvingcapabilitiesofanalyticalmechanicswererecognized andexploitedfollowingdevelopmentofanalyticalmechanics.

Thefullsignificanceandsuperiorityoftheanalyticalvariationalformulationsofclassicalmechanics becamewidelyacceptedfollowingthedevelopmentoftheSpecialTheoryofRelativityin 1905.TheTheory ofRelativityrequiresthatthelawsofnaturebeinvarianttothereferenceframe.Thisisnotsatisfiedby theNewtonianformulationofmechanicswhichassumes oneabsoluteframeofreferenceandaseparationof spaceandtime.Incontrast,theLagrangianandHamiltonianformulationsoftheprincipleofleastaction remainvalidintheTheoryofRelativity,iftheLagrangianiswritteninarelativistically-invariantform inspace-time.Thecompleteinvarianceofthevariationalapproachtocoordinateframesispreciselythe formalismnecessaryforhandlingrelativisticmechanics.Hamiltonianmechanics,whichisexpressedinterms oftheconjugatevariables (q p),relatesclassicalmechanicsdirectlytotheunderlyingphysicsofquantum mechanicsandquantum fieldtheory.Asaconsequence,thephilosophi caloppositiontoexploitingvariational principlesnolongerexists,andHamiltonianmechanicshasbecomethepreeminentformulationofmodern classicalmechanics.Thereaderisfreetodrawtheirownconclusionsregardingthephilosophicalquestion "istheprincipleofeconomyafundamentallawofclassicalmechanics,orisitafortuitousconsequenceof thefundamentallawsofnature?"

Fromthelateseventeenthcentury,untilthedawnofmodernphysicsatthestartofthetwentiethcentury,classicalmechanicsremainedaprimarydrivingforceinthedevelopmentofphysics.Classicalmechanics embracesanunusuallybroadrangeoftopicsspanningmotionofmacroscopicastronomicalbodiestomicroscopicparticlesinnuclearandparticlephysics,atvelocitiesrangingfromzerotonearthevelocityof light,fromone-bodytostatisticalmany-bodysystems,aswellashavingextensionstoquantummechanics. IntroductionoftheSpecialTheoryofRelativityin 1905,andtheGeneralTheoryofRelativityin 1916, necessitatedmodificationstoclassicalmechanicsforrelativisticvelocities,andcanbeconsideredtobean extendedtheoryofclassicalmechanics.Sincethe 19200 s,quantalphysicshassupersededclassicalmechanics inthemicroscopicdomain.Althoughquantumphysicshasplayedtheleadingroleinthedevelopmentof physicsduringmuchofthepastcentury,classicalmechanicsstillisavibrant fieldofphysicsthatrecently hasledtoexcitingdevelopmentsassociatedwithnon-linearsystemsandchaostheory.Thishasspawned newbranchesofphysicsandmathematicsaswellaschangingournotionofcausality.

Goals: Theprimarygoalofthisbookistointroducethereadertothepowerfulvariationalapproachesthat playsuchapivotalroleinclassicalmechanics,plusmanyotherbranchesofmodernscienceandengineering. Figure 1 givesahistoricalroadmapoftheevolutionofclassicalmechanicsfromNewton,tothevariational approachesofEuler,Lagrange,HamiltonandJacobi.Thisbookemphasizestheintellectualbeautyofthese remarkabledevelopmentsaswellasstressingthephilosophicalimplicationsthathavehadatremendous impactonmodernscience.Asecondarygoalistoapplyvariationalprinciplestosolveadvancedapplications inclassicalmechanicsinordertointroducemanysophisticatedandpowerfulmathematicaltechniquesthat underliemuchofmodernphysics.

Theconnectionsandapplicationsofclassicalmechanicstomodernphysicsareemphasizedthroughout thebookinanefforttospanthechasmthatdividestheNewtonianvector-differentialformulationandthe integralvariationalformulationofclassicalmechanics,andthecorrespondingchasmthatexistsbetween classicalandquantummechanics.Notethatthesevariationalprinciples,developedinthe fieldofclassical mechanics,nowareusedinadiverseandwiderangeof fieldsincludingeconomics,meteorology,engineering, andcomputing.

Thisstudyofclassicalmechanicsinvolvesclimbingavastmountainofknowledge,andthepathwayto thetopleadstoelegantandbeautifultheoriesthatunderliemuchofmodernphysics.Thesetheoriesare appliedtofourmajortopicsinclassicalmechanics.Inaddition,beingsoclosetothesummitprovidesthe opportunityforthisbooktotakeafewextrastepsbeyondthenormalundergraduateclassicalmechanics syllabustoprovideaglimpseoftheexcitingphysics foundatthesummit.Thisnewphysicsincludestopics suchasquantum,relativistic,andstatisticalmechanics..

Chapter1

Abriefhistoryofclassicalmechanics

1.1Introduction

Thischapterbrieflyreviewsthehistoricalevolutionofclassicalmechanicssinceconsiderableinsightcanbe gainedfromstudyofthehistoryofscience.Therearetwodramaticallydifferentapproachesusedinclassical mechanics.The firstisthevectorialapproachofNewtonwhichisbasedonvectorquantitieslikemomentum, force,andacceleration.ThesecondistheanalyticalapproachofLagrange,Euler,Hamilton,andJacobi, thatisbasedontheconceptofleastactionandvariationalcalculus.ThemoreintuitiveNewtonianpicture reignedsupremeinclassicalmechanicsuntilthestartofthetwentiethcentury.Variationalprinciples,which weredevelopedduringthenineteenthcentury,neverarousedmuchenthusiasminscientificcirclesdueto philosophicalobjectionstotheunderlyingconcepts;thisapproachwasmerelytoleratedasanefficienttool forexploitingclassicalmechanics.Adramaticadvanceinthephilosophyofscientificthinkingoccurredat thestartofthe 20 centuryleadingtowidespreadacceptanceofthesuperiorityofvariationalprinciples.

1.2Prehistoricastronomy

Astronomyistheearliestbranchofclassicalmechanics.Astronomicalobservatoriesdatebacktoaround 4900BCwhenwoodensolarobservatories,calledhenges,werebuiltinEurope.StonehengeinEngland isawell-knownexamplewhichwasbuilt ∼ 3000 .Themesopotamianpeople,wholivedintheland betweentheTigressandEuphratesrivers,developedcuneiformwritingandrecordedaccuratenumerical dataaround 3500 3000 .Theyrecognizedthatthemotionofthe planetswasperiodicasreportedin Babyloniantablets.After2700BCtheEgyptiansbuiltpyramidsthatarealignedtothepolestarandthey madesignificantadvancesinastronomy,mathematicsandmedicine.

1.3Greekantiquity

ThegreatphilosophersinancientGreeceplayedakeyrolebyusingtheastronomicalworkoftheBabylonians todevelopscientifictheoriesofmechanics. ThalesofMiletus(624-547BC),the firstoftheseven greatgreekphilosophers,developedgeometryandishailedasthe firsttruemathematician. Pythagorus (570-495BC) developedmathematicsandpostulatedthattheearthisspherical. Democritus(460370BC) hasbeencalledthefatherofmodernscience,while Socrates(469-399BC) isrenownedforhis contributionstoethics. Plato(427-347B.C.) whowasamathematicianandstudentofSocrates,wrote importantphilosophicaldialogues.HefoundedtheAcademyinAthenswhichwasthe firstinstitutionof higherlearningintheWesternworldthathelpedlaythefoundationsofWesternphilosophyandscience. Aristotle(384-322B.C.) isanimportantfounderofWesternphilosophyencompassingethics,logic, science,andpolitics.Hisviewsonthephysicalsciencesprofoundlyinfluencedmedievalscholarshipthat extendedwellintotheRenaissance.Hepresentedthe firstimpliedformulationoftheprincipleofvirtual workinstaticsandhisstatementthat"whatislostinvelocityisgainedinforce"isaveiledreferenceto kineticandpotentialenergy.HeadoptedanEarthcenteredmodeloftheuniverse. Aristarchus(310-240 B.C.) arguedthattheEarthorbitedtheSunandusedmeasurementstoimplytherelativedistancesofthe

MoonandtheSun.Thegreekphilosopherswererelativelyadvancedinlogicandmathematicsanddeveloped conceptsthatenabledthemtocalculateareasandperimeters.Unfortunatelytheirphilosophicalapproach neglectedcollectingquantitativeandsystematicdatathatisanessentialingredienttotheadvancementof science.

Archimedes(287-212B.C.) representedtheculminationofscienceinancientGreece.Asanengineer hedesignedmachinesofwarwhileasascientisthemadesignificantcontributionstohydrostaticsandthe principleofthelever.Asamathematicianheappliedinfinitessimalsinawaythatisreminiscentofmodern integralcalculuswhichheusedtoderiveavaluefor  Unfortunatelymuchoftheworkofthebrilliant Archimedessubsequentlyfellintooblivion. HeroofAlexandria(10-70A.D.) describedtheprinciple ofreflectionthatlighttakestheshortestpath.Thisisanearlyillustrationofvariationalprincipleof leasttime. Ptolemy(83-161A.D.) wroteseveralscientifictreatisesthatgreatlyinfluencedsubsequent philosophers.Unfortunatelyheadoptedtheincorrectgeocentricsolarsystemincontrasttotheheliocentric modelofAristarchusandothers.

1.4MiddleAges

ThedeclineandfalloftheRomanEmpirein ∼410A.D.markstheendofClassicalAntiquityandthe beginningoftheDarkAgesinWesternEurope(Christendom)whiletheMuslimscholarsinEasternEurope continuedtomakeprogressinastronomyandmathematics.Forexample,inEgypt, Alhazen(965-1040 A.D.) expandedtheprincipleofleasttimetoreflectionandrefraction.TheDarkAgesinvolvedalong scientificdeclineinWesternEuropethatlanguishedforabout900years.Sciencewasdominatedbyreligious dogma,allwesternscholarsweremonks,andtheimportantscientificachievementsofGreekantiquitywere forgotten.TheworksofAristotlewerereintroducedtoWesternEuropebyArabsintheearly 13 century leadingtotheconceptsofforcesinstaticsystemswhichweredevelopedduringthefourteenthcentury. Thisincludedconceptsoftheworkdonebyaforce,andthevirtualworkinvolvedinvirtualdisplacements. LeonardodaVinci(1452-1519) wasaleaderinmechanicsatthattime.Hemadeseminalcontributions toscience,inadditiontohiswellknowncontributionstoarchitecture,engineering,sculpture,andart. NicolausCopernicus(1473-1543) rejectedthegeocentrictheoryofPtolomyandformulatedascientificallybasedheliocentriccosmologythatdisplacedtheEarthfromthecenteroftheuniverse.ThePtolomicview wasthatheavenrepresentedtheperfectunchangingdivinewhiletheearthrepresentedchangepluschaos andthecelestialbodiesmovedrelativetothe fixedheavens.Thebook,"Derevolutionibusorbiumcoelestium "(OntheRevolutionsoftheCelestialSpheres),publ ishedbyCopernicusin1543,isregardedasthestarting pointofmodernastronomyandthedefiningepiphanythatbegantheScientificRevolution.Thebook"De Magnete "writtenin1600bytheEnglishphysician WilliamGilbert(1540-1603) presentedtheresultsof well-plannedstudiesofmagnetismandstronglyinfluencedtheintellectual-scientificevolutionatthattime.

JohannesKepler(1571-1630), aGermanmathematician,astronomerandastrologer,wasakey figureinthe17thcenturyScientificRevolution.Heisbestknownforrecognizingtheconnectionbetweenthe motionsintheskyandphysics.Hislawsofplanetarymotionweredevelopedbylaterastronomersbasedon hiswrittenwork"Astronomianova ","HarmonicesMundi ",and"EpitomeofCopernicanAstrononomy ". Keplerwasanassistantto TychoBrahe(1546-1601) whoformanyyearsrecordedaccurateastronomical datathatplayedakeyroleinthedevelopmentofKepler’stheoryofplanetarymotion.Kepler’swork providedthefoundationforIsaacNewton’stheoryofuniversalgravitation.UnfortunatelyKeplerdidnot recognizethetruenatureofthegravitationalforce.

GalileoGalilei(1564-1642) builtontheAristotleprinciplebyrecognizingthelawofinertia,the persistenceofmotionifnoforcesact,andtheproportionalitybetweenforceandacceleration.Thisamounts torecognitionofworkastheproductofforcetimesdisplacementinthedirectionoftheforce.Heapplied virtualworktotheequilibriumofabodyonaninclinedplane.Healsoshowedthatthesameprinciple appliestohydrostaticpressurethathadbeenestablishedbyArchimedes,buthedidnotapplyhisconcepts inclassicalmechanicstotheconsiderableknowledgebaseonplanetarymotion.Galileoisfamousforthe apocryphalstorythathedroppedtwocannonballsofdifferentmassesfromtheTowerofPisatodemonstrate thattheirspeedofdescentwasindependentoftheirmass.

1.5AgeofEnlightenment

TheAgeofEnlightenmentisatermusedtodescribeaphaseinWesternphilosophyandculturallifein whichreasonwasadvocatedastheprimarysourceandlegitimacyforauthority.Itdevelopedsimultaneously inGermany,France,Britain,theNetherlands,and Italyaroundthe1650’sandlasteduntiltheFrench Revolutionin1789.Theintellectualandphilosophicaldevelopmentsledtomoral,social,andpolitical reforms.Theprinciplesofindividualrights,reason,commonsense,anddeismwerearevolutionarydeparture fromtheexistingtheocracy,autocracy,oligarchy,aristocracy,andthedivinerightofkings.Itledtopolitical revolutionsinFranceandtheUnitedStates.ItmarksadramaticdeparturefromtheEarlyModernperiod whichwasnotedforreligiousauthority,absolutestatepower,guild-basedeconomicsystems,andcensorshipof ideas.Itopenedaneweraofrationaldiscourse, liberalism,freedomofexpression,andscientificmethod.This newenvironmentledtotremendousadvancesinbothscienceandmathematicsinadditiontomusic(Johann SebastianBach,Mozart),literature(Goethe),philosophy(Spinoza,Kant)andart(Rubens).Scientific developmentduringthe 17 centuryincludedthepivotaladvancesmadebyNewtonandLeibnizatthe beginningoftherevolutionaryAgeofEnlightenment,culminatinginthedevelopmentofvariationalcalculus andanalyticalmechanicsbyEulerandLagrange.Thescientificadvancesofthisageincludepublicationof twomonumentalbooks"PhilosophiaeNaturalisPrincipiaMathematica" byNewtonin1687and Mécanique analytique byLagrangein1788.Thesearethedefinitivetwobooksuponwhichclassicalmechanicsisbuilt. RenéDescartes(1596-1650) attemptedtoformulatethelawsofmotionin1644.Hetalkedabout conservationofmotion(momentum)inastraightline butdidnotrecognizethevectorcharacterofmomentum. PierredeFermat(1601-1665) andRenéDescartesweretwoleadingmathematiciansinthe first halfofthe 17 century.Independentlytheydiscoveredtheprinciplesofanalyticgeometryanddeveloped someinitialconceptsofcalculus.Fermatand BlaisePascal(1623-1662) werethefoundersofthetheory ofprobability.Fermatrevivedtheprincipleofleasttime,whichstatesthat"lighttravelsbetweentwogiven pointsalongthepathofshortesttime "andwasusedtoderiveSnell’slawin1657.Thisenunciationofvariationalprinciplesinphysicsplayedakeyroleinthehistoricaldevelopmentoftheprincipleofleastaction thatunderliestheanalyticalformulationsofclassicalmechanics.

IsaacNewton(1642-1727) madepioneeringcontributionstophysicsandmathematicsaswellas beingatheologian.At 18 hewasadmittedtoTrinityCollegeCambridgewherehereadthewritingsof modernphilosopherslikeDescartes,andastronomerslikeCopernicus,Galileo,andKepler.By1665he haddiscoveredthegeneralizedbinomialtheorem,andbegandevelopinginfinitessimalcalculus.Duetoa plague,theuniversityclosedfortwoyearsin1665duringwhichNewtonworkedathomedevelopingthe theoryofcalculusthatbuiltupontheearlierworkofBarrowandDescartes.HewaselectedLucasian ProfessorofMathematicsin1669attheageof26.From1670Newtonfocussedonopticsleadingtohis "HypothesisofLight" publishedin1675andhisbook"Opticks" in1704.Newtondescribedlightasbeing madeupofa flowofextremelysubtlecorpusclesthatalsohadassociatedwavelikepropertiestoexplain diffractionandopticalinterferencethathestudied.Newtonreturnedtomechanicsin1677bystudying planetarymotionandgravitationthatappliedthe calculushehaddeveloped.In1687hepublishedhis monumentaltreatiseentitled"PhilosophiaeNaturalisPrincipiaMathematica" whichestablishedhisthree universallawsofmotion,theuniversaltheoryofgravitation,derivationofKepler’sthreelawsofplanetary motion,andwashis firstpublicationofthedevelopmentofcalculuswhichhecalled"thescienceof fluxions". Newton’slawsofmotionarebasedontheconceptsofforceandmomentum,thatis,forceequalstherateof changeofmomentum.Newton’spostulateofaninvisibleforceabletoactovervastdistancesledhimtobe criticizedforintroducing"occultagencies"intoscience.Inaremarkableachievement,Newtoncompletely solvedthelawsofmechanics.Histheoryofclassicalmechanicsandofgravitationreignedsupremeuntilthe developmentoftheTheoryofRelativityin1905.ThefollowersofNewtonenvisionedtheNewtonianlaws tobeabsoluteanduniversal.ThisdogmaticreverenceofNewtonianmechanicspreventedphysicistsfrom anunprejudicedappreciationoftheanalyticvariationalapproachtomechanicsdevelopedduringthe 17 through 19 centuries.Newtonwasthe firstscientisttobeknightedandwasappointedpresidentofthe RoyalSociety.Newtonhadanunpleasantcharacterandwasnotoriousfortheheateddisputesheprovoked withotheracademics.Eventuallyheleftacademiaandb ecameactiveinpolitics.Thisledtohisappointment asWardenoftheRoyalMintwhereheconductedamajorcampaignagainstcounterfeitingthatsentseveral mentotheirdeathonthegallows.

GottfriedLeibniz(1646-1716) wasabrilliantGermanphilosopher,acontemporaryofNewton,who workedonbothcalculusandmechanics.Leibnizstarteddevelopmentofcalculusin1675,tenyearsafter Newton,butLeibnizpublishedhisworkin1684,whichwas threeyearsbeforeNewton’sPrincipia.Leibniz madesignificantcontributionstointegralcalculusandwasresponsibleforthecalculusnotationcurrently used.HeintroducedthenamecalculusbasedontheLatinwordforthesmallstoneusedforcounting. NewtonandLeibnizwereinvolvedinaprotractedargumentoverwhooriginatedcalculus.Itappearsthat LeibnizsawdraftsofNewton’sworkoncalculusduringavisittoEngland.Throughouttheirargument Newtonwastheghostwriterofmostofthearticlesinsupportofhimselfandhehadthempublishedunder non-de-plumeofhisfriends.LeibnizmadethetacticalerrorofappealingtotheRoyalSocietytointercedeon hisbehalf.Newton,aspresidentoftheRoyalSociety,appointedhisfriendstoan"impartial"committeeto investigatethisissue,thenhewrotethecommittee’sreportthataccusedLeibnizofplagiarismofNewton’s workoncalculus,afterwhichhehaditpublishedbytheRoyalSociety.Stillunsatisfiedhethenwrotean anonymousreviewofthereportintheRoyalSociety’sownperiodical.Thisbitterdisputelasteduntilthe deathofLeibniz.WhenLeibnizdiedhisworkwaslargely discredited.Thefactthathefalselyclaimedtobe anoblemanandaddedtheprefixvontohisname,coupledwithNewton’svitriolicattacks,didnothelphis credibility.Newtonisreportedtohavedeclaredthathetookgreatsatisfactionin"breakingLeibniz’sheart." Studiesduringthe 20 centuryhavelargelyrevivedthereputationofLeibnizandheisacknowledgedto havemademajorcontributionstothedevelopmentofcalculus.

Leibnizmadesignificantcontributionstoclassicalmechanics.IncontrasttoNewton’slawsofmotion, whicharebasedontheconceptofmomentum,Leibnizdevisedanewtheoryofdynamicsbasedonkinetic andpotentialenergythatanticipatestheanalytical variationalapproachofLagrangeandHamilton.Leibniz arguedforaquantitycalledthe"visviva", whichisLatinfor "livingforce" thatequalstwicethekinetic energy.Leibnizarguedthatthechangeinkineticenergyisequaltotheworkdone.In 1687 Leibniz proposedthattheoptimumpathisbasedonminimizingthetimeintegralofthevisvivawhichisequivalent totheactionintegral.Leibnizusedbothphilosophicalandcausalargumentsinhisworkwhichwereequally acceptableduringtheAgeofEnlightenment.UnfortunatelyforLeibniz,hisanalyticalapproachbasedon energies,whicharescalars,appearedcontradictorytoNewton’sintuitivevectorialtreatmentofforceand momentum.Therewasconsiderableprejudiceandphilosophicaloppositiontothevariationalapproachwhich assumesthatnatureisthriftyinallofitsactions.Thevariationalapproachwasconsideredtobespeculative and"metaphysical"incontrasttothecausalargumentssupportingNewtonianmechanics.Thisopposition delayedfullappreciationofthevariationalapproachuntilthestartofthe 20 century.

JohannBernoulli(1667-1748) wasaSwissmathematicianwhowasastudentofLeibniz’scalculus,and sidedwithLeibnizintheNewton-Leibnizdisputeoverthecreditfordevelopingcalculus.AlsoBernoullisided withtheDescartes’vortextheoryofgravitationwhichdelayedacceptanceofNewton’stheoryofgravitation inEurope.Bernoullipioneereddevelopmentofthecalculusofvariationsbysolvingtheproblemsofthe catenary,thebrachistochrone,andFermat’sprinciple.TheBernoullifamilyisfamousforitscontributions tomathematicsandscience;Johann’ssonDanielplayedasignificantroleinthedevelopmentofthewellknownBernoulliPrincipleinhydrodynamics.

PierreLouisMaupertuis(1698-1759) wasastudentofJohannBernoulliandconceivedtheuniversal hypothesisthatinnaturethereisacertainquantitycalledactionwhichisminimized.Althoughthisbold assumptioncorrectlyanticipatesthedevelopmentofthevariationalapproachtoclassicalmechanics,he obtainedhishypothesisbyanentirelyincorrectmethod.Hewasadilettantewhosemathematicalprowess wasfarbehindthehighstandardsofthattime,andhecouldnotestablishsatisfactorilythequantitytobe minimized.Histeleological1 argumentwasinfluencedbyFermat’sprincipleandthecorpuscletheoryoflight thatimpliedacloseconnectionbetweenopticsandmechanics.

LeonhardEuler(1707-1783) wasthepreeminentSwissmathematicianofthe 18 centuryandwas astudentofJohannBernoulli.Eulerdeveloped,withfullmathematicalrigor,thecalculusofvariations followinginthefootstepsofJohannBernoulli.Eulerusedvariationalcalculustosolveminimum/maximum isoperimetricproblemswhichhadattractedandchallengedtheearlydevelopersofcalculus,Newton,Leibniz, andBernoulli.Euleralsowasthe firsttosolvetherigid-bodyrotationproblemusingthethreecomponents oftheangularvelocityaskinematicalvariables.Eulerbecameblindinbotheyesby1766butthatdidnot hinderhisprolificoutputinmathematicsduetohisremarkable memoryandmentalcapabilities.Euler’s contributionstomathematicsareremarkableinqualityandquantity;forexampleduring1775hepublished

1 Teleologyisanyphilosophicalaccountthatholdsthat fi nalcausesexistinnature,meaningthat–analogoustopurposes foundinhumanactions–natureinherentlytendstowardde fi niteends.

onemathematicalpaperperweekinspiteofbeingblind.Eulerimplicitlyimpliedtheprincipleofleast actionusingvisvisawhichisnottheexactformexplicitlydevelopedbyLagrange.

JeanleRondd’Alembert(1717-1785) wasaFrenchmathematicianandphysicistwhohadthe cleverideaofextendinguseoftheprincipleofvirtualworkfromstaticstodynamics.D’Alembert’sPrinciple rewritestheprincipleofvirtualworkintheform

wheretheinertialreactionforce ˙ p issubtractedfromthecorrespondingforce F.Thisextensionofthe principleofvirtualworkappliesequallytobothstaticsanddynamicsleadingtoasinglevariationalprinciple.

JosephLouisLagrange(1736-1813) wasanItalianmathematicianwhowasastudentofLeonhard EulerandhisworkparalleledthatofEuler.In1788Lagrangepublishedhismonumentaltreatiseonanalyticalmechanicsentitled"MécaniqueAnalytique" whichdescribeshisnew,immenselypowerful,analytical techniquethatcansolveanymechanicalproblemwithoutresorttogeometricalconsiderations.Histheory onlyrequiredtheanalyticalformofthescalarquantitieskineticandpotentialenergy.Intheprefaceof hisbookherefersmodestlytohisextraordinaryachievementswiththestatement"Thereaderwill findno figuresinthework.ThemethodswhichIsetforthdonot requireeitherconstructionsorgeometricalor mechanicalreasonings:butonlyalgebraicoperations,subjecttoaregularanduniformruleofprocedure." Lagrangealsointroducedtheconceptofundetermined multiplierstohandleauxiliaryconditionswhichplays avitalpartoftheoreticalmechanics.WilliamHamilton,anoutstanding figureintheanalyticalformulation ofclassicalmechanics,calledLagrangethe"Shakespeareofmathematics,"onaccountoftheextraordinary beauty,elegance,anddepthoftheLagrangianmethods.Lagrangealsopioneerednumeroussignificant contributionstomathematics.Forexample,Euler,Lagrange,andd’Alembertdevelopedmuchofthemathematicsofpartialdifferentialequations.LagrangesurvivedtheFrenchRevolutionand,inspiteofbeinga foreigner,NapoleonnamedLagrangetotheLegionofHonourandmadehimaCountoftheEmpirein1808. LagrangewashonouredbybeingburiedinthePantheon.

JeanBaptisteJosephFourier(1768-1830) wasaFrenchmathematicianandphysicistwhowasa studentofLagrange.FourierismostfamousforthedevelopmentofFourieranalysiswhichincludesFourier series,andFouriertransforms.Hisworkhasmanyapplicationstoclassicalmechanicssuchasallformsof wavemotion,signalprocessing,andsolving fortheeigenfunctionsoflinearequations.

1.6 19 century

Thezenithindevelopmentofthevariationalapproachtoclassicalmechanicsoccurredduringthe 19 century primarilyduetotheworkofHamiltonandJacobi.

CarlFriedrichGauss(1777-1855) wasaGermanchildprodigywhomademanysignificantcontributionstomathematics,astronomyandphysics.Hedidnotworkdirectlyonthevariationalapproach,but Gauss’slaw,thedivergencetheorem,andtheGaussianstatisticaldistributionareimportantexamplesof conceptsthathedevelopedandwhichfeatureprominentlyinclassicalmechanicsaswellasotherbranches ofphysics,andmathematics.

SimeonPoisson(1781-1840),wasabrilliantmathematicianwhowasastudentofLagrange.He developedthePoissonstatisticaldistributionaswell asthePoissonequationthatfeaturesprominentlyin electromagneticandother fieldtheories.Hismajorcontributiontoclassicalmechanicsisdevelopment,in 1809,ofthePoissonbracketformalismwhichfeaturedprominentlyindevelopmentofHamiltonianmechanics andquantummechanics.

WilliamHamilton(1805-1865) wasabrilliantIrishphysicist,astronomerandmathematicianwhowas appointedprofessorofastronomyatDublinwhenhewasbarely22yearsold.HedevelopedtheHamiltonian mechanicsformalismofclassicalmechanicswhichnowplaysapivotalroleinmodernclassicalandquantum mechanics.HeopenedanentirelynewworldbeyondthedevelopmentsofLagrange.WhereastheLagrange equationsofmotionarecomplicatedsecond-orderdifferentialequations,Hamiltonsucceededintransforming themintoasetof first-orderdifferentialequationswithtwiceasmanyvariablesthatconsidermomentaand theconjugatepositionsasindependentvariables.ThedifferentialequationsofHamiltonarelinear,have separatedderivatives,andrepresentthesimplestandmostdesirableformpossiblefordifferentialequationsto beusedinavariationalapproach.Hencethename"canonicalvariables"givenbyJacobi.Hamiltonexploited

thed’Alembertprincipletogivethe firstexactformulationoftheprincipleofleastactionwhichunderliesthe variationalprinciplesusedinanalyticalmechanics.TheformderivedbyEulerandLagrangeemployedthe principleinawaythatappliesonlyforconservative(scleronomic)cases.AsignificantdiscoveryofHamilton ishisrealizationthatclassicalmechanicsandgeometricalopticscanbehandledfromoneunifiedviewpoint. Inbothcasesheusesa"characteristic"functionthathasthepropertythat,bymeredifferentiation,the pathofthebody,orlightray,canbedeterminedbythesamepartialdifferentialequations.Thissolutionis equivalenttothesolutionoftheequationsofmotion.

CarlGustaveJacobJacobi(1804-1851),aPrussianmathematicianandcontemporaryofHamilton, significantlydevelopedHamiltonianmechanics.Hewasoneofthefewwhoimmediatelyrecognizedthe extraordinaryimportanceoftheHamiltonianformulationofmechanics.Jacobidevelopedcanonicaltransformationtheoryandshowedthatthefunction,usedbyHamilton,isonlyonespecialcaseoffunctionsthat generatesuitablecanonicaltransformations.Heprovedthatanycompletesolutionofthepartialdifferentialequation,withoutthespecificboundaryconditionsappliedbyHamilton,issufficientforthecomplete integrationoftheequationsofmotion.ThisgreatlyextendstheusefulnessofHamilton’spartialdifferential equations.In 1843 JacobidevelopedboththePoissonbrackets, andtheHamilton-Jacobi,formulationsof Hamiltonianmechanics.Thelattergivesasingle, first-orderpartialdifferentialequationfortheactionfunctionintermsofthe  generalizedcoordinateswhichgreatlysimplifiessolutionoftheequationsofmotion. Healsoderivedaprincipleofleastactionfortime-independentcaseswhichhadbeenstudiedbyEulerand Lagrange.Jacobidevelopedasuperiorapproachtothevariationalintegralthat,byeliminatingtimefrom theintegral,determinedthepathwithoutsayinganythingabouthowthemotionoccursintime.

JamesClerkMaxwell(1831-1879) wasaScottishtheoreticalphysicistandmathematician.His mostprominentachievementwasformulatingaclassicalelectromagnetictheorythatunitedallpreviously unrelatedobservations,experimentsandequationsof electricity,magnetismandopticsintooneconsistent theory.Maxwell’sequationsdemonstratedthatelectricity,magnetismandlightareallmanifestationsofthe samephenomenon,namelytheelectromagnetic field.Consequently,allotherclassiclawsandequationsof electromagnetismweresimplifiedcasesofMaxwell’sequations.Maxwell’sachievementsconcerningelectromagnetismhavebeencalledthe"secondgreatunificationinphysics".Maxwelldemonstratedthatelectric andmagnetic fieldstravelthroughspaceintheformofwaves,andattheconstantspeedoflight.In 1864 Maxwellwrote"ADynamicalTheoryoftheElectromagneticField"whichproposedthatlightwasinfact undulationsinthesamemediumthatisthecauseofelectricandmagneticphenomena.Hisworkinproducingaunifiedmodelofelectromagnetismisoneofthegreatestadvancesinphysics.Maxwell,incollaboration with LudwigBoltzmann(1844-1906),alsohelpeddeveloptheMaxwell—Boltzmanndistribution,whichis astatisticalmeansofdescribingaspectsofthekinetictheoryofgases.Thesetwodiscoverieshelpedusherin theeraofmodernphysics,layingthefoundationforsuch fieldsasspecialrelativityandquantummechanics. Boltzmannfoundedthe fieldofstatisticalmechanicsandwasanearlystaunchadvocateoftheexistenceof atomsandmolecules.

HenriPoincaré(1854-1912) wasaFrenchtheoreticalphysicistandmathematician.Hewasthe firstto presenttheLorentztransformationsintheirmodernsymmetricformanddiscoveredtheremainingrelativistic velocitytransformations.Althoughthereissimilarityto Einstein’sSpecialTheoryofRelativity,Poincaréand Lorentzstillbelievedintheconceptoftheetheranddidnotfullycomprehendtherevolutionaryphilosophical changeimpliedbyEinstein.Poincaréworkedonthesolutionofthethree-bodyprobleminplanetarymotion andwasthe firsttodiscoverachaoticdeterministicsystemwhichlaidthefoundationsofmodernchaos theory.Itrejectedthelong-helddeterministicviewthatifthepositionandvelocitiesofalltheparticlesare knownatonetime,thenitispossibletopredictthefutureforalltime.

Thelasttwodecadesofthe 19 centurysawtheculminationofclassicalphysicsandseveralimportant discoveriesthatledtoarevolutioninsciencethattoppledclassicalphysicsfromitsthrone.Theendofthe 19 centurywasatimeduringwhichtremendoustechnologicalprogressoccurred, flight,theautomobile, andturbine-poweredshipsweredeveloped,NiagaraFallswasharnessedforpower,etc.Duringthisperiod, HeinrichHertz(1857-1894) producedelectromagneticwavesconfirmingtheirderivationusingMaxwell’s equationsaswellassimultaneouslydiscoveringthephotoelectriceffect.Technicaldevelopments,suchas photography,theinductionsparkcoil,andthevacuumpumpplayedasignificantroleinscientificdiscoveries madeduringthe1890’s.Attheendofthe 19 century,scientiststhoughtthatthebasiclawswereunderstood andworriedthatfuturephysicswouldbeinthe fifthdecimalplace;somescientistsworriedthatlittlewas leftforthemtodiscover.However,thereremainedafew,presumedminor,unexplaineddiscrepanciesplus newdiscoveriesthatledtotherevolutioninsciencethatoccurredatthebeginningofthe 20 century.

1.7The

20 centuryrevolutioninphysics

Thetwogreatestachievementsofmodernphysicsoccurredinthebeginningofthe 20 century.The first wasEinstein’sdevelopmentoftheTheoryofRelativity;theSpecialTheoryofRelativityin1905andthe GeneralTheoryofRelativityin1915.Thiswasfollowedin1925bythedevelopmentofquantummechanics. AlbertEinstein(1879-1955) developedtheSpecialTheoryofRelativityin1905andtheGeneralTheoryofRelativityin1915;bothoftheserevolutionarytheorieshadaprofoundimpactonclassicalmechanics andtheunderlyingphilosophyofphysics.TheNewtonianformulationofmechanicswasshowntobean approximationthatappliesonlyatlowvelocitieswhiletheGeneralTheoryofRelativitysupersededNewton’sLawofGravitationandexplainedtheEquivalencePrinciple.TheNewtonianconceptsofanabsolute frameofreference,plustheassumptionoftheseparationoftimeandspacewereshowntobeinvalidat relativisticvelocities.Einstein’spostulatethatthelawsofphysicsarethesameinallinertialframesrequires arevolutionarychangeinthephilosophyoftime,spaceandreferenceframeswhichleadstoabreakdown intheNewtonianformalismofclassicalmechanics.By contrast,theLagrangeandHamiltonianvariational formalismsofmechanics,plustheprincipleofleastaction,remainintactusingarelativisticallyinvariant Lagrangian.Theindependenceofthevariationalapproachtoreferenceframesispreciselytheformalism necessaryforrelativisticmechanics.Theinvariancetocoordinateframesofthebasic fieldequationsalso mustremaininvariantfortheGeneralTheoryofRelativity.ThusthedevelopmentoftheTheoryofRelativityunambiguouslydemonstratedthesuperiorityofthevariationalformulationofclassicalmechanicsover thevectorialNewtonianformulation,andthustheconsiderableeffortmadebyEuler,Lagrange,Hamilton, Jacobi,andothersindevelopingtheanalyticalvariationalformalismofclassicalmechanics finallycameto fruitionatthestartofthe 20 century.Newton’stwocrowningachievements,theLawsofMotionandthe LawsofGravitation,thathadreignedsupremesincepublishedinthePrincipiain 1687,weretoppledfrom thethronebyEinstein.

EmmyNoether(1882-1935) hasbeendescribedas"thegreatesteverwomanmathematician".In 1915sheproposedatheoremthataconservationlawisassociatedwithanydifferentiablesymmetryofa physicalsystem.Noether’stheoremevolvesnaturallyfromLagrangianandHamiltonianmechanicsand sheappliedittothefour-dimensionalworldofgeneral relativity.Noether’stheoremhashadanimportant impactinguidingthedevelopmentofmodernphysics.

Anotherprofounddevelopmentthathadarevolutionaryimpactonclassicalmechanicswasquantum physicsplusquantum fieldtheory.The1913modelofatomicstructureby NielsBohr(1885-1962) and thesubsequentenhancementsby ArnoldSommerfeld(1868-1951), werebasedcompletelyonclassical Hamiltonianmechanics.Theproposalofwave-particledualityby LouisdeBroglie(1892-1987),made inhis1924thesis,wasthecatalystleadingto thedevelopmentofquantummechanics.In1925 Werner Heisenberg(1901-1976),and MaxBorn(1882-1970) developedamatrixrepresentationofquantum mechanicsusingnon-commutingconjugatepositionandmomentavariables.

PaulDirac(1902-1984) showedinhisPh.D.thesisthatHeisenberg’smatrixrepresentationisbased onthePoissonBracketgeneralizationofHamiltonianmechanics,which,incontrasttoHamilton’scanonicalequations,allowsfornon-commutingconjugatevariables.In1926 ErwinSchrödinger(1887-1961) independentlyintroducedtheoperationalviewpointandreinterpretedthepartialdifferentialequationof Hamilton-Jacobiasawaveequation.Hisstartingpointwastheoptical-mechanicalanalogyofHamilton thatisabuilt-infeatureoftheHamilton-Jacobitheory.Schrödingerthenshowedthatthewavemechanics hedeveloped,andtheHeisenbergmatrixmechanics,areequivalentrepresentationsofquantummechanics. In1928Diracdevelopedhisrelativisticequationofmotionfortheelectronandpioneeredthe fieldofquantumelectrodynamics.DiracalsointroducedtheLagrangianandtheprincipleofleastactiontoquantum mechanicsandtheseideasweredevelopedintothepath-integralformulationofquantummechanicsandthe theoryofelectrodynamicsby RichardFeynman(1918-1988).

Theconceptsofwave-particleduality,andquantizationofobservables,botharebeyondtheclassical notionsofinfinitesubdivisionsinclassicalphysics.Inspiteoftheradicaldepartureofquantummechanics fromearlierclassicalconcepts,thebasicfeatureofthedifferentialequationsofquantalphysicsistheirselfadjointcharacterwhichmeansthattheyarederivablefromavariationalprinciple.ThusboththeTheory ofRelativity,andquantumphysicsareconsistentonlywiththevariationalprincipleofmechanics,and notNewtonianmechanics.AsaconsequenceNewtonianmechanicshasbeendislodgedfromthethrone itoccupiedsince 1687,andtheintellectuallybeautifulandpowerfulvariationalprinciplesofanalytical mechanicshavebeenvalidated

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