Www.lightandmatter.com Lma.pdf

8/13/2019 Www.lightandmatter.com Lma.pdf

1/50


2/50


3/50


4/50

Fullerton, Californiawww.lightandmatter.com

copyright 1998-2010 Benjamin Crowell

rev. August 21, 2013

This book is licensed under the Creative Com-mons Attribution-ShareAlike license, version 3.0,http://creativecommons.org/licenses/by-sa/3.0/, exceptfor those photographs and drawings of which I am notthe author, as listed in the photo credits. If you agreeto the license, it grants you certain privileges that youwould not otherwise have, such as the right to copy thebook, or download the digital version free of charge fromwww.lightandmatter.com.


5/50

Brief Contents

0 Introduction and review 17

1 Scaling and estimation 43

Motion inone dimension

2 Velocity and relative motion 69

3 Acceleration and free fall 93

4 Force and motion 125

5 Analysis of forces 149

Motion inthree dimensions

6 Newtons laws in

three dimensions 181

7 Vectors 193

8 Vectors and motion 209

9 Circular motion 231

10 Gravity 247

Conservation laws

11 Conservation of energy 27912 Simplifying the energy zoo 301

13 Work: the transfer of

mechanical energy 315

14 Conservation of momentum 345

15 Conservation of

angular momentum 377

16 Thermodynamics 413

Vibrations and waves17 Vibrations 437

18 Resonance 451

19 Free waves 475

20 Bounded waves 503

Relativity andelectromagnetism

21 Electricity and circuits 555

22 The nonmechanical universe 611

23 Relativity and magnetism 645

24 Electromagnetism 669

25 Capacitance and inductance 703

26 The atom and E=mc2 721

27 General relativity 783

Optics28 The ray model of light 801

29 Images by reflection 819

30 Images, quantitatively 837

31 Refraction 85532 Wave optics 877

The modern revolutionin physics

33 Rules of randomness 905

34 Light as a particle 929

35 Matter as a wave 947

36 The atom 973


6/50

Contents

0 Introduction and review0.1 The scientific method . . . . . . 17

0.2 What is physics? . . . . . . . . 20

Isolated systems and reductionism, 22.

0.3 How to learn physics. . . . . . . 23

0.4 Self-evaluation . . . . . . . . . 25

0.5 Basics of the metric system . . . . 26

The metric system, 26.The second, 27.The meter, 27.The kilogram, 28.Combinations of metric units, 28.Checking units, 28.

0.6 The Newton, the metric unit of force 30

0.7 Less common metric prefixes . . . 30

0.8 Scientific notation . . . . . . . . 31

0.9 Conversions . . . . . . . . . . 32Should that exponent be positive, ornegative?, 33.

0.10 Significant figures . . . . . . . 34

Summary . . . . . . . . . . . . . 37

Problems . . . . . . . . . . . . . 39

Exercise 0: Models and idealization . . 41

1 Scaling and estimation1.1 Introduction . . . . . . . . . . 43

Area and volume, 43.1.2 Scaling of area and volume . . . . 45

Galileo on the behavior of nature on largeand small scales, 46.Scaling of area andvolume for irregularly shaped ob jects, 49.

1.3 Scaling applied to biology . . . . 53

Organisms of different sizes with the sameshape, 53.Changes in shape to accommo-date changes in size, 55.

1.4 Order-of-magnitude estimates . . . 57

Summary . . . . . . . . . . . . . 60

Problems . . . . . . . . . . . . . 61

Exercise 1: Scaling applied to leaves. . 66

Motion in one dimension

2 Velocity and relative motion2.1 Types of motion . . . . . . . . . 69

Rigid-body motion distinguished from mo-

tion that changes an objects shape,69.Center-of-mass motion as opposed torotation, 69.Center-of-mass motion inone dimension, 73.

2.2 Describing distance and time . . . 73

A point in time as opposed to duration, 74.

2.3 Graphs of motion; velocity . . . . 76

Motion with constant velocity, 76.Motion with changing velocity, 77.Conventions about graphing, 78.

2.4 The principle of inertia . . . . . . 80

Physical effects relate only to a change invelocity, 80.Motion is relative, 81.

2.5 Addition of velocities . . . . . . . 83

Addition of velocities to describe relativemotion, 83.Negative velocities in relativemotion, 83.

2.6 Graphs of velocity versus time. . . 86

2.7

Applications of calculus. . . . . 86

Summary . . . . . . . . . . . . . 88

6


7/50

Problems . . . . . . . . . . . . . 90

3 Acceleration and free fall

3.1 The motion of falling objects . . . 93

How the speed of a falling object increaseswith time, 95.A contradiction in Aristo-tles reasoning, 96.What is gravity?, 96.

3.2 Acceleration . . . . . . . . . . 97

Definition of acceleration for linear v tgraphs, 97.The acceleration of gravity isdifferent in different locations., 98.

3.3 Positive and negative acceleration . 100

3.4 Varying acceleration . . . . . . . 104

3.5 The area under the velocity-time

graph . . . . . . . . . . . . . . . 107

3.6 Algebraic results for constant

acceleration . . . . . . . . . . . . 109

3.7 A test of the principle of inertia . . 112

3.8 Applications of calculus. . . . . 113Summary . . . . . . . . . . . . . 114

Problems . . . . . . . . . . . . . 115

4 Force and motion

4.1 Force . . . . . . . . . . . . . 126

We need only explain changes in motion,not motion itself., 126.Motion changesdue to an interaction between two objects.,127.Forces can all be measured on thesame numerical scale., 127.More thanone force on an object, 128.Objects canexert forces on each other at a distance.,128.Weight, 129.Positive and negativesigns of force, 129.

4.2 Newtons first law . . . . . . . . 129

More general combinations of forces, 131.

4.3 Newtons second law . . . . . . 133

A generalization, 134.The relationshipbetween mass and weight, 135.

4.4 What force is not . . . . . . . . 138

1. Force is not a property of one object.,138.2. Force is not a measure of anobjects motion., 138.3. Force is notenergy., 138.4. Force is not stored orused up., 139.5. Forces need not be ex-erted by living things or machines., 139.

6. A force is the direct cause of a changein motion., 139.

4.5 Inertial and noninertial frames of

reference . . . . . . . . . . . . . 140

Summary . . . . . . . . . . . . . 143

Problems . . . . . . . . . . . . . 144

Exercise 4: Force and motion . . . . . 147


8/50

5 Analysis of forces5.1 Newtons third law . . . . . . . . 149

A mnemonic for using Newtons third lawcorrectly, 152.

5.2 Classification and behavior of forces 154

Normal forces, 157.Gravitational forces,

157.Static and kinetic friction, 158.Fluid friction, 162.

5.3 Analysis of forces . . . . . . . . 163

5.4 Transmission of forces by low-mass

o b j e c t s . . . . . . . . . . . . . . 1 6 6

5.5 Objects under strain . . . . . . . 168

5.6 Simple Machines: the pulley . . . 169

Summary . . . . . . . . . . . . . 171

Problems . . . . . . . . . . . . . 173

Motion in three dimensions

6 Newtons laws in three

dimensions6.1 Forces have no perpendicular effects181

Relationship to relative motion, 183.

6.2 Coordinates and components . . . 184

Projectiles move along parabolas., 187.

6.3 Newtons laws in three dimensions . 187

Summary . . . . . . . . . . . . . 189

Problems . . . . . . . . . . . . . 190

7 Vectors7.1 Vector notation . . . . . . . . . 193

Drawing vectors as arrows, 196.

7.2 Calculations with magnitude and

direction . . . . . . . . . . . . . . 197

7.3 Techniques for adding vectors . . . 200

Addition of vectors given their

components, 200.Addition of vectorsgiven their magnitudes and directions,200.Graphical addition of vectors, 200.

7.4 Unit vector notation . . . . . . 202

7.5 Rotational invariance . . . . . . 203

Summary . . . . . . . . . . . . . 205

Problems . . . . . . . . . . . . . 206

8 Vectors and motion8.1 The velocity vector . . . . . . . 210

8.2 The acceleration vector . . . . . 212

8.3 The force vector and simple machines215

8.4

Calculus with vectors . . . . . 217

Summary . . . . . . . . . . . . . 221

Problems . . . . . . . . . . . . . 222

Exercise 8: Vectors and motion . . . . 228

9 Circular motion9.1 Conceptual framework . . . . . . 231

Circular motion does not produce an out-ward force, 231.Circular motion does notpersist without a force, 232.Uniform andnonuniform circular motion, 233.Only aninward force is required for uniform cir-cular motion., 234.In uniform circularmotion, the acceleration vector is inward.,235.

9.2 Uniform circular motion. . . . . . 238

9.3 Nonuniform circular motion . . . . 241

Summary . . . . . . . . . . . . . 242

Problems . . . . . . . . . . . . . 243

10 Gravity10.1 Keplers laws . . . . . . . . . 248

10.2 Newtons law of gravity . . . . . 250

The suns force on the planets obeys aninverse square law., 250.The forces be-tween heavenly bodies are the same type offorce as terrestrial gravity., 251.Newtons

8


9/50

law of gravity, 252.

10.3 Apparent weightlessness . . . . 256

10.4 Vector addition of gravitational

forces . . . . . . . . . . . . . . . 256

10.5 Weighing the earth . . . . . . . 259

10.6 Dark energy . . . . . . . . . 261

10.7 A gravitational test of Newtons firstlaw . . . . . . . . . . . . . . . . 263

Summary . . . . . . . . . . . . . 265

Problems . . . . . . . . . . . . . 267

Exercise 10: The shell theorem . . . . 275

Conservation laws

11 Conservation of energy11.1 The search for a perpetual motion

M a c h i n e . . . . . . . . . . . . . . 2 7 9

11.2 Energy . . . . . . . . . . . . 280

11.3 A numerical scale of energy . . . 284

How new forms of energy are discovered,287.

11.4 Kinetic energy . . . . . . . . . 289

Energy and relative motion, 290.

11.5 Power . . . . . . . . . . . . 291

Summary . . . . . . . . . . . . . 294

Problems . . . . . . . . . . . . . 296

12 Simplifying the energy zoo

12.1 Heat is kinetic energy . . . . . . 30212.2 Potential energy: energy of distance

or closeness . . . . . . . . . . . . 304

An equation for gravitational potentialenergy, 305.

12.3 All energy is potential or kinetic . . 308

Summary . . . . . . . . . . . . . 310

Problems . . . . . . . . . . . . . 311

13 Work: the transfer of me-

chanical energy

13.1 Work: the transfer of mechanical

energy . . . . . . . . . . . . . . 315

The concept of work, 315.Calculatingwork as force multiplied by distance,316.Machines can increase force, butnot work., 319.No work is done withoutmotion., 319.Positive and negative work,320.

13.2 Work in three dimensions . . . . 322

A force perpendicular to the motion doesno work., 322.Forces at other angles, 323.

13.3 Varying force . . . . . . . . . 326

13.4

Applications of calculus . . . . 329

13.5 Work and potential energy. . . . 330

13.6 When does work equal force timesdistance? . . . . . . . . . . . . . 332

13.7 The dot product . . . . . . . 334

Summary . . . . . . . . . . . . . 336

Problems . . . . . . . . . . . . . 338

14 Conservation of momentum

14.1 Momentum . . . . . . . . . . 346

A conserved quantity of motion, 346.Momentum, 347.Generalization of themomentum concept, 349.Momentumcompared to kinetic energy, 350.

14.2 Collisions in one dimension . . . 353

The discovery of the neutron, 355.

14.3 Relationship of momentum to thecenter of mass . . . . . . . . . . . 357

Momentum in different frames of reference,359.The center of mass frame ofreference, 359.

14.4 Momentum transfer. . . . . . . 360

The rate of change of momentum, 360.

The area under the force-time graph, 362.14.5 Momentum in three dimensions . 363

The center of mass, 364.Counting equa-tions and unknowns, 365.Calculationswith the momentum vector, 366.

14.6


Summary . . . . . . . . . . . . . 370

Problems . . . . . . . . . . . . . 372


10/50

15 Conservation of angular

momentum

15.1 Conservation of angular momentum379

Restriction to rotation in a plane, 383.

15.2 Angular momentum in planetary

motion . . . . . . . . . . . . . . 38315.3 Two theorems about angular

momentum . . . . . . . . . . . . 385

15.4 Torque: the rate of transfer of angu-

lar momentum . . . . . . . . . . . 390

Torque distinguished from force, 390.Relationship between force and torque,391.The torque due to gravity, 393.

15.5 Statics . . . . . . . . . . . . 397

Equilibrium, 397.Stable and unstableequilibria, 400.

15.6 Simple Machines: the lever . . . 401

15.7 Proof of Keplers elliptical orbit law403

Summary . . . . . . . . . . . . . 405

Problems . . . . . . . . . . . . . 407

Exercise 15: Torque . . . . . . . . . 412

16 Thermodynamics

16.1 Pressure and temperature . . . . 414

Pressure, 414.Temperature, 418.

16.2 Microscopic description of an ideal

gas . . . . . . . . . . . . . . . . 421

Evidence for the kinetic theory, 421.Pressure, volume, and temperature, 421.

16.3 Entropy . . . . . . . . . . . . 425

Efficiency and grades of energy, 425.Heat engines, 425.Entropy, 427.

Problems . . . . . . . . . . . . . 431

Vibrations and waves

17 Vibrations17.1 Period, frequency, and amplitude . 43817.2 Simple harmonic motion. . . . . 441

Why are sine-wave vibrations so common?,441.Period is approximately indepen-dent of amplitude, if the amplitude issmall., 442.

17.3 Proofs . . . . . . . . . . . 444

Summary . . . . . . . . . . . . . 446

Problems . . . . . . . . . . . . . 447

Exercise 17: Vibrations . . . . . . . 449

18 Resonance18.1 Energy in vibrations . . . . . . 452

18.2 Energy lost from vibrations. . . . 454

18.3 Putting energy into vibrations . . 456

18.4 Proofs . . . . . . . . . . . 464

Statement 2: maximum amplitude atresonance, 465.Statement 3: amplitudeat resonance proportional to Q, 465.Statement 4: FWHM related to Q, 466.

Summary . . . . . . . . . . . . . 467

Problems . . . . . . . . . . . . . 469

Exercise 18: Resonance . . . . . . . 473

19 Free waves19.1 Wave motion . . . . . . . . . 477

1. Superposition, 477.2. The mediumis not transported with the wave., 479.3.A waves velocity depends on the medium.,480.Wave patterns, 481.

10


11/50

19.2 Waves on a string . . . . . . . 482

Intuitive ideas, 482.Approximatetreatment, 483.Rigorous derivation us-ing calculus (optional), 484.Significanceof the result, 486.

19.3 Sound and light waves . . . . . 486

Sound waves, 486.Light waves, 487

.

19.4 Periodic waves. . . . . . . . . 489

Period and frequency of a periodic wave,489.Graphs of waves as a functionof position, 489.Wavelength, 490.Wave velocity related to frequency andwavelength, 490.Sinusoidal waves, 492.

19.5 The Doppler effect . . . . . . . 493

The Big Bang, 495.What the Big Bangis not, 497.

Summary . . . . . . . . . . . . . 499

Problems . . . . . . . . . . . . . 501

20 Bounded waves20.1 Reflection, transmission, and

absorption . . . . . . . . . . . . . 504

Reflection and transmission, 504.Inverted and uninverted reflections, 507.Absorption, 507.

20.2 Quantitative treatment of reflection511Why reflection occurs, 511.Intensity ofreflection, 512.Inverted and uninvertedreflections in general, 513.

20.3 Interference effects . . . . . . . 514

20.4 Waves bounded on both sides . . 517

Musical applications, 519.Standingwaves, 519.Standing-wave patterns ofair columns, 521.

Summary . . . . . . . . . . . . . 523

Problems . . . . . . . . . . . . . 524

Three essential mathematical skills . . 526

Photo credits for volume 1 . . . . . . 549

Relativity and electromagnetism

21 Electricity and circuits

21.1 The quest for the atomic force . . 55621.2 Electrical forces . . . . . . . . 557

Charge, 557.Conservation of charge,559.Electrical forces involving neutralobjects, 560.

21.3 Current . . . . . . . . . . . . 560

Unity of all types of electricity, 560.Electric current, 561.

21.4 Circuits . . . . . . . . . . . . 563

21.5 Voltage . . . . . . . . . . . . 565

The volt unit, 565.The voltage conceptin general, 565.

21.6 Resistance . . . . . . . . . . 570

Resistance, 570.Superconductors, 572.Constant voltage throughout a conductor,572.Short circuits, 574.Resistors, 575.

21.7


21.8 Series and parallel circuits. . . . 578

Schematics, 578.Parallel resistances andthe junction rule, 579.Series resistances,584.

Summary . . . . . . . . . . . . . 591

Problems . . . . . . . . . . . . . 595

Exercise 21A: Electrical measurements. 603Exercise 21B: Voltage and current . . . 604

Exercise 21C: Reasoning about circuits 609

22 The nonmechanical universe22.1 The stage and the actors . . . . 612

Newtons instantaneous action at adistance, 612.No absolute time, 612.Causality, 613.Time delays in forces ex-erted at a distance, 613.More evidencethat fields of force are real: they carryenergy., 614.

22.2 The gravitational field . . . . . . 616

Sources and sinks, 617.Superposition offields, 617.Gravitational waves, 618.

22.3 The electric field . . . . . . . . 619

Definition, 619.Dipoles, 622.Alternative definition of the electric field,623.Voltage related to electric field, 623.

22.4 Calculating energy in fields . . . 625


12/50

22.5

Voltage for nonuniform fields . . 628

22.6 Two or three dimensions . . . . 629

22.7 Field lines and Gausss law. . . 632

22.8 Electric field of a continuous

charge distribution . . . . . . . . . 634

Summary . . . . . . . . . . . . . 636

Problems . . . . . . . . . . . . . 638Exercise 22: Field vectors . . . . . . 642

23 Relativity and magnetism23.1 Relativistic distortion of space and

time . . . . . . . . . . . . . . . 645

Time distortion arising from mo-tion and gravity, 645.The Lorentztransformation, 647.The G factor, 653.

23.2 Magnetic interactions . . . . . . 660

Relativity requires magnetism, 661.

Summary . . . . . . . . . . . . . 665Problems . . . . . . . . . . . . . 666

24 Electromagnetism24.1 The magnetic field . . . . . . . 669

No magnetic monopoles, 669.Definitionof the magnetic field, 670.Relativity,671.

24.2 Calculating magnetic fields and

forces . . . . . . . . . . . . . . . 672

Magnetostatics, 672.Force on a charge

moving through a magnetic field, 674.Energy in the magnetic field, 675.

24.3 The universal speedc . . . . . . 676

Velocities dont simply add and subtract.,676.A universal speed limit, 677.Lighttravels at c., 677.The Michelson-Morleyexperiment, 677.

24.4 Induction . . . . . . . . . . . 680

The principle of induction, 680.

24.5 Electromagnetic waves . . . . . 682

Polarization, 683.Light is an electro-magnetic wave, 683.The electromagnetic

spectrum, 684.Momentum of light, 686.24.6 Symmetry and handedness . . 686

24.7 Doppler shifts and clock time . . 687

Doppler shifts of light, 687.Clock time,689.

Summary . . . . . . . . . . . . . 692

Problems . . . . . . . . . . . . . 693

Exercise 24: Polarization . . . . . . . 700

25 Capacitance and inductance25.1 Capacitance and inductance . . . 703

Capacitors, 704.Inductors, 704.

25.2 Oscillations . . . . . . . . . . 706

25.3 Voltage and current. . . . . . . 709

25.4 Decay . . . . . . . . . . . . 713

The RC circuit, 713.The RL circuit, 714.25.5 Impedance . . . . . . . . . . 716

Problems . . . . . . . . . . . . . 719

26 The atom and E=mc2

26.1 Atoms . . . . . . . . . . . . 721

The chemical elements, 721.Makingsense of the elements, 722.Direct proofthat atoms existed, 723.

26.2 Quantization of charge . . . . . 724

26.3 The electron. . . . . . . . . . 727

Cathode rays, 727.Were cathode raysa form of light, or of matter?, 727.Thomsons experiments, 728.The cath-ode ray as a subatomic particle: theelectron, 730.The raisin cookie model,730.

26.4 The nucleus . . . . . . . . . . 733

Radioactivity, 733.The planetary model,736.Atomic number, 739.The struc-ture of nuclei, 745.The strong nuclearforce, alpha decay and fission, 748.Theweak nuclear force; beta decay, 751.

26.5 Fusion . . . . . . . . . . . . 753

26.6 Nuclear energy and binding

energies. . . . . . . . . . . . . . 754

26.7 Biological effects of ionizing

radiation. . . . . . . . . . . . . . 756

Units used to measure exposure, 756.Effects of exposure, 757.Effects of highdoses of radiation, 758.Effects of lowdoses radiation, 758.The green case fornuclear power, 759.

26.8 The creation of the elements . . 760Creation of hydrogen and helium in the

Big Bang, 760.We are stardust, 761.Artificial synthesis of heavy elements, 761.

26.9 Relativistic mass and energy. . . 762

Momentum, 762.Equivalence of massand energy, 765.

26.10 Proofs . . . . . . . . . . . 770Transformation of the fields in a light wave,770.Transformation of the volume of alight wave, 771.Transformation of the

12


13/50


14/50

Summary . . . . . . . . . . . . . 869

Problems . . . . . . . . . . . . . 870

Exercise 31: How strong are your

glasses? . . . . . . . . . . . . . 876

32 Wave optics32.1 Diffraction . . . . . . . . . . . 878

32.2 Scaling of diffraction . . . . . . 879

32.3 The correspondence principle . . 880

32.4 Huygens principle . . . . . . . 881

32.5 Double-slit diffraction . . . . . . 882

32.6 Repetition. . . . . . . . . . . 886

32.7 Single-slit diffraction . . . . . . 887

32.8The principle of least time . . 888

Summary . . . . . . . . . . . . . 891

Problems . . . . . . . . . . . . . 893

Exercise 32A: Double-source interference898

Exercise 32B: Single-slit diffraction . . 900

Exercise 32C: Diffraction of light. . . . 902

The modern revolution in

physics

33 Rules of randomness

33.1 Randomness isnt random . . . . 907

33.2 Calculating randomness. . . . . 908

Statistical independence, 908.Additionof probabilities, 909.Normalization,910.Averages, 910.

33.3 Probability distributions . . . . . 912

Average and width of a probabilitydistribution, 913.

33.4 Exponential decay and half-life . . 914

Rate of decay, 917.

33.5


Summary . . . . . . . . . . . . . 922

Problems . . . . . . . . . . . . . 924

34 Light as a particle34.1 Evidence for light as a particle . . 930

34.2 How much light is one photon? . . 932

The photoelectric effect, 932.An unex-pected dependence on frequency, 933.

Numerical relationship between energy andfrequency, 934.

34.3 Wave-particle duality . . . . . . 937

A wrong interpretation: photons interfer-ing with each other, 938.The conceptof a photons path is undefined., 938.Another wrong interpretation: the pilotwave hypothesis, 939.The probabilityinterpretation, 939.

34.4 Photons in three dimensions . . . 942

Summary . . . . . . . . . . . . . 943

Problems . . . . . . . . . . . . . 944

35 Matter as a wave35.1 Electrons as waves . . . . . . . 948

What kind of wave is it?, 951.

35.2Dispersive waves . . . . . . 953

35.3 Bound states . . . . . . . . . 955

35.4 The uncertainty principle . . . . 958

The uncertainty principle, 958.Measurement and Schrodingers cat, 962.

35.5 Electrons in electric fields . . . . 963

Tunneling, 964.

35.6The Schrodinger equation . . 964

Use of complex numbers, 967.

Summary . . . . . . . . . . . . . 968

Problems . . . . . . . . . . . . . 970

36 The atom36.1 Classifying states . . . . . . . 974

14


15/50

36.2 Angular momentum in three

dimensions . . . . . . . . . . . . 975

Three-dimensional angular momentum inclassical physics, 975.Three-dimensionalangular momentum in quantum physics,976.

36.3 The hydrogen atom. . . . . . . 97736.4 Energies of states in hydrogen . 980

History, 980.Approximate treatment,

980.

36.5 Electron spin . . . . . . . . . 982

36.6 Atoms with more than one electron 983

Deriving the periodic table, 985.

Summary . . . . . . . . . . . . . 988

Problems . . . . . . . . . . . . . 990

Exercise 36: Quantum versus classicalrandomness . . . . . . . . . . . . 993

Photo credits for volume 2 . . . . . . 1003


16/50


17/50

The Mars Climate Orbiter is ppared for its mission. The laof physics are the same evewhere, even on Mars, so tprobe could be designed bason the laws of physics as discoered on earth. There is unftunately another reason why t

spacecraft is relevant to the toics of this chapter: it was dstroyed attempting to enter Maatmosphere because engineat Lockheed Martin forgot to co

vert data on engine thrusts fropounds into the metric unit force (newtons) before giving tinformation to NASA. Convsions are important!

Chapter 0

Introduction and review

If you drop your shoe and a coin side by side, they hit the ground atthe same time. Why doesnt the shoe get there first, since gravity ispulling harder on it? How does the lens of your eye work, and whydo your eyes muscles need to squash its lens into different shapes inorder to focus on objects nearby or far away? These are the kindsof questions that physics tries to answer about the behavior of lightand matter, the two things that the universe is made of.

0.1 The scientific method

Until very recently in history, no progress was made in answeringquestions like these. Worse than that, the wrong answers writtenby thinkers like the ancient Greek physicist Aristotle were acceptedwithout question for thousands of years. Why is it that scientificknowledge has progressed more since the Renaissance than it hadin all the preceding millennia since the beginning of recorded his-tory? Undoubtedly the industrial revolution is part of the answer.Building its centerpiece, the steam engine, required improved tech-


18/50

a /Science is a cycle of the-ory and experiment.

b /A satir ical drawing of analchemists laboratory. H. Cock,after a drawing by Peter Brueghelthe Elder (16th century).

niques for precise construction and measurement. (Early on, it wasconsidered a major advance when English machine shops learned tobuild pistons and cylinders that fit together with a gap narrowerthan the thickness of a penny.) But even before the industrial rev-olution, the pace of discovery had picked up, mainly because of theintroduction of the modern scientific method. Although it evolved

over time, most scientists today would agree on something like thefollowing list of the basic principles of the scientific method:

(1) Science is a cycle of theory and experiment. Scientific the-ories 1 are created to explain the results of experiments that werecreated under certain conditions. A successful theory will also makenew predictions about new experiments under new conditions. Even-tually, though, it always seems to happen that a new experimentcomes along, showing that under certain conditions the theory isnot a good approximation or is not valid at all. The ball is thenback in the theorists court. If an experiment disagrees with thecurrent theory, the theory has to be changed, not the experiment.

(2)Theories should both predict and explain. The requirement ofpredictive power means that a theory is only meaningful if it predictssomething that can be checked against experimental measurementsthat the theorist did not already have at hand. That is, a theoryshould be testable. Explanatory value means that many phenomenashould be accounted for with few basic principles. If you answerevery why question with because thats the way it is, then yourtheory has no explanatory value. Collecting lots of data withoutbeing able to find any basic underlying principles is not science.

(3) Experiments should be reproducible. An experiment shouldbe treated with suspicion if it only works for one person, or onlyin one part of the world. Anyone with the necessary skills andequipment should be able to get the same results from the sameexperiment. This implies that science transcends national and eth-nic boundaries; you can be sure that nobody is doing actual sciencewho claims that their work is Aryan, not Jewish, Marxist, notbourgeois, or Christian, not atheistic. An experiment cannot bereproduced if it is secret, so science is necessarily a public enterprise.

As an example of the cycle of theory and experiment, a vital steptoward modern chemistry was the experimental observation that thechemical elements could not be transformed into each other, e.g.,

lead could not be turned into gold. This led to the theory thatchemical reactions consisted of rearrangements of the elements in

1The term theory in science does not just mean what someone thinks, oreven what a lot of scientists think. It means an interrelated set of statementsthat have predictive value, and that have survived a broad set of empiricaltests. Thus, both Newtons law of gravity and Darwinian evolution are scientifictheories. A hypothesis, in contrast to a theory, is any statement of interestthat can be empirically tested. That the moon is made of cheese is a hypothesis,which was empirically tested, for example, by the Apollo astronauts.

18 Chapter 0 Introduction and review


19/50

different combinations, without any change in the identities of theelements themselves. The theory worked for hundreds of years, andwas confirmed experimentally over a wide range of pressures andtemperatures and with many combinations of elements. Only inthe twentieth century did we learn that one element could be trans-formed into one another under the conditions of extremely high

pressure and temperature existing in a nuclear bomb or inside a star.That observation didnt completely invalidate the original theory ofthe immutability of the elements, but it showed that it was only anapproximation, valid at ordinary temperatures and pressures.

self-check A

A psychic conducts seances in which the spirits of the dead speak to

the participants. He says he has special psychic powers not possessed

by other people, which allow him to channel the communications with

the spirits. What part of the scientific method is being violated here?

Answer, p. 542

The scientific method as described here is an idealization, and

should not be understood as a set procedure for doing science. Sci-entists have as many weaknesses and character flaws as any othergroup, and it is very common for scientists to try to discredit otherpeoples experiments when the results run contrary to their own fa-vored point of view. Successful science also has more to do withluck, intuition, and creativity than most people realize, and therestrictions of the scientific method do not stifle individuality andself-expression any more than the fugue and sonata forms stifledBach and Haydn. There is a recent tendency among social scien-tists to go even further and to deny that the scientific method evenexists, claiming that science is no more than an arbitrary social sys-

tem that determines what ideas to accept based on an in-groupscriteria. I think thats going too far. If science is an arbitrary socialritual, it would seem difficult to explain its effectiveness in buildingsuch useful items as airplanes, CD players, and sewers. If alchemyand astrology were no less scientific in their methods than chem-istry and astronomy, what was it that kept them from producinganything useful?

Discussion questions

Consider whether or not the scientific method is being applied in the fol-

lowing examples. If the scientific method is not being applied, are the

people whose actions are being described performing a useful human

activity, albeit an unscientific one?

A Acupuncture is a traditional medical technique of Asian origin inwhich small needles are inserted in the patients body to relieve pain.Many doctors trained in the west consider acupuncture unworthy of ex-perimental study because if it had therapeutic effects, such effects couldnot be explained by their theories of the nervous system. Who is being

more scientific, the western or eastern practitioners?

Section 0.1 The scientific method


20/50

B Goethe, a German poet, is less well known for his theory of color.He published a book on the subject, in which he argued that scientificapparatus for measuring and quantifying color, such as prisms, lensesand colored filters, could not give us full insight into the ultimate meaningof color, for instance the cold feeling evoked by blue and green or theheroic sentiments inspired by red. Was his work scientific?

C A child asks why things fall down, and an adult answers because ofgravity. The ancient Greek philosopher Aristotle explained that rocks fellbecause it was their nature to seek out their natural place, in contact withthe earth. Are these explanations scientific?

D Buddhism is partly a psychological explanation of human suffering,and psychology is of course a science. The Buddha could be said to

have engaged in a cycle of theory and experiment, since he worked bytrial and error, and even late in his life he asked his followers to challengehis ideas. Buddhism could also be considered reproducible, since theBuddha told his followers they could find enlightenment for themselvesif they followed a certain course of study and discipline. Is Buddhism ascientific pursuit?

0.2 What is physics?

Given for one instant an intelligence which could comprehend

all the forces by which nature is animated and the respective

positions of the things which compose it...nothing would be

uncertain, and the future as the past would be laid out before

its eyes.

Pierre Simon de Laplace

Physics is the use of the scientific method to find out the basicprinciples governing light and matter, and to discover the implica-

tions of those laws. Part of what distinguishes the modern outlookfrom the ancient mind-set is the assumption that there are rules bywhich the universe functions, and that those laws can be at least par-tially understood by humans. From the Age of Reason through thenineteenth century, many scientists began to be convinced that thelaws of nature not only could be known but, as claimed by Laplace,those laws could in principle be used to predict everything aboutthe universes future if complete information was available aboutthe present state of all light and matter. In subsequent sections,Ill describe two general types of limitations on prediction using thelaws of physics, which were only recognized in the twentieth century.

Matter can be defined as anything that is affected by gravity,i.e., that has weight or would have weight if it was near the Earthor another star or planet massive enough to produce measurablegravity. Light can be defined as anything that can travel from oneplace to another through empty space and can influence matter, buthas no weight. For example, sunlight can influence your body byheating it or by damaging your DNA and giving you skin cancer.The physicists definition of light includes a variety of phenomena



21/50

c /This telescope picture shotwo images of the same distaobject, an exotic, very luminoobject called a quasar. Thisinterpreted as evidence thatmassive, dark object, possia black hole, happens to between us and it. Light rays th

would otherwise have missed tearth on either side have bebent by the dark objects gravso that they reach us. The actu

direction to the quasar is presuably in the center of the imagbut the light along that central lidoesnt get to us because itabsorbed by the dark object. Tquasar is known by its catal

number, MG1131+0456, or moinformally as Einsteins Ring.

that are not visible to the eye, including radio waves, microwaves,x-rays, and gamma rays. These are the colors of light that do nothappen to fall within the narrow violet-to-red range of the rainbowthat we can see.

self-check B

At the turn of the 20th century, a strange new phenomenon was discov-

ered in vacuum tubes: mysterious rays of unknown origin and nature.These rays are the same as the ones that shoot from the back of your

TVs picture tube and hit the front to make the picture. Physicists in

1895 didnt have the faintest idea what the rays were, so they simply

named them cathode rays, after the name for the electrical contact

from which they sprang. A fierce debate raged, complete with national-

istic overtones, over whether the rays were a form of light or of matter.

What would they have had to do in order to settle the issue?

Answer, p. 542

Many physical phenomena are not themselves light or matter,but are properties of light or matter or interactions between light

and matter. For instance, motion is a property of all light and somematter, but it is not itself light or matter. The pressure that keepsa bicycle tire blown up is an interaction between the air and thetire. Pressure is not a form of matter in and of itself. It is asmuch a property of the tire as of the air. Analogously, sisterhoodand employment are relationships among people but are not peoplethemselves.

Some things that appear weightless actually do have weight, andso qualify as matter. Air has weight, and is thus a form of mattereven though a cubic inch of air weighs less than a grain of sand. Ahelium balloon has weight, but is kept from falling by the force of thesurrounding more dense air, which pushes up on it. Astronauts inorbit around the Earth have weight, and are falling along a curvedarc, but they are moving so fast that the curved arc of their fallis broad enough to carry them all the way around the Earth in acircle. They perceive themselves as being weightless because theirspace capsule is falling along with them, and the floor therefore doesnot push up on their feet.

Optional Topic: Modern Changes in the Definition of Light andMatter

Einstein predicted as a consequence of his theory of relativity that lightwould after all be affected by gravity, although the effect would be ex-tremely weak under normal conditions. His prediction was borne outby observations of the bending of light rays from stars as they passedclose to the sun on their way to the Earth. Einsteins theory also impliedthe existence of black holes, stars so massive and compact that theirintense gravity would not even allow light to escape. (These days thereis strong evidence that black holes exist.)

Einsteins interpretation was that light doesnt really have mass, butthat energy is affected by gravity just like mass is. The energy in a light

Section 0.2 What is physics?


22/50

d /Reductionism.

beam is equivalent to a certain amount of mass, given by the famousequationE = mc2, where c is the speed of light. Because the speedof light is such a big number, a large amount of energy is equivalent toonly a very small amount of mass, so the gravitational force on a lightray can be ignored for most practical purposes.

There is however a more satisfactory and fundamental distinctionbetween light and matter, which should be understandable to you if youhave had a chemistry course. In chemistry, one learns that electronsobey the Pauli exclusion principle, which forbids more than one electronfrom occupying the same orbital if they have the same spin. The Pauli

exclusion principle is obeyed by the subatomic particles of which matteris composed, but disobeyed by the particles, called photons, of which abeam of light is made.

Einsteins theory of relativity is discussed more fully in book 6 of thisseries.

The boundary between physics and the other sciences is notalways clear. For instance, chemists study atoms and molecules,which are what matter is built from, and there are some scientists

who would be equally willing to call themselves physical chemistsor chemical physicists. It might seem that the distinction betweenphysics and biology would be clearer, since physics seems to dealwith inanimate objects. In fact, almost all physicists would agreethat the basic laws of physics that apply to molecules in a test tubework equally well for the combination of molecules that constitutesa bacterium. (Some might believe that something more happens inthe minds of humans, or even those of cats and dogs.) What differ-entiates physics from biology is that many of the scientific theoriesthat describe living things, while ultimately resulting from the fun-damental laws of physics, cannot be rigorously derived from physical

principles.

Isolated systems and reductionism

To avoid having to study everything at once, scientists isolate thethings they are trying to study. For instance, a physicist who wantsto study the motion of a rotating gyroscope would probably preferthat it be isolated from vibrations and air currents. Even in biology,where field work is indispensable for understanding how living thingsrelate to their entire environment, it is interesting to note the vitalhistorical role played by Darwins study of the Galapagos Islands,which were conveniently isolated from the rest of the world. Any

part of the universe that is considered apart from the rest can becalled a system.

Physics has had some of its greatest successes by carrying thisprocess of isolation to extremes, subdividing the universe into smallerand smaller parts. Matter can be divided into atoms, and the be-havior of individual atoms can be studied. Atoms can be split apart



23/50

into their constituent neutrons, protons and electrons. Protons andneutrons appear to be made out of even smaller particles calledquarks, and there have even been some claims of experimental ev-idence that quarks have smaller parts inside them. This methodof splitting things into smaller and smaller parts and studying howthose parts influence each other is called reductionism. The hope is

that the seemingly complex rules governing the larger units can bebetter understood in terms of simpler rules governing the smallerunits. To appreciate what reductionism has done for science, it isonly necessary to examine a 19th-century chemistry textbook. Atthat time, the existence of atoms was still doubted by some, elec-trons were not even suspected to exist, and almost nothing wasunderstood of what basic rules governed the way atoms interactedwith each other in chemical reactions. Students had to memorizelong lists of chemicals and their reactions, and there was no way tounderstand any of it systematically. Today, the student only needsto remember a small set of rules about how atoms interact, for in-stance that atoms of one element cannot be converted into anothervia chemical reactions, or that atoms from the right side of the pe-riodic table tend to form strong bonds with atoms from the leftside.

Discussion questions

A Ive suggested replacing the ordinary dictionary definition of lightwith a more technical, more precise one that involves weightlessness. Itsstill possible, though, that the stuff a lightbulb makes, ordinarily called

light, does have some small amount of weight. Suggest an experimentto attempt to measure whether it does.

B Heat is weightless (i.e., an object becomes no heavier when heated),

and can travel across an empty room from the fireplace to your skin,where it influences you by heating you. Should heat therefore be con-sidered a form of light by our definition? Why or why not?

C Similarly, should sound be considered a form of light?

0.3 How to learn physics

For as knowledges are now delivered, there is a kind of con-

tract of error between the deliverer and the receiver; for he

that delivereth knowledge desireth to deliver it in such a form

as may be best believed, and not as may be best examined;

and he that receiveth knowledge desireth rather present sat-

isfaction than expectant inquiry.

Francis Bacon

Many students approach a science course with the idea that theycan succeed by memorizing the formulas, so that when a problem

Section 0.3 How to learn physics


24/50


25/50

certain topic in the book, and it just doesnt make sense. The worstthing you can possibly do in that situation is to keep on staringat the same page. Every textbook explains certain things badly even mine! so the best thing to do in this situation is to lookat a different book. Instead of college textbooks aimed at the samemathematical level as the course youre taking, you may in some

cases find that high school books or books at a lower math levelgive clearer explanations.

Finally, when reviewing for an exam, dont simply read backover the text and your lecture notes. Instead, try to use an activemethod of reviewing, for instance by discussing some of the discus-sion questions with another student, or doing homework problemsyou hadnt done the first time.

0.4 Self-evaluation

The introductory part of a book like this is hard to write, because

every student arrives at this starting point with a different prepara-tion. One student may have grown up outside the U.S. and so maybe completely comfortable with the metric system, but may havehad an algebra course in which the instructor passed too quicklyover scientific notation. Another student may have already takencalculus, but may have never learned the metric system. The fol-lowing self-evaluation is a checklist to help you figure out what youneed to study to be prepared for the rest of the course.

If you disagree with this state-

ment. . .

you should study this section:

I am familiar with the basic metric

units of meters, kilograms, and sec-onds, and the most common metricprefixes: milli- (m), kilo- (k), andcenti- (c).

section 0.5 Basic of the Metric Sys-

tem

I know about the newton, a unit offorce

section 0.6 The newton, the MetricUnit of Force

I am familiar with these less com-mon metric prefixes: mega- (M),micro- (), and nano- (n).

section 0.7 Less Common MetricPrefixes

I am comfortable with scientific no-tation.

section 0.8 Scientific Notation

I can confidently do metric conver-sions.

section 0.9 Conversions

I understand the purpose and use ofsignificant figures.

section 0.10 Significant Figures

It wouldnt hurt you to skim the sections you think you alreadyknow about, and to do the self-checks in those sections.

Section 0.4 Self-evaluation


26/50

0.5 Basics of the metric system

The metric system

Units were not standardized until fairly recently in history, sowhen the physicist Isaac Newton gave the result of an experimentwith a pendulum, he had to specify not just that the string was 377

/8 inches long but that it was 37 7

/8 London inches long. Theinch as defined in Yorkshire would have been different. Even afterthe British Empire standardized its units, it was still very inconve-nient to do calculations involving money, volume, distance, time, orweight, because of all the odd conversion factors, like 16 ounces ina pound, and 5280 feet in a mile. Through the nineteenth century,schoolchildren squandered most of their mathematical education inpreparing to do calculations such as making change when a customerin a shop offered a one-crown note for a book costing two pounds,thirteen shillings and tuppence. The dollar has always been decimal,and British money went decimal decades ago, but the United Statesis still saddled with the antiquated system of feet, inches, pounds,ounces and so on.

Every country in the world besides the U.S. uses a system ofunits known in English as the metric system.2 This system isentirely decimal, thanks to the same eminently logical people whobrought about the French Revolution. In deference to France, thesystems official name is the Systeme International, or SI, meaningInternational System. (The phrase SI system is therefore redun-dant.)

The wonderful thing about the SI is that people who live incountries more modern than ours do not need to memorize how

many ounces there are in a pound, how many cups in a pint, howmany feet in a mile, etc. The whole system works with a single,consistent set of Greek and Latin prefixes that modify the basicunits. Each prefix stands for a power of ten, and has an abbreviationthat can be combined with the symbol for the unit. For instance,the meter is a unit of distance. The prefix kilo- stands for 103, so akilometer, 1 km, is a thousand meters.

The basic units of the metric system are the meter for distance,the second for time, and the gram for mass.

The following are the most common metric prefixes. You should

memorize them.prefix meaning example

kilo- k 103 60 kg = a persons masscenti- c 102 28 cm = height of a piece of papermilli- m 103 1 ms = time for one vibration of a guitar

string playing the note D

2Liberia and Myanmar have not legally adopted metric units, but use themin everyday life.



27/50

e /The original definition the meter.

The prefix centi-, meaning 102, is only used in the centimeter;a hundredth of a gram would not be written as 1 cg but as 10 mg.The centi- prefix can be easily remembered because a cent is 102

dollars. The official SI abbreviation for seconds is s (not sec)and grams are g (not gm).

The second

When I stated briefly above that the second was a unit of time, itmay not have occurred to you that this was not much of a definition.We can make a dictionary-style definition of a term like time, orgive a general description like Isaac Newtons: Absolute, true, andmathematical time, of itself, and from its own nature, flows equablywithout relation to anything external. . . Newtons characterizationsounds impressive, but physicists today would consider it useless asa definition of time. Today, the physical sciences are based on oper-ational definitions, which means definitions that spell out the actualsteps (operations) required to measure something numerically.

In an era when our toasters, pens, and coffee pots tell us thetime, it is far from obvious to most people what is the fundamentaloperational definition of time. Until recently, the hour, minute, andsecond were defined operationally in terms of the time required forthe earth to rotate about its axis. Unfortunately, the Earths ro-tation is slowing down slightly, and by 1967 this was becoming anissue in scientific experiments requiring precise time measurements.The second was therefore redefined as the time required for a cer-tain number of vibrations of the light waves emitted by a cesiumatoms in a lamp constructed like a familiar neon sign but with theneon replaced by cesium. The new definition not only promises to

stay constant indefinitely, but for scientists is a more convenientway of calibrating a clock than having to carry out astronomicalmeasurements.

self-check C

What is a possible operational definition of how strong a person is?

Answer, p. 542

The meter

The French originally defined the meter as 107 times the dis-tance from the equator to the north pole, as measured through Paris(of course). Even if the definition was operational, the operation of

traveling to the north pole and laying a surveying chain behind youwas not one that most working scientists wanted to carry out. Fairlysoon, a standard was created in the form of a metal bar with twoscratches on it. This was replaced by an atomic standard in 1960,and finally in 1983 by the current definition, which is that the me-ter is the distance traveled by light in a vacuum over a period of(1/299792458) seconds.

Section 0.5 Basics of the metric system


28/50

f /A duplicate of the Pariskilogram, maintained at the Dan-ish National Metrology Institute.

The kilogram

The third base unit of the SI is the kilogram, a unit of mass.Mass is intended to be a measure of the amount of a substance,but that is not an operational definition. Bathroom scales work bymeasuring our planets gravitational attraction for the object beingweighed, but using that type of scale to define mass operationally

would be undesirable because gravity varies in strength from placeto place on the earth.

Theres a surprising amount of disagreement among physics text-books about how mass should be defined, but heres how its actuallyhandled by the few working physicists who specialize in ultra-high-precision measurements. They maintain a physical object in Paris,which is the standard kilogram, a cylinder made of platinum-iridiumalloy. Duplicates are checked against this mother of all kilogramsby putting the original and the copy on the two opposite pans of abalance. Although this method of comparison depends on gravity,the problems associated with differences in gravity in different geo-

graphical locations are bypassed, because the two objects are beingcompared in the same place. The duplicates can then be removedfrom the Parisian kilogram shrine and transported elsewhere in theworld. It would be desirable to replace this at some point with auniversally accessible atomic standard rather than one based on aspecific artifact, but as of 2010 the technology for automated count-ing of large numbers of atoms has not gotten good enough to makethat work with the desired precision.

Combinations of metric units

Just about anything you want to measure can be measured withsome combination of meters, kilograms, and seconds. Speed can bemeasured in m/s, volume in m3, and density in kg/m3. Part of whatmakes the SI great is this basic simplicity. No more funny units likea cord of wood, a bolt of cloth, or a jigger of whiskey. No moreliquid and dry measure. Just a simple, consistent set of units. TheSI measures put together from meters, kilograms, and seconds makeup the mks system. For example, the mks unit of speed is m/s, notkm/hr.

Checking units

A useful technique for finding mistakes in ones algebra is toanalyze the units associated with the variables.



29/50

Checking units example 1

Jae starts from the formula V = 13 Ahfor the volume of a cone,whereA is the area of its base, and h is its height. He wants to

find an equation that will tell him how tall a conical tent has to be

in order to have a certain volume, given its radius. His algebra

goes like this:

V = 1

3Ah[1]

A= r2[2]

V = 1

3r2h[3]

h= r2

3V[4]

Is his algebra correct? If not, find the mistake.

Line 4 is supposed to be an equation for the height, so the unitsof the expression on the right-hand side had better equal meters.

The pi and the 3 are unitless, so we can ignore them. In terms of

units, line 4 becomes

m = m2

m3 =

1

m .

This is false, so there must be a mistake in the algebra. The units

of lines 1, 2, and 3 check out, so the mistake must be in the step

from line 3 to line 4. In fact the result should have been

h= 3V

r2 .

Now the units check: m = m3/m2.

Discussion question

A Isaac Newton wrote, . . . the natural days are truly unequal, though

they are commonly considered as equal, and used for a measure oftime. . . It may be that there is no such thing as an equable motion, whereby

time may be accurately measured. All motions may be accelerated or re-tarded. . . Newton was right. Even the modern definition of the secondin terms of light emitted by cesium atoms is subject to variation. For in-stance, magnetic fields could cause the cesium atoms to emit light witha slightly different rate of vibration. What makes us think, though, that apendulum clock is more accurate than a sundial, or that a cesium atomis a more accurate timekeeper than a pendulum clock? That is, how canone test experimentally how the accuracies of different time standards

compare?

Section 0.5 Basics of the metric system


30/50

g /This is a mnemonic tohelp you remember the most im-

portant metric prefixes. The wordlittle is to remind you that thelist starts with the prefixes usedfor small quantities and buildsupward. The exponent changesby 3, except that of course that

we do not need a special prefixfor 100, which equals one.

0.6 The Newton, the metric unit of force

A force is a push or a pull, or more generally anything that canchange an objects speed or direction of motion. A force is requiredto start a car moving, to slow down a baseball player sliding in tohome base, or to make an airplane turn. (Forces may fail to changean objects motion if they are canceled by other forces, e.g., the

force of gravity pulling you down right now is being canceled by theforce of the chair pushing up on you.) The metric unit of force isthe Newton, defined as the force which, if applied for one second,will cause a 1-kilogram object starting from rest to reach a speed of1 m/s. Later chapters will discuss the force concept in more detail.In fact, this entire book is about the relationship between force andmotion.

In section 0.5, I gave a gravitational definition of mass, but bydefining a numerical scale of force, we can also turn around and de-fine a scale of mass without reference to gravity. For instance, if aforce of two Newtons is required to accelerate a certain object fromrest to 1 m/s in 1 s, then that object must have a mass of 2 kg.From this point of view, mass characterizes an objects resistanceto a change in its motion, which we call inertia or inertial mass.Although there is no fundamental reason why an objects resistanceto a change in its motion must be related to how strongly gravityaffects it, careful and precise experiments have shown that the in-ertial definition and the gravitational definition of mass are highlyconsistent for a variety of objects. It therefore doesnt really matterfor any practical purpose which definition one adopts.

Discussion question

A Spending a long time in weightlessness is unhealthy. One of themost important negative effects experienced by astronauts is a loss ofmuscle and bone mass. Since an ordinary scale wont work for an astro-naut in orbit, what is a possible way of monitoring this change in mass?(Measuring the astronauts waist or biceps with a measuring tape is notgood enough, because it doesnt tell anything about bone mass, or aboutthe replacement of muscle with fat.)

0.7 Less common metric prefixes

The following are three metric prefixes which, while less commonthan the ones discussed previously, are well worth memorizing.

prefix meaning examplemega- M 106 6.4 Mm = radius of the earthmicro- 106 10 m = size of a white blood cellnano- n 109 0.154 nm = distance between carbon

nuclei in an ethane molecule

Note that the abbreviation for micro is the Greek letter mu, a common mistake is to confuse it with m (milli) or M (mega).



31/50

There are other prefixes even less common, used for extremelylarge and small quantities. For instance, 1 femtometer = 1015 m isa convenient unit of distance in nuclear physics, and 1 gigabyte =109 bytes is used for computers hard disks. The international com-mittee that makes decisions about the SI has recently even addedsome new prefixes that sound like jokes, e.g., 1 yoctogram = 1024 g

is about half the mass of a proton. In the immediate future, how-ever, youre unlikely to see prefixes like yocto- and zepto- usedexcept perhaps in trivia contests at science-fiction conventions orother geekfests.

self-check D

Suppose you could slow down time so that according to your perception,

a beam of light would move across a room at the speed of a slow walk.

If you perceived a nanosecond as if it was a second, how would you

perceive a microsecond? Answer, p. 542

0.8 Scientific notation

Most of the interesting phenomena in our universe are not on thehuman scale. It would take about 1,000,000,000,000,000,000,000bacteria to equal the mass of a human body. When the physicistThomas Young discovered that light was a wave, it was back in thebad old days before scientific notation, and he was obliged to writethat the time required for one vibration of the wave was 1/500 ofa millionth of a millionth of a second. Scientific notation is a lessawkward way to write very large and very small numbers such asthese. Heres a quick review.

Scientific notation means writing a number in terms of a product

of something from 1 to 10 and something else that is a power of ten.For instance,

32 = 3.2 101

320 = 3.2 102

3200 = 3.2 103 . . .

Each number is ten times bigger than the previous one.

Since 101 is ten times smaller than 102 , it makes sense to usethe notation 100 to stand for one, the number that is in turn tentimes smaller than 101 . Continuing on, we can write 101 to stand

for 0.1, the number ten times smaller than 100 . Negative exponentsare used for small numbers:

3.2 = 3.2 100

0.32 = 3.2 101

0.032 = 3.2 102 . . .

Section 0.8 Scientific notation


32/50


33/50

we can consider a fraction like

103 g

1 kg

to be a way of expressing the number one. This may bother you. Forinstance, if you type 1000/1 into your calculator, you will get 1000,

not one. Again, different people have different ways of thinkingabout it, but the justification is that it helps us to do conversions,and it works! Now if we want to convert 0.7 kg to units of grams,we can multiply kg by the number one:

0.7 kg 103 g

1 kg

If youre willing to treat symbols such as kg as if they were vari-ables as used in algebra (which theyre really not), you can thencancel the kg on top with the kg on the bottom, resulting in

0.7 kg103 g

1kg = 700 g .

To convert grams to kilograms, you would simply flip the fractionupside down.

One advantage of this method is that it can easily be applied toa series of conversions. For instance, to convert one year to units ofseconds,

1year365

days

1year 24

hours

1day 60

min

1hour 60 s

1min=

= 3.15 107 s .

Should that exponent be positive, or negative?

A common mistake is to write the conversion fraction incorrectly.For instance the fraction

103 kg

1 g (incorrect)

does not equal one, because 103 kg is the mass of a car, and 1 g isthe mass of a raisin. One correct way of setting up the conversion

factor would be 103 kg

1 g (correct) .

You can usually detect such a mistake if you take the time to checkyour answer and see if it is reasonable.

If common sense doesnt rule out either a positive or a negativeexponent, heres another way to make sure you get it right. Thereare big prefixes and small prefixes:

Section 0.9 Conversions


34/50


35/50

thought. In general, your result should not have more than thenumber of sig figs in the least accurate piece of data you startedwith. The calculation above should have been done as follows:

5 cm (1 sig fig)

+0.04 cm (1 sig fig)=5 cm (rounded off to 1 sig fig)

The fact that the final result only has one significant figure thenalerts you to the fact that the result is not very accurate, and wouldnot be appropriate for use in designing the engine.

Note that the leading zeroes in the number 0.04 do not countas significant figures, because they are only placeholders. On theother hand, a number such as 50 cm is ambiguous the zero couldbe intended as a significant figure, or it might just be there as aplaceholder. The ambiguity involving trailing zeroes can be avoided

by using scientific notation, in which 5 101 cm would imply onesig fig of accuracy, while 5.0 101 cm would imply two sig figs.

self-check FThe following quote is taken from an editorial by Norimitsu Onishi in theNew York Times, August 18, 2002.

Consider Nigeria. Everyone agrees it is Africas most populousnation. But what is its population? The United Nations says114 million; the State Department, 120 million. The World Banksays 126.9 million, while the Central Intelligence Agency puts itat 126,635,626.

What should bother you about this? Answer, p. 542

Dealing correctly with significant figures can save you time! Of-ten, students copy down numbers from their calculators with eightsignificant figures of precision, then type them back in for a latercalculation. Thats a waste of time, unless your original data hadthat kind of incredible precision.

The rules about significant figures are only rules of thumb, andare not a substitute for careful thinking. For instance, $20.00 +$0.05 is $20.05. It need not and should not be rounded off to $20.In general, the sig fig rules work best for multiplication and division,and we also apply them when doing a complicated calculation that

involves many types of operations. For simple addition and subtrac-tion, it makes more sense to maintain a fixed number of digits afterthe decimal point.

When in doubt, dont use the sig fig rules at all. Instead, in-tentionally change one piece of your initial data by the maximumamount by which you think it could have been off, and recalculatethe final result. The digits on the end that are completely reshuffledare the ones that are meaningless, and should be omitted.

Section 0.10 Significant figures


36/50

self-check GHow many significant figures are there in each of the following mea-surements?

(1) 9.937 m

(2) 4.0 s

(3) 0.0000000000000037 kg Answer, p. 542



37/50

Summary

Selected vocabularymatter . . . . . . Anything that is affected by gravity.light . . . . . . . . Anything that can travel from one place to an-

other through empty space and can influencematter, but is not affected by gravity.

operational defi-nition . . . . . . .

A definition that states what operationsshould be carried out to measure the thing be-ing defined.

Systeme Interna-tional . . . . . . .

A fancy name for the metric system.

mks system . . . The use of metric units based on the meter,kilogram, and second. Example: meters persecond is the mks unit of speed, not cm/s orkm/hr.

mass . . . . . . . A numerical measure of how difficult it is tochange an objects motion.

significant figures Digits that contribute to the accuracy of ameasurement.

Notationm . . . . . . . . . meter, the metric distance unitkg . . . . . . . . . kilogram, the metric unit of masss . . . . . . . . . . second, the metric unit of timeM- . . . . . . . . . the metric prefix mega-, 106

k- . . . . . . . . . the metric prefix kilo-, 103

m- . . . . . . . . . the metric prefix milli-, 103

- . . . . . . . . . the metric prefix micro-, 106

n- . . . . . . . . . the metric prefix nano-, 109

Summary

Physics is the use of the scientific method to study the behaviorof light and matter. The scientific method requires a cycle of the-ory and experiment, theories with both predictive and explanatoryvalue, and reproducible experiments.

The metric system is a simple, consistent framework for measure-ment built out of the meter, the kilogram, and the second plus a setof prefixes denoting powers of ten. The most systematic method fordoing conversions is shown in the following example:

370 ms 103 s

1 ms = 0.37 s

Mass is a measure of the amount of a substance. Mass can bedefined gravitationally, by comparing an object to a standard masson a double-pan balance, or in terms of inertia, by comparing theeffect of a force on an object to the effect of the same force on astandard mass. The two definitions are found experimentally tobe proportional to each other to a high degree of precision, so we

Summary


38/50

usually refer simply to mass, without bothering to specify whichtype.

A force is that which can change the motion of an object. Themetric unit of force is the Newton, defined as the force required toaccelerate a standard 1-kg mass from rest to a speed of 1 m/s in 1s.

Scientific notation means, for example, writing 3.2 105 ratherthan 320000.

Writing numbers with the correct number of significant figurescorrectly communicates how accurate they are. As a rule of thumb,the final result of a calculation is no more accurate than, and shouldhave no more significant figures than, the least accurate piece ofdata.



39/50

Problems

KeyA computerized answer check is available online.

A problem that requires calculus. A difficult problem.

1 Correct use of a calculator: (a) Calculate 74658

53222+97554 on a cal-culator. [Self-check: The most common mistake results in 97555.40.]

(b) Which would be more like the price of a TV, and which wouldbe more like the price of a house, $3.5 105 or $3.55?2 Compute the following things. If they dont make sense be-cause of units, say so.(a) 3 cm + 5 cm(b) 1.11 m + 22 cm(c) 120 miles + 2.0 hours(d) 120 miles / 2.0 hours

3 Your backyard has brick walls on both ends. You measure adistance of 23.4 m from the inside of one wall to the inside of theother. Each wall is 29.4 cm thick. How far is it from the outsideof one wall to the outside of the other? Pay attention to significantfigures.

4 The speed of light is 3.0 108 m/s. Convert this to furlongsper fortnight. A furlong is 220 yards, and a fortnight is 14 days. Aninch is 2.54 cm.

5 Express each of the following quantities in micrograms:(a) 10 mg, (b) 104 g, (c) 10 kg, (d) 100 103 g, (e) 1000 ng.

6 Convert 134 mg to units of kg, writing your answer in scientificnotation. Solution, p. 528

7 In the last century, the average age of the onset of puberty forgirls has decreased by several years. Urban folklore has it that thisis because of hormones fed to beef cattle, but it is more likely to bebecause modern girls have more body fat on the average and pos-sibly because of estrogen-mimicking chemicals in the environmentfrom the breakdown of pesticides. A hamburger from a hormone-implanted steer has about 0.2 ng of estrogen (about double theamount of natural beef). A serving of peas contains about 300ng of estrogen. An adult woman produces about 0.5 mg of estrogenper day (note the different unit!). (a) How many hamburgers woulda girl have to eat in one day to consume as much estrogen as anadult womans daily production? (b) How many servings of peas?

Problems


40/50

Problem 10.

Problem 12.

8 The usual definition of the mean (average) of two numbersaandbis (a+b)/2. This is called the arithmetic mean. The geometricmean, however, is defined as (ab)1/2 (i.e., the square root ofab). Forthe sake of definiteness, lets say both numbers have units of mass.(a) Compute the arithmetic mean of two numbers that have units

of grams. Then convert the numbers to units of kilograms andrecompute their mean. Is the answer consistent? (b) Do the samefor the geometric mean. (c) If a and b both have units of grams,what should we call the units ofab? Does your answer make sensewhen you take the square root? (d) Suppose someone proposes toyou a third kind of mean, called the superduper mean, defined as(ab)1/3. Is this reasonable? Solution, p. 528

9 In an article on the SARS epidemic, the May 7, 2003 NewYork Times discusses conflicting estimates of the diseases incuba-tion period (the average time that elapses from infection to the firstsymptoms). The study estimated it to be 6.4 days. But other sta-

tistical calculations ... showed that the incubation period could beas long as 14.22 days. Whats wrong here?

10 The photo shows the corner of a bag of pretzels. Whatswrong here?

11 The distance to the horizon is given by the expression

2rh,where r is the radius of the Earth, and h is the observers heightabove the Earths surface. (This can be proved using the Pythagoreantheorem.) Show that the units of this expression make sense. (Seeexample 1 on p. 29 for an example of how to do this.) Dont try toprove the result, just check its units.

12 (a) Based on the definitions of the sine, cosine, and tangent,what units must they have? (b) A cute formula from trigonometrylets you find any angle of a triangle if you know the lengths ofits sides. Using the notation shown in the figure, and letting s =(a+b+c)/2 be half the perimeter, we have

tanA/2 =

(s b)(s c)

s(s a) .

Show that the units of this equation make sense. In other words,check that the units of the right-hand side are the same as your

answer to part a of the question. Solution, p. 528



41/50

Exercise 0: Models and idealization

Equipment:

coffee filters

ramps (one per group)

balls of various sizes

sticky tape

vacuum pump and guinea and feather apparatus (one)

The motion of falling objects has been recognized since ancient times as an important piece ofphysics, but the motion is inconveniently fast, so in our everyday experience it can be hard totell exactly what objects are doing when they fall. In this exercise you will use several techniquesto get around this problem and study the motion. Your goal is to construct a scientificmodeloffalling. A model means an explanation that makes testable predictions. Often models containsimplifications or idealizations that make them easier to work with, even though they are notstrictly realistic.

1. One method of making falling easier to observe is to use objects like feathers that we knowfrom everyday experience will not fall as fast. You will use coffee filters, in stacks of varioussizes, to test the following two hypotheses and see which one is true, or whether neither is true:

Hypothesis 1A: When an object is dropped, it rapidly speeds up to a certain natural fallingspeed, and then continues to fall at that speed. The falling speed is proportionalto the objectsweight. (A proportionality is not just a statement that if one thing gets bigger, the other doestoo. It says that if one becomes three times bigger, the other also gets three times bigger, etc.)

Hypothesis 1B: Different objects fall the same way, regardless of weight.

Test these hypotheses and discuss your results with your instructor.

2. A second way to slow down the action is to let a ball roll down a ramp. The steeper theramp, the closer to free fall. Based on your experience in part 1, write a hypothesis about whatwill happen when you race a heavier ball against a lighter ball down the same ramp, startingthem both from rest.

Hypothesis:

Show your hypothesis to your instructor, and then test it.

You have probably found that falling was more complicated than you thought! Is there morethan one factor that affects the motion of a falling ob ject? Can you imagine certain idealizedsituations that are simpler? Try to agree verbally with your group on an informal model offalling that can make predictions about the experiments described in parts 3 and 4.

3. You have three balls: a standard comparison ball of medium weight, a light ball, and aheavy ball. Suppose you stand on a chair and (a) drop the light ball side by side with thecomparison ball, then (b) drop the heavy ball side by side with the comparison ball, then (c)join the light and heavy balls together with sticky tape and drop them side by side with thecomparison ball.

Use your model to make a prediction:

Test your prediction.

Exercise 0: Models and idealization


42/50


43/50

a /Amoebas this size aseldom encountered.

Life would be very different if y

were the size of an insect.

Chapter 1

Scaling and estimation

1.1 Introduction

Why cant an insect be the size of a dog? Some skinny stretched-

out cells in your spinal cord are a meter tall why does naturedisplay no single cells that are not just a meter tall, but a meterwide, and a meter thick as well? Believe it or not, these are questionsthat can be answered fairly easily without knowing much more aboutphysics than you already do. The only mathematical technique youreally need is the humble conversion, applied to area and volume.

Area and volume

Area can be defined by saying that we can copy the shape ofinterest onto graph paper with 1 cm 1 cm squares and count thenumber of squares inside. Fractions of squares can be estimated by

eye. We then say the area equals the number of squares, in units ofsquare cm. Although this might seem less pure than computingareas using formulae like A = r2 for a circle or A = wh/2 for atriangle, those formulae are not useful as definitions of area becausethey cannot be applied to irregularly shaped areas.

Units of square cm are more commonly written as cm2 in science.Of course, the unit of measurement symbolized by cm is not an


44/50

algebra symbol standing for a number that can be literally multipliedby itself. But it is advantageous to write the units of area that wayand treat the units as if they were algebra symbols. For instance,if you have a rectangle with an area of 6m2 and a width of 2 m,then calculating its length as (6 m2)/(2 m) = 3 m gives a resultthat makes sense both numerically and in terms of units. This

algebra-style treatment of the units also ensures that our methodsof converting units work out correctly. For instance, if we acceptthe fraction

100 cm

1 m

as a valid way of writing the number one, then one times one equalsone, so we should also say that one can be represented by

100 cm

1 m 100 cm

1 m ,

which is the same as10000 cm2

1 m2 .

That means the conversion factor from square meters to square cen-timeters is a factor of 104, i.e., a square meter has 104 square cen-timeters in it.

All of the above can be easily applied to volume as well, usingone-cubic-centimeter blocks instead of squares on graph paper.

To many people, it seems hard to believe that a square meterequals 10000 square centimeters, or that a cubic meter equals amillion cubic centimeters they think it would make more sense ifthere were 100 cm2 in 1 m2, and 100 cm3 in 1 m3, but that would be

incorrect. The examples shown in figure b aim to make the correctanswer more believable, using the traditional U.S. units of feet andyards. (One foot is 12 inches, and one yard is three feet.)

b /Visualizing conversions ofarea and volume using traditionalU.S. units.

self-check A

Based on figure b, convince yourself that there are 9 ft2 in a square yard,

and 27 ft3 in a cubic yard, then demonstrate the same thing symbolically

(i.e., with the method using fractions that equal one). Answer, p.

542

44 Chapter 1 Scaling and estimation


45/50

Solved problem: convertingmm2 tocm2 page 61, problem 10

Solved problem: scaling a liter page 62, problem 19

Discussion question

A How many square centimeters are there in a square inch? (1 inch =

2.54 cm) First find an approximate answer by making a drawing, then de-

rive the conversion factor more accurately using the symbolic method.

c /Galileo Galilei (1564-1642) was a Renaissance Italian who brought thescientific method to bear on physics, creating the modern version of thescience. Coming from a noble but very poor family, Galileo had to dropout of medical school at the University of Pisa when he ran out of money.

Eventually becoming a lecturer in mathematics at the same school, hebegan a career as a notorious troublemaker by writing a burlesque ridi-culing the universitys regulations he was forced to resign, but found anew teaching position at Padua. He invented the pendulum clock, inves-tigated the motion of falling bodies, and discovered the moons of Jupiter.

The thrust of his lifes work was to discredit Aristotles physics by con-fronting it with contradictory experiments, a program that paved the wayfor Newtons discovery of the relationship between force and motion. Inchapter 3 well come to the story of Galileos ultimate fate at the hands of

the Church.

1.2 Scaling of area and volume

Great fleas have lesser fleas

Upon their backs to bite em.

And lesser fleas have lesser still,

And so ad infinitum.

Jonathan Swift

Now how do these conversions of area and volume relate to thequestions I posed about sizes of living things? Well, imagine thatyou are shrunk like Alice in Wonderland to the size of an insect.One way of thinking about the change of scale is that what usedto look like a centimeter now looks like perhaps a meter to you,because youre so much smaller. If area and volume scaled accordingto most peoples intuitive, incorrect expectations, with 1 m2 beingthe same as 100 cm2, then there would be no particular reasonwhy nature should behave any differently on your new, reducedscale. But nature does behave differently now that youre small.For instance, you will find that you can walk on water, and jumpto many times your own height. The physicist Galileo Galilei hadthe basic insight that the scaling of area and volume determineshow natural phenomena behave differently on different scales. Hefirst reasoned about mechanical structures, but later extended hisinsights to living things, taking the then-radical point of view that atthe fundamental level, a living organism should follow the same laws

Section 1.2 Scaling of area and volume


46/50

d /The small boat holds upjust fine.

e /A larger boat built with

the same proportions as thesmall one will collapse under itsown weight.

f /A boat this large needs tohave timbers that are thicker

compared to its size.

of nature as a machine. We will follow his lead by first discussingmachines and then living things.

Galileo on the behavior of nature on large and small scales

One of the worlds most famous pieces of scientific writing isGalileos Dialogues Concerning the Two New Sciences. Galileo was

an entertaining writer who wanted to explain things clearly to laypeo-ple, and he livened up his work by casting it in the form of a dialogueamong three people. Salviati is really Galileos alter ego. Simpliciois the stupid character, and one of the reasons Galileo got in troublewith the Church was that there were rumors that Simplicio repre-sented the Pope. Sagredo is the earnest and intelligent student, withwhom the reader is supposed to identify. (The following excerptsare from the 1914 translation by Crew and de Salvio.)

SAGREDO: Yes, that is what I mean; and I refer especially to

his last assertion which I have always regarded as false. . . ;

namely, that in speaking of these and other similar machines

one cannot argue from the small to the large, because manydevices which succeed on a small scale do not work on

Www.lightandmatter.com Lma.pdf

Documents

Transcript of Www.lightandmatter.com Lma.pdf