simple nature - light and matter · 2020. 5. 21. · simple nature an introduction to physics for...

Simple NatureAn Introduction to Physics for Engineeringand Physical Science StudentsBenjamin Crowellwww.lightandmatter.com

Fullerton, Californiawww.lightandmatter.com

Copyright c©2001-2008 Benjamin Crowell

rev. May 16, 2020

Permission is granted to copy, distribute and/or modify this docu-ment under the terms of the Creative Commons Attribution Share-Alike License, which can be found at creativecommons.org. Thelicense applies to the entire text of this book, plus all the illustra-tions that are by Benjamin Crowell. (At your option, you may alsocopy this book under the GNU Free Documentation License ver-sion 1.2, with no invariant sections, no front-cover texts, and noback-cover texts.) All the illustrations are by Benjamin Crowell ex-cept as noted in the photo credits or in parentheses in the captionof the figure. This book can be downloaded free of charge fromwww.lightandmatter.com in a variety of formats, including editableformats.

Brief Contents

0 Introduction and Review 131 Conservation of Mass 552 Conservation of Energy 733 Conservation of Momentum 1314 Conservation of Angular Momentum 2515 Thermodynamics 3076 Waves 3537 Relativity 3978 Atoms and Electromagnetism 4739 Circuits 531

10 Fields 57911 Electromagnetism 67512 Optics 76513 Quantum Physics 85714 Additional Topics in Quantum Physics 959

5

Contents

0 Introduction and Review0.1 Introduction and review. . . . . . . . . . . . . . . 13

The scientific method, 13.—What is physics?, 16.—How to learnphysics, 19.—Velocity and acceleration, 21.—Self-evaluation, 23.—Basics of the metric system, 24.—Less common metric prefixes,27.—Scientific notation, 27.—Conversions, 28.—Significant figures,30.—A note about diagrams, 32.

0.2 Scaling and order-of-magnitude estimates. . . . . . . 34Introduction, 34.—Scaling of area and volume, 35.—Order-of-magnitudeestimates, 43.

Problems . . . . . . . . . . . . . . . . . . . . . . 47

1 Conservation of Mass1.1 Mass . . . . . . . . . . . . . . . . . . . . . . 55

Problem-solving techniques, 58.—Delta notation, 59.

1.2 Equivalence of gravitational and inertial mass . . . . . 601.3 Galilean relativity . . . . . . . . . . . . . . . . . 62

Applications of calculus, 67.

1.4 A preview of some modern physics. . . . . . . . . . 68Problems . . . . . . . . . . . . . . . . . . . . . . 70

2 Conservation of Energy2.1 Energy . . . . . . . . . . . . . . . . . . . . . 73

The energy concept, 73.—Logical issues, 75.—Kinetic energy, 76.—Power, 80.—Gravitational energy, 81.—Equilibrium and stability,86.—Predicting the direction of motion, 89.

2.2 Numerical techniques . . . . . . . . . . . . . . . 912.3 Gravitational phenomena . . . . . . . . . . . . . . 96

Kepler’s laws, 96.—Circular orbits, 98.—The sun’s gravitationalfield, 99.—Gravitational energy in general, 99.—The shell theorem,102.—?Evidence for repulsive gravity, 108.

2.4 Atomic phenomena . . . . . . . . . . . . . . . . 109Heat is kinetic energy., 110.—All energy comes from particles mov-ing or interacting., 111.—Applications, 113.

2.5 Oscillations . . . . . . . . . . . . . . . . . . . 115Problems . . . . . . . . . . . . . . . . . . . . . . 120Exercises . . . . . . . . . . . . . . . . . . . . . . 128

3 Conservation of Momentum3.1 Momentum in one dimension . . . . . . . . . . . . 132

Mechanical momentum, 132.—Nonmechanical momentum, 135.—

Momentum compared to kinetic energy, 136.—Collisions in onedimension, 138.—The center of mass, 142.—The center of massframe of reference, 147.—Totally inelastic collisions, 148.

3.2 Force in one dimension . . . . . . . . . . . . . . 149Momentum transfer, 149.—Newton’s laws, 150.—What force isnot, 153.—Forces between solids, 155.—Fluid friction, 159.—Analysisof forces, 160.—Transmission of forces by low-mass objects, 162.—Work, 164.—Simple Machines, 171.—Force related to interactionenergy, 172.

3.3 Resonance. . . . . . . . . . . . . . . . . . . . 175Damped, free motion, 176.—The quality factor, 179.—Driven mo-tion, 180.

3.4 Motion in three dimensions . . . . . . . . . . . . . 191The Cartesian perspective, 191.—Rotational invariance, 195.—Vectors,197.—Calculus with vectors, 212.—The dot product, 216.—Gradientsand line integrals (optional), 219.

Problems . . . . . . . . . . . . . . . . . . . . . . 222Exercises . . . . . . . . . . . . . . . . . . . . . . 244

4 Conservation of Angular Momentum4.1 Angular momentum in two dimensions . . . . . . . . 251

Angular momentum, 251.—Application to planetary motion, 256.—Two theorems about angular momentum, 257.—Torque, 260.—Applications to statics, 264.—Proof of Kepler’s elliptical orbit law,268.

4.2 Rigid-body rotation . . . . . . . . . . . . . . . . 271Kinematics, 271.—Relations between angular quantities and mo-tion of a point, 272.—Dynamics, 274.—Iterated integrals, 276.—Finding moments of inertia by integration, 279.

4.3 Angular momentum in three dimensions . . . . . . . 284Rigid-body kinematics in three dimensions, 284.—Angular mo-mentum in three dimensions, 286.—Rigid-body dynamics in threedimensions, 291.


5 Thermodynamics5.1 Pressure, temperature, and heat . . . . . . . . . . . 308

Pressure, 308.—Temperature, 312.—Heat, 315.

5.2 Microscopic description of an ideal gas . . . . . . . . 316Evidence for the kinetic theory, 316.—Pressure, volume, and tem-perature, 317.

5.3 Entropy as a macroscopic quantity . . . . . . . . . . 320Efficiency and grades of energy, 320.—Heat engines, 321.—Entropy,322.

5.4 Entropy as a microscopic quantity . . . . . . . . . . 326A microscopic view of entropy, 326.—Phase space, 328.—Microscopicdefinitions of entropy and temperature, 329.—Equipartition, 333.—The arrow of time, or “this way to the Big Bang”, 337.—Quantum

8 Contents

mechanics and zero entropy, 339.—Summary of the laws of ther-modynamics, 339.

5.5 More about heat engines . . . . . . . . . . . . . . 340Problems . . . . . . . . . . . . . . . . . . . . . . 347

6 Waves6.1 Free waves. . . . . . . . . . . . . . . . . . . . 354

Wave motion, 354.—Waves on a string, 360.—Sound and lightwaves, 363.—Periodic waves, 365.—The Doppler effect, 368.

6.2 Bounded waves . . . . . . . . . . . . . . . . . . 374Reflection, transmission, and absorption, 374.—Quantitative treat-ment of reflection, 379.—Interference effects, 382.—Waves boundedon both sides, 384.—?Some technical aspects of reflection, 389.

Problems . . . . . . . . . . . . . . . . . . . . . . 392

7 Relativity7.1 Time is not absolute . . . . . . . . . . . . . . . . 397

The correspondence principle, 397.—Causality, 397.—Time distor-tion arising from motion and gravity, 398.

7.2 Distortion of space and time . . . . . . . . . . . . 400The Lorentz transformation, 400.—The γ factor, 405.—The uni-versal speed c, 411.—No action at a distance, 416.—The light cone,419.—?The spacetime interval, 420.—?Four-vectors and the innerproduct, 425.—?Doppler shifts of light and addition of velocities,426.

7.3 Dynamics . . . . . . . . . . . . . . . . . . . . 429Momentum, 429.—Equivalence of mass and energy, 433.—?Theenergy-momentum four-vector, 437.—?Proofs, 440.

7.4 ?General relativity . . . . . . . . . . . . . . . . . 443Our universe isn’t Euclidean, 443.—The equivalence principle, 446.—Black holes, 450.—Cosmology, 453.


8 Atoms and Electromagnetism8.1 The electric glue . . . . . . . . . . . . . . . . . 473

The quest for the atomic force, 474.—Charge, electricity and mag-netism, 475.—Atoms, 480.—Quantization of charge, 485.—Theelectron, 488.—The raisin cookie model of the atom, 492.

8.2 The nucleus . . . . . . . . . . . . . . . . . . . 494Radioactivity, 494.—The planetary model, 497.—Atomic number,501.—The structure of nuclei, 506.—The strong nuclear force, al-pha decay and fission, 509.—The weak nuclear force; beta decay,512.—Fusion, 514.—Nuclear energy and binding energies, 517.—Biological effects of ionizing radiation, 518.—?The creation of theelements, 523.


Contents 9

9 Circuits9.1 Current and voltage . . . . . . . . . . . . . . . . 532

Current, 532.—Circuits, 535.—Voltage, 536.—Resistance, 541.—Current-conducting properties of materials, 550.

9.2 Parallel and series circuits . . . . . . . . . . . . . 554Schematics, 554.—Parallel resistances and the junction rule, 555.—Series resistances, 559.


10 Fields10.1 Fields of force . . . . . . . . . . . . . . . . . . 579

Why fields?, 579.—The gravitational field, 581.—The electric field,585.

10.2 Potential related to field . . . . . . . . . . . . . . 592One dimension, 592.—Two or three dimensions, 595.

10.3 Fields by superposition . . . . . . . . . . . . . . 597Electric field of a continuous charge distribution, 597.—The fieldnear a charged surface, 603.

10.4 Energy in fields . . . . . . . . . . . . . . . . . 606Electric field energy, 606.—Gravitational field energy, 611.—Magneticfield energy, 611.

10.5 LRC circuits . . . . . . . . . . . . . . . . . . . 613Capacitance and inductance, 613.—Oscillations, 617.—Voltage andcurrent, 619.—Decay, 624.—Review of complex numbers, 627.—Euler’s formula, 629.—Impedance, 630.—Power, 634.—Impedancematching, 637.—Impedances in series and parallel, 639.

10.6 Fields by Gauss’ law . . . . . . . . . . . . . . . 641Gauss’ law, 641.—Additivity of flux, 645.—Zero flux from outsidecharges, 645.—Proof of Gauss’ theorem, 649.—Gauss’ law as afundamental law of physics, 649.—Applications, 650.

10.7 Gauss’ law in differential form . . . . . . . . . . . 653Gauss’s law as a local law, 653.—Poisson’s equation and Laplace’sequation, 657.—The method of images, 657.


11 Electromagnetism11.1 More about the magnetic field . . . . . . . . . . . 675

Magnetic forces, 675.—The magnetic field, 679.—Some applica-tions, 683.—No magnetic monopoles, 685.—Symmetry and hand-edness, 687.

11.2 Magnetic fields by superposition . . . . . . . . . . 689Superposition of straight wires, 689.—Energy in the magnetic field,693.—Superposition of dipoles, 693.—The g factor (optional), 697.—The Biot-Savart law (optional), 698.

11.3 Magnetic fields by Ampère’s law . . . . . . . . . . 702Ampère’s law, 702.—A quick and dirty proof, 704.—Maxwell’sequations for static fields, 705.

10 Contents

11.4 Ampère’s law in differential form (optional) . . . . . . 707The curl operator, 707.—Properties of the curl operator, 708.

11.5 Induced electric fields . . . . . . . . . . . . . . . 713Faraday’s experiment, 713.—Why induction?, 717.—Faraday’s law,719.

11.6 Maxwell’s equations . . . . . . . . . . . . . . . 724Induced magnetic fields, 724.—Light waves, 726.

11.7 Electromagnetic properties of materials . . . . . . . 736Conductors, 736.—Dielectrics, 737.—Magnetic materials, 739.


12 Optics12.1 The ray model of light . . . . . . . . . . . . . . . 765

The nature of light, 766.—Interaction of light with matter, 769.—The ray model of light, 771.—Geometry of specular reflection,774.—?The principle of least time for reflection, 778.

12.2 Images by reflection . . . . . . . . . . . . . . . 780A virtual image, 780.—Curved mirrors, 783.—A real image, 784.—Images of images, 785.

12.3 Images, quantitatively . . . . . . . . . . . . . . . 790A real image formed by a converging mirror, 790.—Other caseswith curved mirrors, 794.—?Aberrations, 798.

12.4 Refraction. . . . . . . . . . . . . . . . . . . . 801Refraction, 802.—Lenses, 808.—?The lensmaker’s equation, 810.—Dispersion, 811.—?The principle of least time for refraction, 811.—?Microscopic description of refraction, 812.

12.5 Wave optics . . . . . . . . . . . . . . . . . . . 814Diffraction, 814.—Scaling of diffraction, 816.—The correspondenceprinciple, 816.—Huygens’ principle, 817.—Double-slit diffraction,818.—Repetition, 822.—Single-slit diffraction, 823.—Coherence, 825.—?The principle of least time, 827.


13 Quantum Physics13.1 Rules of randomness . . . . . . . . . . . . . . . 857

Randomness isn’t random., 858.—Calculating randomness, 859.—Probability distributions, 863.—Exponential decay and half-life,865.—Applications of calculus, 870.

13.2 Light as a particle . . . . . . . . . . . . . . . . 872Evidence for light as a particle, 873.—How much light is one pho-ton?, 875.—Wave-particle duality, 880.—Nonlocality and entan-glement, 883.—Photons in three dimensions, 889.

13.3 Matter as a wave. . . . . . . . . . . . . . . . . 891Electrons as waves, 892.—Dispersive waves, 896.—Bound states,899.—The uncertainty principle, 902.—Electrons in electric fields,904.—The Schrödinger equation, 906.

Contents 11

13.4 The atom . . . . . . . . . . . . . . . . . . . . 920Classifying states, 920.—Three dimensions, 923.—Quantum num-bers, 925.—The hydrogen atom, 927.—Energies of states in hydro-gen, 929.—Electron spin, 936.—Atoms with more than one elec-tron, 939.


14 Additional Topics in Quantum Physics14.1 The Stern-Gerlach experiment . . . . . . . . . . . 95914.2 Rotation and vibration. . . . . . . . . . . . . . . 962

Types of excitations, 962.—Vibration, 962.—Rotation, 963.—Correctionsto semiclassical energies, 964.

14.3 ?A tiny bit of linear algebra. . . . . . . . . . . . . 96714.4 The underlying structure of quantum mechanics, part 1. 969

The time-dependent Schrödinger equation, 969.—Unitarity, 972.

14.5 Methods for solving the Schrödinger equation. . . . . 973Cut-and-paste solutions, 973.—Separability, 976.

14.6 The underlying structure of quantum mechanics, part 2. 977Observables, 977.—The inner product, 984.—Completeness, 989.—The Schrödinger equation in general, 991.—Summary of the struc-ture of quantum mechanics, 993.

14.7 Applications to the two-state system. . . . . . . . . 993A proton in a magnetic field, 993.—The ammonia molecule, 995.

14.8 Energy-time uncertainty . . . . . . . . . . . . . . 998Classical uncertainty relations, 998.—Energy-time uncertainty, 998.

14.9 Randomization of phase. . . . . . . . . . . . . . 1001Randomization of phase in a measurement, 1001.—Decoherence,1002.

14.10 Quantum computing and the no-cloning theorem . . . 100414.11 More about entanglement . . . . . . . . . . . . 1007Problems . . . . . . . . . . . . . . . . . . . . . . 1011Three essential mathematical skills . . . . . . . . . . . 1018Programming with python . . . . . . . . . . . . . . . 1023Appendix 2: Miscellany 1025Appendix 3: Photo Credits 1031Appendix 4: Useful Data 1070

Notation and terminology, compared with other books, 1070.—Notation and units, 1071.—Fundamental constants, 1071.—Metricprefixes, 1072.—Nonmetric units, 1072.—The Greek alphabet, 1072.—Subatomic particles, 1072.—Earth, moon, and sun, 1073.—Theperiodic table, 1073.—Atomic masses, 1073.

Appendix 5: Summary 1074

12 Contents

The Mars Climate Orbiter is pre-pared for its mission. The lawsof physics are the same every-where, even on Mars, so theprobe could be designed basedon the laws of physics as discov-ered on earth. There is unfor-tunately another reason why thisspacecraft is relevant to the top-ics of this chapter: it was de-stroyed attempting to enter Mars’atmosphere because engineersat Lockheed Martin forgot to con-vert data on engine thrusts frompounds into the metric unit offorce (newtons) before giving theinformation to NASA. Conver-sions are important!

Chapter 0

Introduction and Review

0.1 Introduction and reviewIf you drop your shoe and a coin side by side, they hit the ground atthe same time. Why doesn’t the shoe get there first, since gravity ispulling harder on it? How does the lens of your eye work, and whydo your eye’s 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.

0.1.1 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.

13

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

b / A satirical drawing of analchemist’s laboratory. H. Cock,after a drawing by Peter Brueghelthe Elder (16th century).

Building its centerpiece, the steam engine, required improved tech-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 evolvedover 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 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 that’s 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 indifferent 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 and

14 Chapter 0 Introduction and Review

temperatures 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 highpressure and temperature existing in a nuclear bomb or inside a star.That observation didn’t 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 AA psychic conducts seances in which the spirits of the dead speak tothe participants. He says he has special psychic powers not possessedby other people, which allow him to “channel” the communications withthe spirits. What part of the scientific method is being violated here?. Answer, p. 1057

The scientific method as described here is an idealization, andshould 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 otherpeople’s 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-group’scriteria. I think that’s 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 QuestionsConsider whether or not the scientific method is being applied in the fol-lowing examples. If the scientific method is not being applied, are thepeople whose actions are being described performing a useful humanactivity, albeit an unscientific one?

A Acupuncture is a traditional medical technique of Asian origin inwhich small needles are inserted in the patient’s 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 beingmore scientific, the western or eastern practitioners?

Section 0.1 Introduction and review 15

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 tohave 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?

0.1.2 What is physics?

Given for one instant an intelligence which could comprehend allthe forces by which nature is animated and the respective positionsof the things which compose it...nothing would be uncertain, andthe 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 universe’s future if complete information was available aboutthe present state of all light and matter. In subsequent sections,I’ll 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 physicist’s definition of light includes a variety of phenomenathat are not visible to the eye, including radio waves, microwaves,x-rays, and gamma rays. These are the “colors” of light that do not


c / This telescope picture showstwo images of the same distantobject, an exotic, very luminousobject called a quasar. This isinterpreted as evidence that amassive, dark object, possiblya black hole, happens to bebetween us and it. Light rays thatwould otherwise have missed theearth on either side have beenbent by the dark object’s gravityso that they reach us. The actualdirection to the quasar is presum-ably in the center of the image,but the light along that central linedoesn’t get to us because it isabsorbed by the dark object. Thequasar is known by its catalognumber, MG1131+0456, or moreinformally as Einstein’s Ring.

happen to fall within the narrow violet-to-red range of the rainbowthat we can see.

self-check BAt 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 yourTV’s picture tube and hit the front to make the picture. Physicists in1895 didn’t have the faintest idea what the rays were, so they simplynamed them “cathode rays,” after the name for the electrical contactfrom 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. 1057

Many physical phenomena are not themselves light or matter,but are properties of light or matter or interactions between lightand 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 andMatterEinstein 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. Einstein’s 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.)

Einstein’s interpretation was that light doesn’t really have mass, butthat energy is affected by gravity just like mass is. The energy in a lightbeam is equivalent to a certain amount of mass, given by the famousequation E = mc2, where c is the speed of light. Because the speed


d / Reductionism.

of 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 Pauliexclusion 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.

Einstein’s 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 scientistswho 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 physicalprinciples.

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 Darwin’s study of the Galápagos Islands,which were conveniently isolated from the rest of the world. Anypart 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 apartinto 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 isthat 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 I’ve suggested replacing the ordinary dictionary definition of lightwith a more technical, more precise one that involves weightlessness. It’sstill 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?

0.1.3 How to learn physics

For as knowledges are now delivered, there is a kind of contract oferror between the deliverer and the receiver; for he that deliverethknowledge desireth to deliver it in such a form as may be best be-lieved, and not as may be best examined; and he that receivethknowledge desireth rather present satisfaction than expectant in-quiry.

Francis Bacon

Many students approach a science course with the idea that theycan succeed by memorizing the formulas, so that when a problemis assigned on the homework or an exam, they will be able to plugnumbers in to the formula and get a numerical result on their cal-culator. Wrong! That’s not what learning science is about! Thereis a big difference between memorizing formulas and understandingconcepts. To start with, different formulas may apply in differentsituations. One equation might represent a definition, which is al-ways true. Another might be a very specific equation for the speed


of an object sliding down an inclined plane, which would not be trueif the object was a rock drifting down to the bottom of the ocean.If you don’t work to understand physics on a conceptual level, youwon’t know which formulas can be used when.

Most students taking college science courses for the first timealso have very little experience with interpreting the meaning of anequation. Consider the equation w = A/h relating the width of arectangle to its height and area. A student who has not developedskill at interpretation might view this as yet another equation tomemorize and plug in to when needed. A slightly more savvy stu-dent might realize that it is simply the familiar formula A = whin a different form. When asked whether a rectangle would havea greater or smaller width than another with the same area buta smaller height, the unsophisticated student might be at a loss,not having any numbers to plug in on a calculator. The more ex-perienced student would know how to reason about an equationinvolving division — if h is smaller, and A stays the same, then wmust be bigger. Often, students fail to recognize a sequence of equa-tions as a derivation leading to a final result, so they think all theintermediate steps are equally important formulas that they shouldmemorize.

When learning any subject at all, it is important to become asactively involved as possible, rather than trying to read throughall the information quickly without thinking about it. It is a goodidea to read and think about the questions posed at the end of eachsection of these notes as you encounter them, so that you know youhave understood what you were reading.

Many students’ difficulties in physics boil down mainly to diffi-culties with math. Suppose you feel confident that you have enoughmathematical preparation to succeed in this course, but you arehaving trouble with a few specific things. In some areas, the briefreview given in this chapter may be sufficient, but in other areasit probably will not. Once you identify the areas of math in whichyou are having problems, get help in those areas. Don’t limp alongthrough the whole course with a vague feeling of dread about some-thing like scientific notation. The problem will not go away if youignore it. The same applies to essential mathematical skills that youare learning in this course for the first time, such as vector addition.

Sometimes students tell me they keep trying to understand acertain topic in the book, and it just doesn’t 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 you’re taking, you may in somecases find that high school books or books at a lower math level


give clearer explanations.

Finally, when reviewing for an exam, don’t 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 hadn’t done the first time.

0.1.4 Velocity and acceleration

Calculus was invented by a physicist, Isaac Newton, becausehe needed it as a tool for calculating velocity and acceleration; inyour introductory calculus course, velocity and acceleration wereprobably presented as some of the first applications.

If an object’s position as a function of time is given by the func-tion x(t), then its velocity and acceleration are given by the firstand second derivatives with respect to time,

v =dx

dt

and

a =d2 x

dt2.

The notation relates in a logical way to the units of the quantities.Velocity has units of m/s, and that makes sense because dx is inter-preted as an infinitesimally small distance, with units of meters, anddt as an infinitesimally small time, with units of seconds. The seem-ingly weird and inconsistent placement of the superscripted twos inthe notation for the acceleration is likewise meant to suggest theunits: something on top with units of meters, and something on thebottom with units of seconds squared.

Velocity and acceleration have completely different physical in-terpretations. Velocity is a matter of opinion. Right now as you sitin a chair and read this book, you could say that your velocity waszero, but an observer watching the Earth rotate would say that youhad a velocity of hundreds of miles an hour. Acceleration representsa change in velocity, and it’s not a matter of opinion. Accelerationsproduce physical effects, and don’t occur unless there’s a force tocause them. For example, gravitational forces on Earth cause fallingobjects to have an acceleration of 9.8 m/s2.

Constant acceleration example 1. How high does a diving board have to be above the water if thediver is to have as much as 1.0 s in the air?

. The diver starts at rest, and has an acceleration of 9.8 m/s2.We need to find a connection between the distance she travelsand time it takes. In other words, we’re looking for information


about the function x(t), given information about the acceleration.To go from acceleration to position, we need to integrate twice:

x =∫ ∫

a dt dt

=∫

(at + vo) dt [vo is a constant of integration.]

=∫

at dt [vo is zero because she’s dropping from rest.]

=12

at2 + xo [xo is a constant of integration.]

=12

at2 [xo can be zero if we define it that way.]

Note some of the good problem-solving habits demonstrated here.We solve the problem symbolically, and only plug in numbers atthe very end, once all the algebra and calculus are done. Oneshould also make a habit, after finding a symbolic result, of check-ing whether the dependence on the variables make sense. Agreater value of t in this expression would lead to a greater valuefor x ; that makes sense, because if you want more time in theair, you’re going to have to jump from higher up. A greater ac-celeration also leads to a greater height; this also makes sense,because the stronger gravity is, the more height you’ll need in or-der to stay in the air for a given amount of time. Now we plug innumbers.

x =12

(9.8 m/s2

)(1.0 s)2

= 4.9 m

Note that when we put in the numbers, we check that the unitswork out correctly,

(m/s2

)(s)2 = m. We should also check that

the result makes sense: 4.9 meters is pretty high, but not unrea-sonable.

The notation dq in calculus represents an infinitesimally smallchange in the variable q. The corresponding notation for a finitechange in a variable is ∆q. For example, if q represents the valueof a certain stock on the stock market, and the value falls fromqo = 5 dollars initially to qf = 3 dollars finally, then ∆q = −2dollars. When we study linear functions, whose slopes are constant,the derivative is synonymous with the slope of the line, and dy/dxis the same thing as ∆y/∆x, the rise over the run.

Under conditions of constant acceleration, we can relate velocityand time,

a =∆v

∆t,

or, as in the example 1, position and time,

x =1

2at2 + vot+ xo.


It can also be handy to have a relation involving velocity and posi-tion, eliminating time. Straightforward algebra gives

v2f = v2o + 2a∆x,

where vf is the final velocity, vo the initial velocity, and ∆x thedistance traveled.

. Solved problem: Dropping a rock on Mars page 49, problem 17

. Solved problem: The Dodge Viper page 50, problem 19

0.1.5 Self-evaluation

The introductory part of a book like this is hard to write, becauseevery 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 takenvector calculus, but may have never learned the metric system. Thefollowing self-evaluation is a checklist to help you figure out whatyou need 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 metricunits of meters, kilograms, and sec-onds, and the most common metricprefixes: milli- (m), kilo- (k), andcenti- (c).

subsection 0.1.6 Basic of the MetricSystem

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

subsection 0.1.7 Less Common Met-ric Prefixes

I am comfortable with scientific no-tation.

subsection 0.1.8 Scientific Notation

I can confidently do metric conver-sions.

subsection 0.1.9 Conversions

I understand the purpose and use ofsignificant figures.

subsection 0.1.10 Significant Figures

It wouldn’t hurt you to skim the sections you think you alreadyknow about, and to do the self-checks in those sections.


0.1.6 Basics of the metric system

The metric system

Every country in the world besides the U.S. uses a system ofunits known in English as the “metric system.1” This system isentirely decimal, thanks to the same eminently logical people whobrought about the French Revolution. In deference to France, thesystem’s official name is the Système International, or SI, meaningInternational System. The system uses a single, consistent set ofGreek and Latin prefixes that modify the basic units. Each prefixstands for a power of ten, and has an abbreviation that can becombined with the symbol for the unit. For instance, the meter isa unit of distance. The prefix kilo- stands for 103, so a kilometer, 1km, is a thousand meters.

The basic units of the SI are the meter for distance, the secondfor time, and the kilogram (not the gram) for mass.

The following are the most common metric prefixes. You shouldmemorize them.

prefix meaning examplekilo- k 103 60 kg = a person’s masscenti- c 10−2 28 cm = height of a piece of papermilli- m 10−3 1 ms = time for one vibration of a guitar

string playing the note D

The prefix centi-, meaning 10−2, 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 10−2

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 Newton’s: “Absolute, true, andmathematical time, of itself, and from its own nature, flows equablywithout relation to anything external. . . ” Newton’s 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 for

1Liberia and Myanmar have not legally adopted metric units, but use themin everyday life.


e / The original definition ofthe meter.

f / A duplicate of the Pariskilogram, maintained at the Dan-ish National Metrology Institute.As of 2019, the kilogram is nolonger defined in terms of aphysical standard.

the earth to rotate about its axis. Unfortunately, the Earth’s 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 tostay constant indefinitely, but for scientists is a more convenientway of calibrating a clock than having to carry out astronomicalmeasurements.

self-check CWhat is a possible operational definition of how strong a person is? .Answer, p. 1057

The meter

The French originally defined the meter as 10−7 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 oftraveling 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 speedof light has a defined value in units of m/s.

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 planet’s gravitational attraction for the object beingweighed, but using that type of scale to define mass operationallywould be undesirable because gravity varies in strength from placeto place on the earth. The kilogram was for a long time definedby a physical artifact (figure f), but in 2019 it was redefined bygiving a defined value to Planck’s constant (p. 877), which plays afundamental role in the description of the atomic world.

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 one’s algebra is toanalyze the units associated with the variables.

Checking units example 2. Jae starts from the formula V = 13Ah for the volume of a cone,where A is the area of its base, and h is its height. He wants tofind an equation that will tell him how tall a conical tent has to bein order to have a certain volume, given its radius. His algebragoes like this:

V =13

Ah[1]

A = πr2[2]

V =13πr2h[3]

h =πr2

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 ofunits, line 4 becomes

m =m2

m3=

1m

.

This is false, so there must be a mistake in the algebra. The unitsof lines 1, 2, and 3 check out, so the mistake must be in the stepfrom line 3 to line 4. In fact the result should have been

h =3Vπr2

.

Now the units check: m = m3/m2.

Discussion Question

A Isaac Newton wrote, “. . . the natural days are truly unequal, thoughthey are commonly considered as equal, and used for a measure oftime. . . It may be that there is no such thing as an equable motion, wherebytime 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 standardscompare?


g / This is a mnemonic tohelp you remember the most im-portant metric prefixes. The word“little” is to remind you that thelist starts with the prefixes usedfor small quantities and buildsupward. The exponent changesby 3, except that of course thatwe do not need a special prefixfor 100, which equals one.

0.1.7 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- µ 10−6 10 µm = size of a white blood cellnano- n 10−9 0.154 nm = distance between carbon

nuclei in an ethane molecule

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

There are other prefixes even less common, used for extremelylarge and small quantities. For instance, 1 femtometer = 10−15 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 = 10−24 gis about half the mass of a proton. In the immediate future, how-ever, you’re unlikely to see prefixes like “yocto-” and “zepto-” usedexcept perhaps in trivia contests at science-fiction conventions orother geekfests.

self-check DSuppose 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 youperceive a microsecond? . Answer, p. 1057

0.1.8 Scientific notation

Most of the interesting phenomena in our universe are not onthe human 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. Here’s a quick review.

Scientific notation means writing a number in terms of a productof 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 use


the notation 100 to stand for one, the number that is in turn tentimes smaller than 101 . Continuing on, we can write 10−1 to standfor 0.1, the number ten times smaller than 100 . Negative exponentsare used for small numbers:

3.2 = 3.2× 100

0.32 = 3.2× 10−1

0.032 = 3.2× 10−2 . . .

A common source of confusion is the notation used on the dis-plays of many calculators. Examples:

3.2× 106 (written notation)3.2E+6 (notation on some calculators)3.26 (notation on some other calculators)

The last example is particularly unfortunate, because 3.26 reallystands for the number 3.2 × 3.2 × 3.2 × 3.2 × 3.2 × 3.2 = 1074, atotally different number from 3.2 × 106 = 3200000. The calculatornotation should never be used in writing. It’s just a way for themanufacturer to save money by making a simpler display.

self-check EA student learns that 104 bacteria, standing in line to register for classesat Paramecium Community College, would form a queue of this size:

The student concludes that 102 bacteria would form a line of this length:

Why is the student incorrect? . Answer, p. 1057

0.1.9 Conversions

Conversions are one of the three essential mathematical skills,summarized on pp.1018-1020, that you need for success in this course.

I suggest you avoid memorizing lots of conversion factors be-tween SI units and U.S. units, but two that do come in handy are:

1 inch = 2.54 cm

An object with a weight on Earth of 2.2 pounds-force has amass of 1 kg.

The first one is the present definition of the inch, so it’s exact. Thesecond one is not exact, but is good enough for most purposes. (U.S.units of force and mass are confusing, so it’s a good thing they’renot used in science. In U.S. units, the unit of force is the pound-


force, and the best unit to use for mass is the slug, which is about14.6 kg.)

More important than memorizing conversion factors is under-standing the right method for doing conversions. Even within theSI, you may need to convert, say, from grams to kilograms. Differ-ent people have different ways of thinking about conversions, butthe method I’ll describe here is systematic and easy to understand.The idea is that if 1 kg and 1000 g represent the same mass, thenwe 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 you’re willing to treat symbols such as “kg” as if they were vari-ables as used in algebra (which they’re really not), you can thencancel the kg on top with the kg on the bottom, resulting in

0.7��kg×103 g

1��kg= 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,

1��year×365��

�days

1��year× 24

��hours

1��day× 60�

��min

1��hour× 60 s

1��min=

= 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 be10−3 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 doesn’t rule out either a positive or a negativeexponent, here’s another way to make sure you get it right. Thereare big prefixes and small prefixes:

big prefixes: k Msmall prefixes: m µ n

(It’s not hard to keep straight which are which, since “mega” and“micro” are evocative, and it’s easy to remember that a kilometeris bigger than a meter and a millimeter is smaller.) In the exampleabove, we want the top of the fraction to be the same as the bottom.Since k is a big prefix, we need to compensate by putting a smallnumber like 10−3 in front of it, not a big number like 103.

. Solved problem: a simple conversion page 47, problem 6

. Solved problem: the geometric mean page 47, problem 8

Discussion Question

A Each of the following conversions contains an error. In each case,explain what the error is.

(a) 1000 kg× 1 kg1000 g = 1 g

(b) 50 m× 1 cm100 m = 0.5 cm

(c) “Nano” is 10−9, so there are 10−9 nm in a meter.

(d) “Micro” is 10−6, so 1 kg is 106 µg.

0.1.10 Significant figures

The international governing body for football (“soccer” in theUS) says the ball should have a circumference of 68 to 70 cm. Tak-ing the middle of this range and dividing by π gives a diameter ofapproximately 21.96338214668155633610595934540698196 cm. Thedigits after the first few are completely meaningless. Since the cir-cumference could have varied by about a centimeter in either direc-tion, the diameter is fuzzy by something like a third of a centimeter.We say that the additional, random digits are not significant figures.If you write down a number with a lot of gratuitous insignificant fig-ures, it shows a lack of scientific literacy and imples to other peoplea greater precision than you really have.

As a rule of thumb, the result of a calculation has as manysignificant figures, or “sig figs,” as the least accurate piece of datathat went in. In the example with the soccer ball, it didn’t do us anygood to know π to dozens of digits, because the bottleneck in theprecision of the result was the figure for the circumference, which


was two sig figs. The result is 22 cm. The rule of thumb works bestfor multiplication and division.

For calculations involving multiplication and division, a givenfractional or “percent” error in one of the inputs causes the samefractional error in the output. The number of digits in a numberprovides a rough measure of its possible fractional error. These arecalled significant figures or “sig figs.” Examples:

3.14 3 sig figs

3.1 2 sig figs

0.03 1 sig fig, because the zeroes are just placeholders

3.0× 101 2 sig figs30 could be 1 or 2 sig figs, since we can’t tell if the

0 is a placeholder or a real sig fig

In such calculations, your result should not have more than thenumber of sig figs in the least accurate piece of data you startedwith.

Sig figs in the area of a triangle example 3. A triangle has an area of 6.45 m2 and a base with a width of4.0138 m. Find its height.

. The area is related to the base and height by A = bh/2.

h =2Ab

= 3.21391200358762 m (calculator output)= 3.21 m

The given data were 3 sig figs and 5 sig figs. We’re limited by theless accurate piece of data, so the final result is 3 sig figs. Theadditional digits on the calculator don’t mean anything, and if wecommunicated them to another person, we would create the falseimpression of having determined h with more precision than wereally obtained.

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 Africa’s 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. 1058

Dealing correctly with significant figures can save you time! Of-


h / A diagram of a tomato.

ten, students copy down numbers from their calculators with eightsignificant figures of precision, then type them back in for a latercalculation. That’s a waste of time, unless your original data hadthat kind of incredible precision.

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. 1058

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 sometimes also apply them when doing a complicated calcu-lation that involves many types of operations. For simple additionand subtraction, it makes more sense to maintain a fixed number ofdigits after the decimal point.

When in doubt, don’t 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.

A nonlinear function example 4. How many sig figs are there in sin 88.7◦?

. We’re using a sine function, which isn’t addition, subtraction,multiplication, or division. It would be reasonable to guess thatsince the input angle had 3 sig figs, so would the output. But ifthis was an important calculation and we really needed to know,we would do the following:

sin 88.7◦ = 0.999742609322698sin 88.8◦ = 0.999780683474846

Surprisingly, the result appears to have as many as 5 sig figs, notjust 3:

sin 88.7◦ = 0.99974,

where the final 4 is uncertain but may have some significance.The unexpectedly high precision of the result is because the sinefunction is nearing its maximum at 90 degrees, where the graphflattens out and becomes insensitive to the input angle.

0.1.11 A note about diagrams

A quick note about diagrams. Often when you solve a problem,the best way to get started and organize your thoughts is by draw-ing a diagram. For an artist, it’s desirable to be able to draw a


recognizable, realistic, perspective picture of a tomato, like the oneat the top of figure h. But in science and engineering, we usuallydon’t draw solid figures in perspective, because that would make itdifficult to label distances and angles. Usually we want views orcross-sections that project the object into its planes of symmetry,as in the line drawings in the figure.


a / Amoebas this size areseldom encountered.

0.2 Scaling and order-of-magnitude estimates0.2.1 Introduction

Why can’t 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 byeye. 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 = πr2 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 analgebra 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. Thisalgebra-style treatment of the units also ensures that our methodsof converting units work out correctly. For instance, if we acceptthe fraction

100 cm

1 mas 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 a


million 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 beincorrect. 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 HBased 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.1058

. Solved problem: converting mm2 to cm2 page 51, problem 31

. Solved problem: scaling a liter page 52, problem 40

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.

0.2.2 Scaling of area and volume

Great fleas have lesser fleasUpon 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 you’re so much smaller. If area and volume scaled accordingto most people’s 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 you’re small.For instance, you will find that you can walk on water, and jump

Section 0.2 Scaling and order-of-magnitude estimates 35

c / Galileo Galilei (1564-1642).

d / The small boat holds upjust fine.

e / A larger boat built withthe same proportions as thesmall one will collapse under itsown weight.

f / A boat this large needs tohave timbers that are thickercompared to its size.

to 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 lawsof 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 world’s most famous pieces of scientific writing isGalileo’s Dialogues Concerning the Two New Sciences. Galileo wasan 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 Galileo’s 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 tohis last assertion which I have always regarded as false. . . ;namely, that in speaking of these and other similar machinesone cannot argue from the small to the large, because manydevices which succeed on a small scale do not work on alarge scale. Now, since mechanics has its foundations in ge-ometry, where mere size [ is unimportant], I do not see thatthe properties of circles, triangles, cylinders, cones and othersolid figures will change with their size. If, therefore, a largemachine be constructed in such a way that its parts bear toone another the same ratio as in a smaller one, and if thesmaller is sufficiently strong for the purpose for which it isdesigned, I do not see why the larger should not be able towithstand any severe and destructive tests to which it may besubjected.

Salviati contradicts Sagredo:

SALVIATI: . . . Please observe, gentlemen, how facts whichat first seem improbable will, even on scant explanation, dropthe cloak which has hidden them and stand forth in naked andsimple beauty. Who does not know that a horse falling from aheight of three or four cubits will break his bones, while a dogfalling from the same height or a cat from a height of eightor ten cubits will suffer no injury? Equally harmless would bethe fall of a grasshopper from a tower or the fall of an ant fromthe distance of the moon.

The point Galileo is making here is that small things are sturdier


in proportion to their size. There are a lot of objections that could beraised, however. After all, what does it really mean for something tobe “strong”, to be “strong in proportion to its size,” or to be strong“out of proportion to its size?” Galileo hasn’t given operationaldefinitions of things like “strength,” i.e., definitions that spell outhow to measure them numerically.

Also, a cat is shaped differently from a horse — an enlargedphotograph of a cat would not be mistaken for a horse, even if thephoto-doctoring experts at the National Inquirer made it look like aperson was riding on its back. A grasshopper is not even a mammal,and it has an exoskeleton instead of an internal skeleton. The wholeargument would be a lot more convincing if we could do some iso-lation of variables, a scientific term that means to change only onething at a time, isolating it from the other variables that might havean effect. If size is the variable whose effect we’re interested in see-ing, then we don’t really want to compare things that are differentin size but also different in other ways.

SALVIATI: . . . we asked the reason why [shipbuilders] em-ployed stocks, scaffolding, and bracing of larger dimensionsfor launching a big vessel than they do for a small one; and[an old man] answered that they did this in order to avoid thedanger of the ship parting under its own heavy weight, a dan-ger to which small boats are not subject?

After this entertaining but not scientifically rigorous beginning,Galileo starts to do something worthwhile by modern standards.He simplifies everything by considering the strength of a woodenplank. The variables involved can then be narrowed down to thetype of wood, the width, the thickness, and the length. He alsogives an operational definition of what it means for the plank tohave a certain strength “in proportion to its size,” by introducingthe concept of a plank that is the longest one that would not snapunder its own weight if supported at one end. If you increasedits length by the slightest amount, without increasing its width orthickness, it would break. He says that if one plank is the sameshape as another but a different size, appearing like a reduced orenlarged photograph of the other, then the planks would be strong“in proportion to their sizes” if both were just barely able to supporttheir own weight.


h / Galileo discusses planksmade of wood, but the conceptmay be easier to imagine withclay. All three clay rods in thefigure were originally the sameshape. The medium-sized onewas twice the height, twice thelength, and twice the width ofthe small one, and similarly thelarge one was twice as big asthe medium one in all its lineardimensions. The big one hasfour times the linear dimensionsof the small one, 16 times thecross-sectional area when cutperpendicular to the page, and64 times the volume. That meansthat the big one has 64 times theweight to support, but only 16times the strength compared tothe smallest one.

g / 1. This plank is as long as it can be without collapsing underits own weight. If it was a hundredth of an inch longer, it would collapse.2. This plank is made out of the same kind of wood. It is twice as thick,twice as long, and twice as wide. It will collapse under its own weight.

Also, Galileo is doing something that would be frowned on inmodern science: he is mixing experiments whose results he has ac-tually observed (building boats of different sizes), with experimentsthat he could not possibly have done (dropping an ant from theheight of the moon). He now relates how he has done actual ex-periments with such planks, and found that, according to this op-erational definition, they are not strong in proportion to their sizes.The larger one breaks. He makes sure to tell the reader how impor-tant the result is, via Sagredo’s astonished response:

SAGREDO: My brain already reels. My mind, like a cloudmomentarily illuminated by a lightning flash, is for an instantfilled with an unusual light, which now beckons to me andwhich now suddenly mingles and obscures strange, crudeideas. From what you have said it appears to me impossibleto build two similar structures of the same material, but ofdifferent sizes and have them proportionately strong.

In other words, this specific experiment, using things like woodenplanks that have no intrinsic scientific interest, has very wide impli-cations because it points out a general principle, that nature actsdifferently on different scales.

To finish the discussion, Galileo gives an explanation. He saysthat the strength of a plank (defined as, say, the weight of the heav-iest boulder you could put on the end without breaking it) is pro-portional to its cross-sectional area, that is, the surface area of thefresh wood that would be exposed if you sawed through it in themiddle. Its weight, however, is proportional to its volume.2

How do the volume and cross-sectional area of the longer plankcompare with those of the shorter plank? We have already seen,

2Galileo makes a slightly more complicated argument, taking into accountthe effect of leverage (torque). The result I’m referring to comes out the sameregardless of this effect.


i / The area of a shape isproportional to the square of itslinear dimensions, even if theshape is irregular.

while discussing conversions of the units of area and volume, thatthese quantities don’t act the way most people naively expect. Youmight think that the volume and area of the longer plank would bothbe doubled compared to the shorter plank, so they would increasein proportion to each other, and the longer plank would be equallyable to support its weight. You would be wrong, but Galileo knowsthat this is a common misconception, so he has Salviati address thepoint specifically:

SALVIATI: . . . Take, for example, a cube two inches on aside so that each face has an area of four square inchesand the total area, i.e., the sum of the six faces, amountsto twenty-four square inches; now imagine this cube to besawed through three times [with cuts in three perpendicularplanes] so as to divide it into eight smaller cubes, each oneinch on the side, each face one inch square, and the totalsurface of each cube six square inches instead of twenty-four in the case of the larger cube. It is evident therefore,that the surface of the little cube is only one-fourth that ofthe larger, namely, the ratio of six to twenty-four; but the vol-ume of the solid cube itself is only one-eighth; the volume,and hence also the weight, diminishes therefore much morerapidly than the surface. . . You see, therefore, Simplicio, thatI was not mistaken when . . . I said that the surface of a smallsolid is comparatively greater than that of a large one.

The same reasoning applies to the planks. Even though theyare not cubes, the large one could be sawed into eight small ones,each with half the length, half the thickness, and half the width.The small plank, therefore, has more surface area in proportion toits weight, and is therefore able to support its own weight while thelarge one breaks.

Scaling of area and volume for irregularly shaped objects

You probably are not going to believe Galileo’s claim that thishas deep implications for all of nature unless you can be convincedthat the same is true for any shape. Every drawing you’ve seen sofar has been of squares, rectangles, and rectangular solids. Clearlythe reasoning about sawing things up into smaller pieces would notprove anything about, say, an egg, which cannot be cut up into eightsmaller egg-shaped objects with half the length.

Is it always true that something half the size has one quarterthe surface area and one eighth the volume, even if it has an irreg-ular shape? Take the example of a child’s violin. Violins are madefor small children in smaller size to accomodate their small bodies.Figure i shows a full-size violin, along with two violins made withhalf and 3/4 of the normal length.3 Let’s study the surface area of

3The customary terms “half-size” and “3/4-size” actually don’t describe the


j / The muffin comes out ofthe oven too hot to eat. Breakingit up into four pieces increasesits surface area while keepingthe total volume the same. Itcools faster because of thegreater surface-to-volume ratio.In general, smaller things havegreater surface-to-volume ratios,but in this example there is noeasy way to compute the effectexactly, because the small piecesaren’t the same shape as theoriginal muffin.

the front panels of the three violins.

Consider the square in the interior of the panel of the full-sizeviolin. In the 3/4-size violin, its height and width are both smallerby a factor of 3/4, so the area of the corresponding, smaller squarebecomes 3/4×3/4 = 9/16 of the original area, not 3/4 of the originalarea. Similarly, the corresponding square on the smallest violin hashalf the height and half the width of the original one, so its area is1/4 the original area, not half.

The same reasoning works for parts of the panel near the edge,such as the part that only partially fills in the other square. Theentire square scales down the same as a square in the interior, andin each violin the same fraction (about 70%) of the square is full, sothe contribution of this part to the total area scales down just thesame.

Since any small square region or any small region covering partof a square scales down like a square object, the entire surface areaof an irregularly shaped object changes in the same manner as thesurface area of a square: scaling it down by 3/4 reduces the area bya factor of 9/16, and so on.

In general, we can see that any time there are two objects withthe same shape, but different linear dimensions (i.e., one looks like areduced photo of the other), the ratio of their areas equals the ratioof the squares of their linear dimensions:

A1A2

=

(L1L2

)2.

Note that it doesn’t matter where we choose to measure the linearsize, L, of an object. In the case of the violins, for instance, it couldhave been measured vertically, horizontally, diagonally, or even fromthe bottom of the left f-hole to the middle of the right f-hole. Wejust have to measure it in a consistent way on each violin. Since allthe parts are assumed to shrink or expand in the same manner, theratio L1/L2 is independent of the choice of measurement.

It is also important to realize that it is completely unnecessaryto have a formula for the area of a violin. It is only possible toderive simple formulas for the areas of certain shapes like circles,rectangles, triangles and so on, but that is no impediment to thetype of reasoning we are using.

Sometimes it is inconvenient to write all the equations in termsof ratios, especially when more than two objects are being compared.A more compact way of rewriting the previous equation is

A ∝ L2.

sizes in any accurate way. They’re really just standard, arbitrary marketinglabels.


k / Example 5. The big trian-gle has four times more area thanthe little one.

l / A tricky way of solving ex-ample 5, explained in solution #2.

The symbol “∝” means “is proportional to.” Scientists and engi-neers often speak about such relationships verbally using the phrases“scales like” or “goes like,” for instance “area goes like length squared.”

All of the above reasoning works just as well in the case of vol-ume. Volume goes like length cubed:

V ∝ L3.

self-check IWhen a car or truck travels over a road, there is wear and tear on theroad surface, which incurs a cost. Studies show that the cost C per kilo-meter of travel is related to the weight per axle w by C ∝ w4. Translatethis into a statement about ratios. . Answer, p. 1058

If different objects are made of the same material with the samedensity, ρ = m/V , then their masses, m = ρV , are proportional toL3. (The symbol for density is ρ, the lower-case Greek letter “rho.”)

An important point is that all of the above reasoning aboutscaling only applies to objects that are the same shape. For instance,a piece of paper is larger than a pencil, but has a much greatersurface-to-volume ratio.

Scaling of the area of a triangle example 5. In figure k, the larger triangle has sides twice as long. Howmany times greater is its area?

Correct solution #1: Area scales in proportion to the square of thelinear dimensions, so the larger triangle has four times more area(22 = 4).

Correct solution #2: You could cut the larger triangle into four ofthe smaller size, as shown in fig. (b), so its area is four timesgreater. (This solution is correct, but it would not work for a shapelike a circle, which can’t be cut up into smaller circles.)

Correct solution #3: The area of a triangle is given by

A = bh/2, where b is the base and h is the height. The areas ofthe triangles are

A1 = b1h1/2A2 = b2h2/2

= (2b1)(2h1)/2= 2b1h1

A2/A1 = (2b1h1)/(b1h1/2)= 4

(Although this solution is correct, it is a lot more work than solution#1, and it can only be used in this case because a triangle is asimple geometric shape, and we happen to know a formula for itsarea.)


m / Example 6. The big spherehas 125 times more volume thanthe little one.

n / Example 7. The 48-point“S” has 1.78 times more areathan the 36-point “S.”

Correct solution #4: The area of a triangle is A = bh/2. Thecomparison of the areas will come out the same as long as theratios of the linear sizes of the triangles is as specified, so let’sjust say b1 = 1.00 m and b2 = 2.00 m. The heights are then alsoh1 = 1.00 m and h2 = 2.00 m, giving areas A1 = 0.50 m2 andA2 = 2.00 m2, so A2/A1 = 4.00.

(The solution is correct, but it wouldn’t work with a shape forwhose area we don’t have a formula. Also, the numerical cal-culation might make the answer of 4.00 appear inexact, whereassolution #1 makes it clear that it is exactly 4.)

Incorrect solution: The area of a triangle is A = bh/2, and if youplug in b = 2.00 m and h = 2.00 m, you get A = 2.00 m2, sothe bigger triangle has 2.00 times more area. (This solution isincorrect because no comparison has been made with the smallertriangle.)

simple nature - light and matter · 2020. 5. 21. · simple nature an introduction to physics for...

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