How Things Work: The Physics of Everyday Life

594 Pages·2013·36.86 MB·English

by  Louis A. Bloomfield

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5 How
th
Edition
Things
Work
THE PHYSICS OF EVERYDAY LIFE
Louis A. Bloomfi eld
The University of Virginia
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5 How
th
Edition
Things
Work
THE PHYSICS OF EVERYDAY LIFE
Louis A. Bloomfi eld
The University of Virginia
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To Aaron, Elana, and Rich, for being everything a father could want
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To the students of the University of Virginia, for making teaching, research, and writing fun
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Foreword
I
n today’s world we are surrounded by science physics was discovered in the fi rst place: people asked
and by the technology that has grown out of that why the world around them behaved as it did and as a
science. For most of us, this is making the world result discovered the principles that explained and pre-
increasingly mysterious and somewhat ominous as tech- dicted what they observed.
nology becomes ever more powerful. For instance, we are I have been using this book in my classes for sev-
confronted by many global environmental questions such eral years, and I continue to be impressed with how
as the dangers of greenhouse gases and the best choices Lou can take seemingly highly complex devices and
of energy sources. These are questions that are funda- strip away the complexity to show how at their heart are
mentally technical in nature and there is a bewildering simple physics ideas. Once these ideas are understood,
variety of claims and counterclaims as to what is “the they can be used to understand the behavior of many
truth” on these and similar important scientifi c issues. devices we encounter in our daily lives, and often even
For many people, the reaction is to throw up their hands fi x things that before had seemed impossibly complex.
in hopeless frustration and accept that the modern world In the process of teaching from this book, I have in-
is impossible to understand and one can only huddle in creased my own understanding of the physics behind
helpless ignorance at the mercy of its mysterious and in- much of the world around me. In fact, after consulting
explicable behavior. How Things Work, I have had the confi dence to con-
In fact, much of the world around us and the tech- front both plumbers and airconditioner repairmen to
nology of our everyday lives is governed by a few basic tell them (correctly as it turned out) that their diagnosis
physics principles, and once these principles are under- did not make sense and they needed to do something
stood, the world and the vast array of technology in our different to solve my plumbing and AC problems. Now
lives become understandable and predictable. How does I am regularly amused at the misconceptions some
your microwave oven heat up food? Why is your radio trained physicists have about some of the physics they
reception bad in some places and not others? And why encounter in their daily lives, such as how a microwave
can birds happily land on a high-voltage electrical wire? oven works and why it can be made out of metal walls,
The answers to questions like these are obvious once but putting aluminum foil in it is bad. It has convinced
you know the relevant physics. Unfortunately, you are me that we need to take the approach used in this book
not likely to learn that from a standard physics course or in far more of our science texts.
physics textbook. There is a large body of research show- Of course, the most important impact is on the stu-
ing that, instead of providing this improved understanding dents in my classes that use this book. These are typi-
of everyday life, most introductory physics courses are cally nonscience students majoring in fi elds such as fi lm
doing quite the opposite. In spite of the best intentions of studies, classics, English, business, etc. They often come
the teachers, most students are “learning” that physics is to physics with considerable trepidation. It is inspiring to
abstract, uninteresting, and unrelated to the world around see many of them discover to their surprise that physics is
them. very different from what they thought—that physics can
How Things Work is a dramatic step toward changing actually be interesting and useful and makes the world
that by presenting physics in a new way. Instead of start- a much less mysterious and more understandable place.
ing out with abstract principles that leave the reader with I remember many examples of seeing this in action: the
the idea that physics is about artifi cial and uninteresting student who, after learning how both speakers and TVs
ideas, Lou Bloomfi eld starts out talking about real objects work, was suddenly able to understand that it was not
and devices that we encounter in our everyday lives. He magic that putting his large speaker next to the TV dis-
then shows how these seemingly magical devices can be torted the picture but in fact it was just physics, and now
understood in terms of the basic physics principles that he knew just how to fi x it; the young woman scuba diver
govern their behavior. This is much the way that most who, after learning about light and color, suddenly interrupted
v
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vi FOREWORD
class to announce that now she understood why it was way, something that happens far too seldom in science
that you could tell how deep you were by seeing what courses.
color lobsters appeared; or the students who announced Whether a curious layperson, a trained physicist, or
that suddenly it made sense that the showers on the fi rst a beginning physics student, most everyone will fi nd this
fl oor of the dorm worked better than those on the second book an interesting and enlightening read and will go
fl oor. In addition, of course everyone is excited to learn away comforted in that the world is not so strange and
how a microwave oven works and why there are these inexplicable after all.
strange rules as to what you can and cannot put in it.
These examples are particularly inspiring to a teacher, Carl Wieman
because they tell you that the students are not just learn- Nobel Laureate in Physics 2001
ing the material presented in class but they are then able CASE/Carnegie US University Professor
to apply that understanding to new situations in a useful of the Year 2004
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Contents
Foreword v
Preface xiv
CHAPTER 1 THE LAWS OF MOTION, PART 1 1
Experiment: Removing a Tablecloth from a Table 1
Chapter Itinerary 2
1.1 Skating 3
(inertia, coasting, vector quantities, position, velocity, force, acceleration, mass,
net force, Newton’s fi rst and second laws, inertial frames of reference, units)
1.2 Falling Balls 14
(gravity, weight, constant acceleration, projectile motion, vector components)
1.3 Ramps 24
(support forces, Newton’s third law, energy, work, conservation of energy, kinetic and
potential energies, gravitational potential energy, ramps, mechanical advantage)
Epilogue for Chapter 1 35 / Explanation: Removing a Tablecloth from a
Table 35 / Chapter Summary 35 / Exercises 37 / Problems 38
CHAPTER 2 THE LAWS OF MOTION, PART 2 40
Experiment: A Spinning Pie Dish 40
Chapter Itinerary 41
2.1 Seesaws 42
(rotational inertia; angular velocity; torque; angular acceleration; rotational mass;
net torque; Newton’s fi rst, second, and third laws of rotation; centers of mass and
gravity; levers; balance)
2.2 Wheels 57
(friction, traction, ordered and thermal energies, wheels, bearings, kinetic energy,
power, rotational work)
2.3 Bumper Cars 69
(momentum, impulse, conservation of momentum, angular momentum, angular impulse,
conservation of angular momentum, gradients, potential energy, acceleration, and forces)
Epilogue for Chapter 2 81 / Explanation: A Spinning Pie Dish 81 / Chapter
Summary 81 / Exercises 83 / Problems 84
vii
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viii CONTENTS
CHAPTER 3 MECHANICAL OBJECTS, PART 1 86
Experiment: Swinging Water Overhead 86
Chapter Itinerary 87
3.1 Spring Scales 88
(equilibrium, stable equilibrium, restoring force, Hooke’s law, elastic potential
energy, oscillation, calibration)
3.2 Ball Sports: Bouncing 94
(collisions, energy transfers, elastic and inelastic collisions, vibration)
3.3 Carousels and Roller Coasters 102
(uniform circular motion, feeling of acceleration, apparent weight, centripetal acceleration)
Epilogue for Chapter 3 111 / Explanation: Swinging Water Overhead 112 /
Chapter Summary 112 / Exercises 113 / Problems 115
CHAPTER 4 MECHANICAL OBJECTS, PART 2 116
Experiment: High-Flying Balls 116
Chapter Itinerary 117
4.1 Bicycles 118
(stable, neutral, and unstable equilibriums; static and dynamic stability; precession)
4.2 Rockets and Space Travel 126
(reaction forces, law of universal gravitation, elliptical orbits, escape velocity,
Kepler’s laws, speed of light, special and general relativity, equivalence principle)
Epilogue for Chapter 4 139 / Explanation: High-Flying Balls 139 / Chapter
Summary 139 / Exercises 140 / Problems 142
CHAPTER 5 FLUIDS 143
Experiment: A Cartesian Diver 143
Chapter Itinerary 144
5.1 Balloons 145
(pressure, density, temperature, thermal motion, absolute zero, Archimedes’
principle, buoyant force, ideal gas law)
5.2 Water Distribution 157
(hydrostatics, Pascal’s principle, hydraulics, hydrodynamics, steady state fl ow,
streamlines, pressure potential energy, Bernoulli’s equation)
Epilogue for Chapter 5 167 / Explanation: A Cartesian Diver 167 / Chapter
Summary 167 / Exercises 168 / Problems 169