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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Discover the power of physics!
Physics I For Dummies, 4th Edition explains the basic principles of physics simply and clearly. This useful guide includes easy-to-follow explanations of motion, energy, the states of matter, thermodynamics, electromagnetism, relativity, and beyond. It breaks down complex concepts into manageable chunks and uses relatable examples to you readers understand how physics applies to everyday situations. You'll get a handle on physics in no time!
Inside:
- Work through everything in an intro physics class, explained in plain English
- Understand physics formulas and how to use them
- Learn the basics of motion, force, work, heat, and more—and pass your test!
- Make learning easier with examples that connect physics to the world
Perfect for anyone looking for a course supplement or for those simply curious about physics and the way it intersects with our everyday life, Physics I For Dummies takes the work out of learning physics.
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Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Introduction
About This Book
Foolish Assumptions
Icons Used in This Book
Beyond the Book
Where to Go from Here
Part 1: Putting Physics into Motion
Chapter 1: Using Physics to Understand Your World
What Physics Is All About
Observing Objects in Motion
When Push Comes to Shove: Forces
Feeling the Heat with Thermodynamics
Chapter 2: Reviewing the Basics: Math and Measurement
Measuring the World around You and Making Predictions
Eliminating Some Zeros: Using Scientific Notation
Checking the Accuracy and Precision of Measurements
Arming Yourself with Basic Algebra
Tackling a Little Trig
Interpreting Equations as Real-World Ideas
Chapter 3: Exploring the Need for Speed
Going the Distance with Displacement
Speed Specifics: What Is Speed, Anyway?
Speeding Up (or Down): Acceleration
Relating Acceleration, Time, and Displacement
Linking Velocity, Acceleration, and Displacement
Chapter 4: Following Directions: Motion in Two Dimensions
Visualizing Vectors
Putting Vectors on the Grid
A Little Trig: Breaking Up Vectors into Components
Displacing and Accelerating in Two Dimensions
Accelerating Downward: Motion under the Influence of Gravity
Part 2: May the Forces of Physics Be with You
Chapter 5: When Push Comes to Shove: Force
Newton’s First Law: Resisting with Inertia
Newton’s Second Law: Relating Force, Mass, and Acceleration
Newton’s Third Law: Looking at Equal and Opposite Forces
Chapter 6: Getting Down with Gravity, Inclined Planes, and Friction
Acceleration Due to Gravity: One of Life’s Little Constants
Finding a New Angle on Gravity with Inclined Planes
Getting Sticky with Friction
Let’s Get Fired Up! Sending Objects Airborne
Chapter 7: Circling Around Rotational Motion and Orbits
Centripetal Acceleration: Changing Direction to Move in a Circle
Seeking the Center: Centripetal Force
Getting Angular with Displacement, Velocity, and Acceleration
Letting Gravity Supply Centripetal Force
Looping the Loop: Vertical Circular Motion
Chapter 8: Go with the Flow: Looking at Pressure in Fluids
Mass Density: Getting Some Inside Information
Applying Pressure
Buoyancy: Floating Your Boat with Archimedes’s Principle
Fluid Dynamics: Going with Fluids in Motion
Getting Up to Speed on Flow and Pressure
Part 3: Manifesting the Energy to Work
Chapter 9: Getting Some Work Out of Physics
Looking for Work
Making a Move: Kinetic Energy
Energy in the Bank: Potential Energy
Choose Your Path: Conservative versus Nonconservative Forces
Keeping the Energy Up: The Conservation of Mechanical Energy
Powering Up: The Rate of Doing Work
Chapter 10: Putting Objects in Motion: Momentum and Impulse
Looking at the Impact of Impulse
Gathering Momentum
The Impulse-Momentum Theorem: Relating Impulse and Momentum
When Good Objects Go Wild: Conserving Momentum
When Worlds (or Cars) Collide: Elastic and Inelastic Collisions
Chapter 11: Winding Up with Angular Kinetics
Going from Linear to Rotational Motion
Understanding Tangential Motion
Applying Vectors to Rotation
Doing the Twist: Torque
Spinning at Constant Velocity: Rotational Equilibrium
Chapter 12: Round and Round with Rotational Dynamics
Rolling Up Newton’s Second Law into Angular Motion
Moments of Inertia: Looking into Mass Distribution
Wrapping Your Head around Rotational Work and Kinetic Energy
Can’t Stop This: Angular Momentum
Chapter 13: Springs ’n’ Things: Simple Harmonic Motion
Bouncing Back with Hooke’s Law
Getting Around to Simple Harmonic Motion
Factoring Energy into Simple Harmonic Motion
Swinging with Pendulums
Part 4: Laying Down the Laws of Thermodynamics
Chapter 14: Turning Up the Heat with Thermodynamics
Measuring Temperature
The Heat Is On: Thermal Expansion
Heat: Going with the Flow (of Thermal Energy)
Chapter 15: Here, Take My Coat: How Heat Is Transferred
Convection: Letting the Heat Flow
Too Hot to Handle: Getting in Touch with Conduction
Radiation: Riding the (Electromagnetic) Wave
Chapter 16: Best of the Best: The Ideal Gas Law
Digging into Molecules and Moles with Avogadro’s Number
Relating Pressure, Volume, and Temperature with the Ideal Gas Law
Tracking Ideal Gas Molecules with the Kinetic Energy Formula
Chapter 17: Heat and Work: The Laws of Thermodynamics
Getting Temperature with Thermal Equilibrium: The Zeroth Law
Conserving Energy: The First Law of Thermodynamics
Flowing from Hot to Cold: The Second Law of Thermodynamics
Going Cold: The Third (and Absolute Last) Law of Thermodynamics
Part 5: The Part of Tens
Chapter 18: Ten Ways Physics Runs Your Day
Waking Up
Turning On the Lights
Checking Your Morning Email
Brewing the Coffee
Getting to Work
Sitting Down in the Office
Climbing the Stairs
Idling the Day Away
Heading Home
Calling It a Day
Chapter 19: Ten Physics Heroes
Galileo Galilei
Sir Isaac Newton
Charles-Augustin de Coulomb
William Thomson (Lord Kelvin)
Marie Salomea Skłodowska Curie
Albert Einstein
Emmy Noether
Maria Goeppert Mayer
Chien-Shiung Wu
Jocelyn Bell Burnell
Glossary
Index
About the Authors
Dedication
Authors’ Acknowledgments
Connect with Dummies
End User License Agreement
List of Tables
Chapter 2
TABLE 2-1 Units of Measurement in the MKS System
Chapter 5
TABLE 5-1 Units of Force
Chapter 7
TABLE 7-1 Linear and Angular Motion Formulas
Chapter 8
TABLE 8-1 Densities of Common Materials
Chapter 12
TABLE 12-1 Moments of Inertia for Various Shapes and Solids
Chapter 15
TABLE 15-1 Thermal Conductivities for Various Materials
List of Illustrations
Chapter 2
FIGURE 2-1: A labeled triangle that you can use to find trig values.
Chapter 3
FIGURE 3-1: Examining displacement of a golf ball.
FIGURE 3-2: A ball moving in two dimensions.
FIGURE 3-3: Applying the Pythagorean theorem to calculate magnitude of displace...
FIGURE 3-4: A trip from Ohio to Michigan.
FIGURE 3-5: Increasing velocity under constant acceleration.
Chapter 4
FIGURE 4-1: A vector, represented by an arrow, has both a direction and a magni...
FIGURE 4-2: Equal vectors have the same length and direction but may have diffe...
FIGURE 4-3: Going from the tail of one vector to the head of a second vector ge...
FIGURE 4-4: Take the sum of two vectors by creating a new vector.
FIGURE 4-5: Subtracting two vectors by putting their feet together and drawing ...
FIGURE 4-6: Use vector coordinates to make handling vectors easy.
FIGURE 4-7: Breaking a vector into components allows you to add or subtract the...
FIGURE 4-8: Using the angle created by a vector to get to a hotel.
FIGURE 4-9: A baseball diamond is a series of vectors created by the
x
-axis and...
FIGURE 4-10: You can use acceleration and change in time to find a change in ve...
FIGURE 4-11: A golf ball about to roll off a cliff.
FIGURE 4-12: A kicked soccer ball.
Chapter 5
FIGURE 5-1: Accelerating a hockey puck.
FIGURE 5-2: A ball in flight may face many forces that act on it.
FIGURE 5-3: The net force vector factors in all forces to determine the ball’s ...
FIGURE 5-4: A free-body diagram of all the forces acting on a football at one t...
FIGURE 5-5: Equal forces acting on a car tire and the road during acceleration.
FIGURE 5-6: Pulling a heavy puck with a rope to overcome friction.
FIGURE 5-7: Using a pulley to exert force.
FIGURE 5-8: Using a pulley at an angle to keep a mass stationary.
FIGURE 5-9: Hanging a sign requires equilibrium from the involved forces.
Chapter 6
FIGURE 6-1: Racing a cart down a ramp.
FIGURE 6-2: The angle of the direction perpendicular to the ramp surface from t...
FIGURE 6-3: The forces acting on a bar of gold.
FIGURE 6-4: You must battle different types of force and friction to push an ob...
FIGURE 6-5: All the forces acting on an object sliding down a ramp.
FIGURE 6-6: Shooting a cannon at a particular angle with respect to the ground.
Chapter 7
FIGURE 7-1: Velocity constantly changes direction when an object is in circular...
FIGURE 7-2: A golf ball on a string traveling with constant speed.
FIGURE 7-3: The forces acting on a car banking around a turn.
FIGURE 7-4: A circular arc extends an angle of one radian.
FIGURE 7-5: The force and velocity of a ball on a circular track.
Chapter 8
FIGURE 8-1: A cube of water has different pressures on the top and bottom faces...
FIGURE 8-2: A hydraulic system magnifies force.
FIGURE 8-3: A raft in water.
FIGURE 8-4: A streamline shows the directions of flow.
FIGURE 8-5: A cube of fluid flowing through a pipe.
Chapter 9
FIGURE 9-1: To do work on this gold ingot, you have to push with enough force t...
FIGURE 9-2: More force is required to do the same amount of work if you pull at...
FIGURE 9-3: You find the net force acting on an object to find its speed at the...
FIGURE 9-4: Kinetic energy converted to potential energy and then back to kinet...
Chapter 10
FIGURE 10-1: Examining force versus time gives you the impulse you apply on obj...
FIGURE 10-2: The average force over a time interval depends on the values the f...
FIGURE 10-3: Shooting a wooden block on a string allows you to experiment with ...
FIGURE 10-4: Before, during, and after a collision between two balls moving in ...
Chapter 11
FIGURE 11-1: A ball in circular motion has angular speed with respect to the ra...
FIGURE 11-2: Angular velocity points in a direction perpendicular to the wheel.
FIGURE 11-3: Angular acceleration in the same direction as the angular velocity...
FIGURE 11-4: Angular acceleration in the direction opposite the angular velocit...
FIGURE 11-5: Angular acceleration perpendicular to the angular velocity tilts t...
FIGURE 11-6: A seesaw demonstrates torque in action.
FIGURE 11-7: The torque you exert on a door depends on where you push it.
FIGURE 11-8: You produce a useful angle of a lever arm by exerting force in the...
FIGURE 11-9: A turning motion toward larger positive angles indicates a positiv...
FIGURE 11-10: A schematic of the forces acting on Hercules’s arm.
FIGURE 11-11: Hanging a heavy flag requires some serious torque.
FIGURE 11-12: Keeping a ladder upright requires friction and rotational equilib...
Chapter 12
FIGURE 12-1: A tangential force applied to a ball on a string.
FIGURE 12-2: The shapes corresponding to the moments of inertia in Table 12-1.
FIGURE 12-3: You use the torque you apply and the angular motion of the pulley ...
FIGURE 12-4: Exerting a force to turn a tire.
FIGURE 12-5: A solid cylinder and a hollow cylinder ready to race down a ramp.
Chapter 13
FIGURE 13-1: The direction of force exerted by a spring.
FIGURE 13-2: A ball on a spring, influenced by gravity.
FIGURE 13-3: Tracking a ball’s simple harmonic motion over time.
FIGURE 13-4: The vertical component of the displacement of an object moving in ...
FIGURE 13-5: A reference circle helps you analyze simple harmonic motion.
FIGURE 13-6: A pendulum moves in simple harmonic motion.
Chapter 14
FIGURE 14-1: Linear expansion usually takes place when you apply heat to solids...
FIGURE 14-2: Phase changes of water.
Chapter 15
FIGURE 15-1: You can see convection in action by boiling a pot of oil.
FIGURE 15-2: Conduction heats the pot that holds the boiling water.
FIGURE 15-3: Conducting heat in a bar of steel.
FIGURE 15-4: An incandescent light bulb radiates heat into its environment.
Chapter 16
FIGURE 16-1: For an ideal gas, pressure is directly proportional to temperature...
Chapter 17
FIGURE 17-1: An isobaric system may feature a change in volume, but the pressur...
FIGURE 17-2: Pressure and volume in an isobaric system.
FIGURE 17-3: An isochoric system features a constant volume as other quantities...
FIGURE 17-4: Because volume is constant in an isochoric process, no work is don...
FIGURE 17-5: An isothermal system maintains a constant temperature amidst other...
FIGURE 17-6: The area under the curve shows the work done in an isothermal proc...
FIGURE 17-7: An adiabatic system doesn’t allow heat to escape or enter.
FIGURE 17-8: An adiabatic graph of pressure versus volume.
FIGURE 17-9: A heat engine turns heat into work.
Guide
Cover
Table of Contents
Title Page
Copyright
Begin Reading
Glossary
Index
About the Authors
Dedication
Authors’ Acknowledgments
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Physics I For Dummies®, 4thEdition
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Introduction
Physics is what it’s all about. What what’s all about? Everything. Physics is present in every action around you. And because physics is everywhere, it gets into some tricky places, which means it can be hard to follow. Studying physics can be even worse when you’re reading some dense textbook that’s somewhat less than captivating.
For most people who come into contact with physics, textbooks that land with 1,200-page whumps on desks are their only exposure to this amazingly rich and rewarding field. And what follows are weary struggles as the readers try to scale the awesome bulwarks of these massive tomes. What’s vastly different about this physics book is that it’s written from the reader’s point of view, and contains plenty of real-world examples.
About This Book
Physics I For Dummies, 4th Edition, is all about physics from your point of view. We know that most physics students share one common trait: confusion. As in, “I’m confused about what I did to deserve such torture.”
This book is different. Instead of writing it from the physicist’s or professor’s point of view, we wrote it from the reader’s point of view. We’ve taken great care to jettison the top-down kinds of explanations in the usual book presentation of this topic. You don’t survive one-on-one tutoring sessions for long unless you get to know what really makes sense to people — what they want to see from their points of view. In other words, this book is designed to be crammed full of the good stuff — and only the good stuff. We tell you what you need to know, exactly how you need to know it — and work out plenty of sample problems to help you figure out how to put these ideas into action. You’ll also discover unique ways of looking at physics problems that professors and teachers use to simplify the process of solving these problems.
Some books have a dozen conventions that you need to know before you can start. Not this one. All you need to know is that variables and new terms appear in italics, like this, and that vectors — items that have both a magnitude and a direction — appear in bold. Web addresses appear in monofont.
We provide two elements in this book that you don’t have to read at all if you’re not interested in the inner workings of physics — sidebars and paragraphs marked with a Technical Stuff icon.
Sidebars provide a little more insight into what’s going on with a particular topic. They give you more of the story, such as how some famous physicist made a discovery or an unexpected real-life application of the point under discussion. You can skip these sidebars, if you like, without missing any essential physics.
The Technical Stuff material gives you technical insights into a topic, but you don’t miss any information that you need to solve a problem. Your guided tour of the world of physics won’t suffer at all should you choose to skip over these sections.
The natural world is, well, big. And to handle it, physics breaks the world down into different parts.
The following list shows you the various parts you’ll find in this book.
Part 1
: Putting Physics into Motion:
You usually start your physics journey with motion, because describing motion — including acceleration, velocity, and displacement — is a fantastic, easy introduction into physics and problem-solving. You have only a few equations to deal with, and you can get them under your belt in no time at all. Examining motion is a great way to understand how physics works, both in measuring and in predicting what’s going on in the world.
Part 2
: May the Forces of Physics Be with You:
“For every action, there is an equal and opposite reaction.” Ever heard that one? This law of physics (and its accompanying implications) comes up in this part. Without forces, the motion of objects wouldn’t change at all, which would make for a very boring world. Thanks to Sir Isaac Newton, physics is particularly good at explaining what happens when you apply forces. You also take a look at the motion of fluids.
Part 3
: Manifesting the Energy to Work:
If you apply a force to an object, moving it around and making it go faster, what are you really doing? You’re doing work, and that work becomes the kinetic energy of that object. Together, work and energy explain a whole lot about the whirling world around you, which is why we dedicate
Part 3
to these topics.
Part 4
: Laying Down the Laws of Thermodynamics:
What happens when you stick your finger in a candle flame and hold it there? You get a burned finger, that’s what. And you complete an experiment in heat transfer, one of the topics you see in
Part 4
, which is a roundup of thermodynamics — the physics of heat and heat flow. You also see how heat-based engines work, how ice melts, how an ideal gas behaves, and more.
Part 5
: The Part of Tens:
This part is made up of fast-paced lists of ten items each. You discover all kinds of amazing topics here, like some far-out physics — everything from black holes and the Big Bang to wormholes in space — as well as a look at all the down-to-Earth ways in which physics both explains and makes possible your daily routine.
Foolish Assumptions
In writing this book, we made some assumptions about you:
You have no or very little prior knowledge of physics.
You have some math prowess. In particular, you know algebra and a little trigonometry. You don’t need to be an algebra pro, but you should know how to move items from one side of an equation to another and how to solve for values. If you need a refresher, we suggest
Algebra 1 For Dummies
(Wiley).
You want physics concepts explained clearly and concisely, and you want examples that let you see those concepts in action.
Icons Used in This Book
Throughout this book, icons in the margins highlight certain types of valuable information that call out for your attention. Here are the icons you’ll encounter and a brief description of each.
When you run across this icon, be prepared to find a shortcut in the math or info designed to help you understand a topic better.
This icon marks information to remember, such as an application of a law of physics or a particularly useful equation.
This icon means that the info is technical, insider stuff. You don’t have to read it if you don’t want to, but if you want to become a physics pro (and who doesn’t?), take a look.
This icon highlights common mistakes people make when studying physics and solving problems.
Beyond the Book
In addition to what you’re reading right now, this book comes with a free online Cheat Sheet you can access anytime you need a quick physics refresher on important constants and equations. To get this Cheat Sheet, simply go to www.dummies.com and type Physics I For Dummies Cheat Sheet in the search box.
Where to Go from Here
You can leaf through this book; you don’t have to read it from beginning to end. Like other For Dummies books, this one was designed to let you skip around as you like. This is your book, and physics provides a menu of opportunities. You can jump into Chapter 1, which is where all the action starts; you can head to Chapter 2 for a discussion of the necessary algebra and trig you will need; or you can jump in anywhere you like if you know exactly what topic you want to study. And when you’re ready for more advanced topics, from electromagnetism to relativity to nuclear physics, move on over to Physics II For Dummies (Wiley).
Part 1
Putting Physics into Motion
IN THIS PART …
Find out how physics shows up in the world around you.
Check out the basic mathematics and measurement concepts that physicists use.
Dive into the topic of motion and see how easily you can solve related problems with just a few equations.
Apply the laws of motion (and some equations) to figure out velocity, speed, and acceleration.
Chapter 1
Using Physics to Understand Your World
IN THIS CHAPTER
Recognizing the physics in your world
Understanding motion
Handling the force and energy around you
Warming up to thermodynamics
Physics is the study of the world and universe around you. Luckily, the behavior of matter and energy — the stuff of this universe — is not completely unruly. Instead, it strictly obeys laws that we can understand through the careful application of the scientific method, which relies on experimental evidence and rigorous reasoning. In this way, physicists have been uncovering more and more of the beauty that lies at the heart of the workings of the universe, from the infinitely small to the mind-bogglingly large.
Physics is an all-encompassing science. You can study various aspects of the natural world (in fact, the word physics is derived from the Greek word physika, which means “natural things”), and accordingly, you can study different fields in physics: The physics of objects in motion, of energy, of forces, of gases, of heat and temperature, and so on. This book exposes you to the study of all these topics and many more. In this chapter, we give you an overview of physics — what it is, what it deals with, and why mathematical calculations are important to it.
What Physics Is All About
Thinking about physics makes most of us a little nervous. The sheer number of equations, symbols, and other terms makes physics seem like a language in and of itself. Guess what? It is! And by reading this book, you can pick up the key to understanding this language. Physics exists to help you make sense of the world, and it’s a human adventure — undertaken on behalf of everyone — into the way the universe works.
At its root, physics is all about becoming aware of your world and using mental and mathematical models to explain it. The gist of physics is this: You start by making an observation, you create a model to simulate that situation, and then you add some math to fill it out — and voilà! You have the power to predict what will happen in the real world. All this math exists to help you see what happens and why.
In this section, we explain how real-world observations fit in with the math. The later sections take you on a brief tour of the key topics that comprise basic physics.
Observing the world
The complexity of today’s world is an excellent starting point for observations of motion. Leaves are waving, the sun is shining, light bulbs are glowing, driverless cars are moving, 3-D printers are making objects, people are walking and riding bikes, streams are flowing, and so on. When you stop to examine these actions, your natural curiosity gives rise to endless questions such as these:
Why do I slip when I try to climb that snowbank?
How distant are other stars, and how long would it take to get there?
How can a thermos flask keep hot things warm
and
keep cold things cool?
Why does an enormous cruise ship float when a paper clip sinks?
Why does water roll around when it boils?
Any law of physics comes from very close observation of the world, and any theory that a physicist comes up with has to stand up to experimental measurements. Physics goes beyond qualitative statements about physical actions — “If I push the child on the swing harder, then she swings higher,” for example. With the laws of physics, you can predict precisely how much higher the child will swing.
Making predictions
Physics is simply about modeling the world (although an alternative viewpoint claims that physics actually uncovers the truth about the workings of the world; it doesn’t just model it). You can use these mental models (abstract representations of physical phenomena) to describe how the world works: How blocks slide down ramps, how stars form and shine, how black holes trap light so it can’t escape, what happens when cars collide, and so on.
Initially, physics models are relatively number-free; they focus on explaining and understanding scenarios at a high level. Here’s an example: How are stars created? You could start by saying that stars are made up of this layer and then that layer, and as a result, this reaction takes place, followed by that one. And pow! — you have a star.
As time goes on, those models become more numerically inclined. Physics class would be a cinch if you could simply say, “That cart is going to roll down that hill, and as it gets toward the bottom, it’s going to roll faster and faster.” But the story is more involved than that: Not only can you say that the cart is going to go faster, but by exerting your grasp of the physical world, you can also say how much faster it’ll go.
Think about the power of physics this way: You can start with a qualitative, intuitive explanation of some physical phenomenon that just makes sense to you — such as the harder you throw a ball, the further it will go. Applying physics takes that intuitive understanding into a quantitative result: If you know the force with which you throw the ball, you can predict how far it will travel!
There’s a delicate interplay between theory — formulated with math — and experimental measurements. Often experimental measurements not only verify theories but also suggest ideas for new theories, which in turn suggest new experiments. Theories and measurements feed off each other and lead to further discovery.
Many people approaching the technical side of physics may think of math as something tedious and overly abstract. However, in the context of physics, math comes to life. While quadratic equations may seem like something you’d rather skip over, don’t rush to judgement — they’re key to understanding concepts such as the correct angle to fire a rocket for the perfect trajectory. Chapter 2 explains all the math you need to know to perform basic physics calculations.
Reaping the rewards
So what do you get out of studying physics? If you want to pursue a career in physics or in a related field such as engineering, the answer is clear: You’ll need this knowledge on an everyday basis. But even if you’re not planning to embark on a physics-related career, you can get a lot out of studying the subject. You can apply much of what you discover in an introductory physics course to real life for these reasons:
In a sense, all other sciences are based upon physics.
For example, the structure and electrical properties of atoms determine chemical reactions; therefore, all of chemistry is governed by the laws of physics. In fact, you could argue that everything ultimately boils down to the laws of physics!
Physics does deal with some pretty cool phenomena.
Many videos of physical phenomena have gone viral on TikTok (or other social media); take a look for yourself. Do a search for “non-Newtonian fluid,” and you can watch the creeping, oozing dance of a cornstarch/water mixture on a speaker cone.
The applications of physics arm you with problem-solving skills for approaching any kind of problem.
Physics problems train you to stand back, consider your options for attacking the issue, select your method, and then solve the problem in the easiest way possible.
Observing Objects in Motion
Some of the most fundamental questions you may have about the world deal with objects in motion. Will that boulder rolling toward you slow down? How fast do you have to move to get out of its way? (Grab your calculator …) Evaluating motion was one of the earliest explorations of physics.
When you take a look around, you see that the motion of objects changes all the time. You see a motorcycle coming to a halt at a stop sign. You see a leaf falling and then stopping when it hits the ground, only to be picked up again by the wind. You see a pool ball hitting other balls in just the wrong way so that they all move without going where they should. Part 1 of this book handles objects in motion — from balls to railroad cars and most objects in between. In this section, we introduce motion in a straight line, rotational motion, and the cyclical motion of springs and pendulums.
Measuring speed, direction, velocity, and acceleration
Speeds are big with physicists — how fast is an object going? Is 35 miles per hour not fast enough? How about 3,500? No problem when you’re dealing with physics. Besides speed, the direction an object is going is important if you want to describe its motion. If the home team is carrying a football down the field, you want to make sure that they’re going in the right direction.
When you put speed and direction together, you get a vector — the velocity vector. Vectors are a very useful kind of quantity. Anything that has both magnitude (an amount) and direction is best described with a vector. Vectors are often represented as arrows, where the length of the arrow tells you the magnitude (size), and the direction of the arrow tells you, well, the direction. For a velocity vector, the length corresponds to the speed of the object, and the arrow points in the direction the object is moving. (To find out how to use vectors, head to Chapter 4.)
Everything has a velocity, so velocity is great for describing the world around you. Even if an object is at rest with respect to the ground, it’s still on the Earth, which itself has a velocity due to its orbit around the Sun. (And the Sun’s in motion around the center of the galaxy — hope you aren’t dizzy now!)
If you’ve ever ridden in a car, you know that velocity isn’t the end of the story. Cars don’t start off traveling at 60 miles per hour; stepping on the accelerator pedal kicks off a chain of events that leads to a car accelerating until it reaches the speed limit (and maybe beyond — but we won’t tell!). Like velocity, acceleration has not only a magnitude but also a direction, so acceleration is a vector in physics as well. We cover speed, velocity, and acceleration in Chapter 4.
Round and round: Rotational motion
Plenty of things go round and round in the everyday world: Figure skaters, tires, pitchers’ arms, clothes in a dryer, roller coasters doing the loop, or just little kids spinning from joy in their first snowstorm. That being the case, physicists want to get in on the action with measurements. Just as you can have a car moving and accelerating in a straight line, its tires can rotate and accelerate in a circle.
Fortunately, switching from the linear to the rotational world is eminently doable thanks to a handy physics analog (a fancy word for “equivalent”) for everything linear in the rotational world. For example, distance traveled (linear) becomes angle turned (rotational). Speed in meters per second becomes angular speed in angle turned per second. Even linear acceleration becomes rotational acceleration.
Once you’re familiar with linear motion, rotational motion will quickly become second nature. You use the same equations for both linear and angular motion — just different symbols with slightly different meanings (angle replaces distance, for example). You’ll be looping the loop in no time. Chapter 7 has the details.
Springs and pendulums: Simple harmonic motion
Have you ever watched something bouncing up and down on a spring? That kind of motion puzzled physicists for a long time, until they did what they do best: Derive equations! They discovered that when you stretch a spring, the force isn’t constant. The spring pulls back, and the more you pull the spring, the stronger it pulls back.
So how does the force compare to the distance you pull a spring? The force is directly proportional to the amount you stretch the spring. Double the amount you stretch the spring, and you double the amount of force with which the spring pulls back. (Just don’t overstretch it; damaged springs may not work as expected.)
Physicists were overjoyed — this was the kind of math they understood. Force proportional to distance? Great — you can put that relationship into an equation, and you can use that equation to describe the motion of the object tied to the spring. Physicists got results that revealed just how objects tied to springs would move — another triumph of physics.
This particular triumph of physics is called simple harmonic motion. It’s simple because force is directly proportional to distance, so the result is simple. It’s harmonic because it repeats over and over again as the object on the spring bounces up and down. Physicists were able to derive simple equations that could tell you exactly where the object would be at any given time.
But that’s not all. Simple harmonic motion applies to many objects in the real world, not just things on springs. For example, pendulums also move in simple harmonic motion. Say you have a stone that’s swinging back and forth on a string. As long as the arc it swings through isn’t too high, the stone on a string is a pendulum; therefore, it follows simple harmonic motion. If you know how long the string is and how big of an angle the swing covers, you can predict where the stone will be at any time. We discuss simple harmonic motion in Chapter 13.
When Push Comes to Shove: Forces
Forces are a particular favorite in physics. You need forces to get motionless things moving — literally. Consider a stone on the ground. Many physicists (except, perhaps, geophysicists) would disregard it altogether. It’s just sitting there. What fun is that? What can you measure about that? After physicists had measured its size and mass, they’d lose interest.
But kick the stone — that is, apply a force — and watch the physicists come running over. Now something is happening! The stone started at rest, but now it’s moving. You can find all kinds of numbers associated with this motion. For instance, you can connect the force you apply to the stone to its mass and get its acceleration. And physicists love numbers because numbers help describe what’s happening in the physical world.
Physicists are experts in applying forces to objects and predicting the results. Got a refrigerator to push up a ramp and want to know if it’ll go? Ask a physicist. Have a rocket to launch? Same thing.
Absorbing the energy around you
You don’t have to look far to find your next occurrence of physics. (But, then again, you never do.) As you exit your house in the morning, for example, you may hear a crash up the street. Two cars have collided at a high speed, and, while locked together, they’re sliding your way. Thanks to the physics of energy and momentum (presented in Part 3 of this book), you can make the necessary measurements and predictions to know exactly how far you have to move to get out of the way.
Your newfound grasp of energy and momentum allows you to solve these problems (which, we hope, are entirely theoretical.) You use these ideas to describe the motion of objects with mass. The energy of motion is called kinetic energy, and when you accelerate a car from 0 to 60 miles per hour in 10 seconds, the car ends up with plenty of kinetic energy.
Where does the kinetic energy come from? It comes from work, the phenomenon of a force moving an object through a distance. The energy can also come from potential energy, the energy stored in the object. Potential energy comes from the work done by a particular kind of force, such as gravity or electrical forces. Using gasoline, for example, an engine does work on the car to get it up to speed. However, that’s not the end of the story. You need a force to accelerate something, and the way the engine does work on the car, surprisingly, is to use the force of friction with the road. Without friction, the wheels would simply spin, but because of a frictional force, the tires impart a force on the road.
For every force between two objects, there is a reactive force of equal size but in the opposite direction. In the preceding car example, just as the car exerts a force onto the road, the road exerts forces onto the car. In one direction, this force causes the car to accelerate, and in the other, it prevents the car from falling through the road!
Or say that you’re moving a piano up the stairs of your new place. After you move it up the stairs, your piano has potential energy, simply because you put in a lot of work against gravity to get the piano up those six floors. Unfortunately, your roommate hates pianos and drops yours out the window. What happens next? The potential energy of the piano — due to its height in a gravitational field — is converted into kinetic energy, the energy of motion. You decide to calculate the final speed of the piano as it hits the street. (Next, you calculate the bill for the piano, hand it to your roommate, and go back downstairs to get your drum set.)
Getting weighed down with pressures in fluids
Did you ever notice that when you’re 5,000 feet down in the ocean, the pressure is different from at the surface? You’ve never been 5,000 feet beneath the ocean waves? Then you may have noticed the difference in pressure when you dive into a swimming pool. The deeper you go, the higher the pressure is because of the weight of the water above you exerting a force downward. Pressure is defined as force per unit area.
If you have a swimming pool, any physicists worth their salt can tell you the approximate pressure at the bottom if you tell them how deep the pool is. When working with fluids, you have all kinds of other quantities to measure, such as the velocity of fluids through small holes, a fluid’s density, and so on. Once again, physics responds with grace under pressure (so to speak). You can read about forces in fluids in Chapter 8.
Feeling the Heat with Thermodynamics
Heat and cold are parts of your everyday life. Did you ever take a look at the beads of condensation on a cold glass of water in a warm room? Water vapor in the air is being cooled when it touches the glass, and it condenses into liquid water in the form of condensation beads. The condensing water vapor passes thermal energy to the glass, which passes thermal energy to the cold drink, which ends up getting warmer as a result.
Thermodynamics can tell you how much heat you’re radiating away on a cold day, how many bags of ice you need to cool a lava pit, and anything else that deals with heat energy. You can also take the study of thermodynamics beyond planet Earth. Why is the darkness of space cold? Here on Earth, air insulates you and the world stays warm even after the Sun sets. In space, there is no air, and temperatures can drop hundreds of degrees as you go from sunlight to shadow (although it takes time for spacecraft, moons, and people who forgot their space suits to cool off).
Radiating heat is just one of the three ways heat can be transferred. You can discover plenty more about heat, whether created by a heat source like the Sun or by friction, through the topics in Part 4.
