Physics Essentials For Dummies

Chapter 1

Viewing the World through the Lens of Physics

In This Chapter

Recognizing the physics in your world

Getting a handle on motion and energy

Wrapping your head around relativity

Mastering measurements

Physics is the study of your world and the world and universe around you. You may think of physics as a burden — an obligation placed on you in school. But in truth, physics is a study that you undertake naturally from the moment you open your eyes.

Nothing falls beyond the scope of physics; it’s an all-encompassing science. You can study various aspects of the natural world, and, accordingly, you can study different fields in physics: the physics of objects in motion, of forces, of what happens when you start going nearly as fast as the speed of light, and so on. You enjoy the study of all these topics and many more in this book.

Figuring Out What Physics Is About

You can observe plenty going on around you all the time in the middle of your complex world. Leaves are waving, the sun is shining, the stars are twinkling, light bulbs are glowing, cars are moving, computer printers are printing, 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:

Why do I slip when I try to climb that snow bank?

What are those stars all about? Or are they planets? Why do they seem to move?

What’s the nature of this speck of dust?

Are there hidden worlds I can’t see?

Why do blankets make me warm?

What’s the nature of matter?

What happens if I touch that high-tension line? (You know the answer to that one; as you can see, a little knowledge of physics can be a lifesaver.)

Physics is an inquiry into the world and the way it works, from the most basic (like coming to terms with the inertia of a dead car that you’re trying to push) to the most exotic (like peering into the very tiniest of worlds inside the smallest of particles to try to make sense of the fundamental building blocks of matter). At root, physics is all about getting conscious about your world.

Paying Attention to 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 will you have to move to get out of its way? (Hang on just a moment while I get out my calculator . . .) Motion was one of the earliest explorations of physics, and physics has proved great at coming up with answers.

This book handles objects in motion — from balls to railroad cars and most objects in between. Motion is a fundamental fact of life and one that most people already know a lot about. You put your foot on the accelerator, and the car takes off.

But there’s more to the story. Describing motion and how it works is the first step in really understanding physics, which is all about observations and measurements and making mental and mathematical models based on those observations and measurements. This process is unfamiliar to most people, which is where this book comes in.

Studying motion is fine, but it’s just the very beginning of the beginning. 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 the 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.

Motion changes all the time as the result of force. You may know the basics of force, but sometimes it takes an expert to really know what’s going on in a measurable way. In other words, sometimes it takes a physicist like you.

Getting Energized

You don’t have to look far to find your next piece of physics. 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, locked together, they’re sliding your way.

Thanks to physics you can make the necessary measurements and predictions to know exactly how far you have to move to get out of the way. You know that it’s going to take a lot to stop the cars. But a lot of what?

It helps to have the ideas of energy and momentum mastered at such a time. 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? Not from nowhere — if it did, you wouldn’t have to worry about the price of gas. Using gas, the engine does work on the car to get it up to speed.

Or say, for example, that you don’t have the luxury of an engine when you’re moving a piano up the stairs of your new place. But there’s always time for a little physics, so you whip out your calculator to calculate how much work you have to do to carry it up the six floors to your new apartment.

After you move up the stairs, your piano will have what’s called 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. It’s an interesting process to watch, and 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.

Moving as Fast as You Can: Special Relativity

Even when you start with the most mundane topics in physics, you quickly get to the most exotic. In Chapter 12, you discover ten amazing insights into Einstein’s theory of special relativity.

But what exactly did Einstein say? What does the famous E = mc2 equation really mean? Does it really say that matter and energy are equivalent — that you can convert matter into energy and energy into matter? Yep, sure does.

And stranger things happen when matter starts moving near the speed of light, as predicted by your buddy Einstein.

“Watch that spaceship,” you say as a rocket goes past at nearly the speed of light. “It appears compressed along its direction of travel — it’s only half as long as it would be at rest.”

“What spaceship?” your friends all ask. “It went by too fast for us to see anything.”

“Time measured on that spaceship goes more slowly than time here on Earth, too,” you explain. “For us, it will take 200 years for the rocket to reach the nearest star. But for the rocket, it will take only 2 years.”

“Are you making this up?” everyone asks.

Physics is all around you, in every commonplace action. But if you want to get wild, physics is the science to do it.

Measuring Your World

Physics excels at measuring and predicting the physical world — after all, that’s why it exists. Measuring is the starting point — part of observing the world so that you can then model and predict it. You have several different measuring sticks at your disposal: some for length, some for weight, some for time, and so on. Mastering those measurements is part of mastering physics.

To keep like measurements together, physicists and mathematicians have grouped them into measurement systems. The most common measurement systems you see in physics are the centimeter-gram-second (CGS) and meter-kilogram-second (MKS) systems, together called SI (short for Système International d’Unités). But you may also come across the foot-pound-inch (FPI) system. For reference, Table 1-1 shows you the primary units of measurement in the MKS system, which I use for most of the book. (Don’t bother memorizing the ones you’re not familiar with now; come back to them later as needed.)