A Brief Survey of Physics…
Look around you, inside a building, or outdoors. Most things are still, forming a big frame or backdrop, and within that frame some things move. A pen moves across a page, a leaf blows through the air and falls downward, a living being stirs, breathing, blood pumping within…A large truck drives by, a tiny insect flies along, comes to a stop on some surface, crawls along slowly …
Mechanics is the study of the motion of things, large and small. We describe their motion, in terms of distance traveled and the time that passes while they are traveling, and we also explain why they get their motion and lose their motion—what the cause of motion is.
We find that to get a pretty good picture of what’s going on it is important to keep track of these four quantities: the time, the object’s position, how fast it is going, and how quickly it is speeding up or slowing down.
We find that there are many regularities in nature—under the same conditions, things move the same way. These are called laws of physics. One of the most important principles we know is called the principle of relativity: that the laws of physics are the same regardless of how fast you are moving. For example, if you are throwing a ball back and forth in a moving train car it is pretty much the same as if you are throwing a ball back and forth in a parked train car.
It’s different if the train is speeding up or slowing down. You’ll have to shift your weight or you’ll fall. You’ll have to adjust your aim when you throw the ball, and compensate when you try to catch it. Speeding up and slowing down is very important in physics; new phenomena occur.
The best way to describe what’s going on is in terms of objects pushing against other objects. All over, objects are pushing and pulling each other, in different amounts, and in different directions. We say “pushing and pulling”, though a pull is just a push in the opposite direction. Normally all the pushes and pulls balance out, and nothing speeds up or slows down. When a new push is introduced, and it is not balanced out, then something speeds up or slows down.
There are many kinds of pushes and pulls. One of the simplest is your hand pushing on an object. If that object is touching another object, it will push on that other object, and the push gets transmitted. In fact, inside the object one part will push against another part—the atoms push on other atoms. A spring gives a vivid example of this—you can see the whole thing stretching, and the harder you pull the ends apart, the more it stretches.
When you drop a ball, what makes it fall to earth? Gravity is a fascinating source of a pull between two objects that aren’t even touching. The ball and the earth pull on each other, and the ball speeds up, toward the earth. This phenomenon is universal, as all objects pull on each other. We notice it between the earth and the moon, between the sun and the earth, and between the sun and each of the planets.
Mechanics was the first branch of physics to be figured out. It is the foundation for much of the rest of physics. Other physical laws, for example those describing electricity and those describing wave motion, have to mesh with the description of nature formulated in the branch of physics called mechanics.
Mechanics problems can be solved by keeping track of the pushes and pulls between objects. Another way of analyzing motion is in terms of a quantity that is usually invisible but seems to have a fundamental importance…energy.
Energy is not something we would think of paying attention to, except for three reasons: 1) it is necessary for motion, 2) it can take different forms, and especially 3) there is always the same limited amount of it.
Let’s look at each reason.
1) Energy is necessary for motion.
There is a way of calculating how much energy of motion something has, based upon how much matter is moving, and how fast it is going. The more matter, the more energy. The faster it goes, the more energy. Alone, this is no big deal at all, but we also find that…
2) Energy can take different forms.
When a moving object is slowed down or stopped by rubbing against another object, we notice that it heats up. Less motion, but more heat. Likewise, when an object slows down as it is traveling upward, such as what happens when you throw a ball straight up, it’s obvious that with less motion, there is more altitude. We find similar regularities: less motion along with more sound, less motion accompanied by more light, less motion at the same time as more reactive chemicals. It seems to be a physical law that any time an object slows down and loses some speed, it gains something else in compensation. If we are clever about our calculations, we can always find an equation for the “something else” that it gains, so that we can calculate an amount of “height energy,” or chemical energy, or sound energy, or light energy. These calculations show that…
3) There is always the same, limited, total amount of energy.
Whenever we look very carefully at some process, and add up all the different kinds of energy, we find that the total stays the same, regardless of what kinds of changes take place. This happens so regularly that we have a lot of confidence that it will always happen, and that it is a fundamental law of physics.
This law turns out to be a nice shortcut for calculating things like how high a ball will go if we throw it upward at a certain speed, or how much light will be given off when a certain number of electrons pass through a wire at a certain speed.
Energy can be “stored”—put into a form where it can be easily kept, and maybe easily moved from one place to another. Electric batteries and chemical fuels come to mind.
This very abstract and somewhat crazy idea of a quantity that we can’t hold or see but somehow always stays in the same amount is cherished by physicists because it ties together a lot of theoretical ideas about different branches of physics, and because it has very practical uses.
Electricity and Magnetism
We learned that gravity is a phenomenon in which objects pull on each other, even though they are not touching, and may be very far away from each other. There is another phenomena that is similar, and it has to do with electricity.
As far as gravity goes, there is only one type of matter, and the more matter there is in either object, the greater the pull of gravity between the objects. There seem to be two different kinds of matter when it comes to electricity, though. A better way of describing it is to say that matter itself has different properties: a gravitational property, and an electric property.
There is only one kind of gravitational property, but two kinds of electric property. Two objects that have the same kind of electric property will push away from each other. Two objects that have the different kinds of electric property will pull toward each other. We say “likes repel”, and “opposites attract.”
When we look at matter on a very small scale, very tiny pieces, we find that matter is made of small, stable structures. “Structures,” because they are built from even smaller objects. “Stable,” because the structures seem to form naturally. They usually stay together quite strongly. These little structures are called “atoms,” the building blocks of matter.
The even smaller objects that atoms are built from sometimes do escape from an atom, and we know a lot about them. The protons are bigger, and carry one type of electric property. The electrons are smaller, and have the other type of electrical property. And there is a third thing called a neutron, which has both types of electrical property in equal amounts, so they cancel out.
A large-scale object that has more protons than electrons will have one type of electric property. If an object has more electrons than protons it will have the other type of electric property. The electrons escape from atoms easily, and they can pass through the spaces in atoms and between atoms.
We can control the flow of these electrons—we make a pump (such as a battery) that keeps pushing the electrons through a metal wire loop. Interesting things happen—the loop gets hot. Sometimes it glows. And two of these loops will push or pull each other—we call this kind of push or pull magnetism!
Why magnetism? Does this have anything to do with the ordinary magnets we are familiar with? The answer is yes—all magnets get their magnetism from electrons going around in loops. It is a natural effect, that just seems to happen whenever electrons move around. The magnetism of ordinary magnets happens because of electrons moving around in atoms of iron and certain other kinds of atoms.
Another effect just seems to happen when any magnet is moved—nearby electrons get pushed or pulled around. There is a symmetrical relationship. When electrons move it gives a magnetic push to any magnets in the area. When a magnet moves it gives an electric push to any electrons in the area.
Engineers take advantage of these phenomena to make electric motors and electric generators, among other things. As important as these are, nature itself has used these phenomena to make something even better. Light itself is a cycle of electric and magnetic pushes that feeds on itself, moving through space in a wave motion. Here we see many different physical phenomena combine to create something really wonderful but so common that most of the time we take it for granted.
An interesting way that something happening here can affect another object over there is by wave motion. For example, drop a rock into a pond, and the ripples will make a leaf floating in the water far away bob up and down a bit.
The water itself doesn’t move very far. Instead, water moving up and down in one place makes the water all around it move up and down, and all that water makes the water around it move up and down, and this spreads out symmetrically over the surface of all the water—a ripple, or wave. It’s a little bit like falling dominoes, except unlike dominoes the water resets itself, and is ready to carry waves again.
We find that sound works in this way. When sound moves through air the air molecules bounce off each other, each one moving just a little bit until it bounces into the next one. The place where the bouncing takes place moves along…it is one edge of a sound wave.
Certain funny effects occur when two waves from different sources meet and combine. Sometimes the resulting wave becomes stronger than either individual wave, but other times it actually becomes weaker, as though the two waves cancel each other out. The canceling out effect with water waves is the result of one wave being above the water surface at the same time that the other is below the surface. This is such a “wavelike” phenomenon that when we observe two light beams canceling each other out in places and making dark areas we have to conclude that light also has a fundamental wave nature.
We find that the observable properties of light are related to its wave properties. Adjusting how close together the “ripples” of light are changes the color of the light, for example. Pursuing this further, we find that there are many colors of light that we can’t even see: radio waves, infrared waves, ultraviolet waves, and more.
Wave motion is a way for energy to travel from one place to another while the matter in between just moves up and down or back and forth only a little bit.
In the final chapter of physics we take a closer look at atoms. What is the connection between atoms and light? Why are atoms such stable structures—in other words, why do protons, neutrons, and electrons combine to form atoms? What are protons, neutrons, and electrons, anyway?
The electrons and protons have opposite electrical properties, so they attract one another. There seems, however, to be some sort of natural effect that keeps them from getting too close to one another. An electron can very easily settle in to a circular path around a proton or a group of protons, similar to how a planet travels in a path around the sun, pulled by gravity. The electron’s travel is different, and kind of strange, because it seems that the electron actually has a wave motion as it goes around the proton. We know this because the electron shows the funny wave effects that sound, water waves, and light do. There are ways to get two electrons to combine to form…no electrons, in certain areas. In an atom the electron wraps around and does this trick with itself, making for a very natural set of very specific paths it can travel in an atom. These are the orbitals studied in chemistry.
The electron has different amounts of electrical energy and energy of motion in the different paths. When it goes from a higher to a lower energy path it gives off a little energy, in the form of light energy. When it goes from a lower to a higher energy path it has to take energy from somewhere, such as from some light that is shining on the atom.
The nucleus of an atom is made of protons and neutrons. There is a natural pull among protons and neutrons, holding them all close together. This is in addition to the gravitational, electrical, and magnetic pulls we have already read about. The proton and neutron pull is called a nuclear pull, and it is stronger than the electrical push of protons away from each other.
Physicists build instruments to detect protons, neutrons, and electrons, and they find many other kind kinds of particles, too. Sometimes they occur naturally, and sometimes they are made by shooting particles at other particles at high speeds.
All these other particles are unstable, in that they don’t form atoms or any other kind of structure, and they turn into other particles in a short time. We do find some order to their properties, though, and it leads us to think that all of the particles fall into two groups: those that are electrons or similar to electrons, and those that are made of small, fundamental particles that combines in twos or threes. Protons and neutrons are in this second group. They are each made of three of the fundamental particles.
This brief survey of physics was intended to show you some of the most important physical laws and phenomena, which we will study in more depth this year. It should also have given you a sense of the way that different parts of physics fit together.
We will learn technical terms for many of the ideas introduced here, and clarify them a great deal. A large part of that process involves quantifying the relationships: in other words, learning and using equations.
Answer any five. Put some thought into your work. As much as possible, phrase things in your own words, and especially in your own way. You will hand in your work, so don’t do it on this paper.
- List the five topics (branches) of physics we will study this year.
- Give a good description of how sound travels through air.
- What is the connection between pushes and pulls and the speed of an object?
- Why is energy an important quantity? Don’t just say that it’s because we need energy for cars, planes, etc.—give a structured, reasoned explanation.
- If a new physical phenomenon were discovered today, how would physicists decide whether or not it had something to do with waves?
- Compare and contrast gravity and electricity.
- How could you get a rock to make a leaf, 20 meters away, move, without throwing the rock at it?
- Give two different but correct answers to the question “What is matter made of?”
- There are two protons in the nucleus of a helium atom. There are three different kinds of pushes and pulls between the protons. List all three, and state whether they are pushes, or pulls. Also, which of the three is the strongest?