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2. Science and the Universe and the ideas it supports (or does not support). In doing so, you will gain experience in using the scientific method as you learn about our modern understanding of our surroundings. The task of physical science is to describe the entire universe, from its tiniest components to its largest collections of matter, living and nonliving, and to understand the rules governing its behavior. To begin, we will sketch a description of the universe and show how the universe is constructed from a few simple components. You may think of this description as a kind of map of the material we will discuss in this book. Each step in the description will be elaborated in subsequent chapters, where we will elaborate each level of description, explain the rules governing the changes that occur, and present some of the relevant evidence. Humans have always been curious about their environment and how they relate to and control it. Earthquakes, volcanoes, hurricanes, and drought are examples of natural phenomena that have affected lives in important ways. People have sought to control these phenomena, or at least their impact on human lives. Over the years people have built “models” or “schema” of how natural phenomena worked. In earlier times these models often claimed a supernatural relationship among humans, gods, and natural phenomena. Our models of the physical world today have evolved significantly from those of our ancestors of just a few generations ago. Whatever the motivation, we now know more about the universe than our ancestors did. Their curiosity and study helped unveil a structure and order that is more profound, yet simpler, than they could have imagined. We truly do live in the age of science. Our lives are partly controlled and greatly enriched by the fruits of our knowledge, and science gives us the power to continue improving the conditions under which we live. Those who have little control over their society might argue that they need not pay attention to the knowledge and ideas of science. However, in a free society, citizens are often able to make decisions about the interaction of science and their lives. Wrong choices might unleash a destructive mechanism or might deny them the use of a technology that could be the basis of future prosperity and peace. The freedom to choose implies the responsibility to understand. If we use our knowledge unwisely, we have the power to destroy our civilization. Our purposes in this book are to describe the universe and the rules that govern it and to help you gain some experience with the scientific method of thinking. We will do this without using sophisticated mathematical notation even though the description is more elegant in that form. We cannot describe every detail in a book of this size, so we have chosen those parts of the universe that seem to us most interesting and important and those rules or laws that have the broadest range of application. Further, we will explain some of the evidence that leads us to believe that what we describe is valid. You will gain the most from your study if you make sure you understand the relationship between the evidence The World Around Us As we go through life we encounter a dazzling array of objects and materials. Bricks, rocks, sand, glass, soil, air, cans, footballs, rain, mountains, trees, dogs, and many other things are forms of matter that enrich our lives. And there is motion all around. Rain falls, rivers flow, the wind blows, cars and people start and stop, waves move across a lake, objects fall to the ground, smoke rises, the sun and stars move through the heavens, and the grass grows. Matter also seems to change form in arbitrary ways. Wood burns and disappears, whereas water does not burn but may disappear all the same. This variety in motion and matter at first seems unfathomable. How can mere humans, so limited in senses and mobility, hope to comprehend it all? Can any order exist in such diversity? Are there rules which govern change? The answers have come through the centuries, little by little. Gifted and persistent people have learned to ask the right questions and how to induce nature to yield the answers. Each stands upon the shoulders of those who went before and thereby gains a more complete view. We together stand at the apex of a great pyramid of giants from which we view the truth more completely than people in any other age. What we see is astounding. Much of the physical 11 Some objects have a characteristic called electric charge. Charge may be positive or negative and is a characteristic associated specifically with the electromagnetic force. Objects with like charges are repelled by the electromagnetic force while objects with opposite charges attract one another. Objects may also have a characteristic called mass. Objects with mass are attracted (never repelled) by the force of gravity. world can be understood simply. Matter is made up of only a few kinds of pieces, which can be arranged in countless ways. The motion we see around us depends on just a few simple rules. Changes in form and substance are also easy to understand in terms of a few comparatively simple ideas. When these rules and ideas are understood, chaos becomes order. Order and law really do govern our world. Even living things seem to operate on the same principles. The laws of force and motion and chemical change govern the processes of life as well as the behavior of nonliving objects. But the view is not yet complete. As we consider our knowledge and observations, we encounter questions for which the answers are not yet known. Perhaps we have not asked the right questions. Perhaps we are just not yet wise enough to understand the answers. At any rate, asking and trying to solve the puzzle is half the fun. We will try to let you share the mysteries as well as the answers as we proceed. A distinctive nomenclature is worth noting. When we speak of objects too small to be seen without a microscope, we refer to them as microscopic objects. Atoms, molecules, and their constituents are microscopic, as are most living cells. Objects large enough to be seen without the aid of a microscope are macroscopic. Thus, one way to characterize this chapter is to say that we are describing the macroscopic parts of the universe in terms of its microscopic constituents (a strategy called reductionism). This, as you will see, is the key to understanding the structure and behavior of the universe in terms of a few simple ideas. It is often useful in the study of physical objects to categorize and compare them on the basis of their size and the forces that hold them together. The size of a physical object may be given in terms of its spatial dimensions. People-sized objects have typical dimensions of a meter or a few meters or a fraction of a meter. Much smaller objects, such as cells in the human body, have typical dimensions of micrometers (millionths of a meter). The extremely small nuclei of atoms typically have dimensions of milli-micro-micrometers (thousandth-millionth-millionths of a meter). Buildings have dimensions of a few tens to a few hundreds of meters. The earth is approximately spherical in shape with a diameter of about 13,000 kilometers. The earth moves about the sun in an approximately circular orbit with a diameter of about 300 million kilometers. The Milky Way has a diameter of about 100,000 light years. (A light year is approximately 10 million million kilometers.) There are four basic forces in nature: strong force, electromagnetic force, weak force, and gravity. In some structures these four forces may be at work simultaneously and may even have opposite effects. The strong force is operative only over very short distances while the electromagnetic force and gravity, in contrast, reach much further, although they weaken with distance. Nuclear Matter All matter as we currently understand it is made up of elementary particles, point-like objects without size or structure. Among these particles we number quarks and electrons. The electron carries a unit of negative electric charge. Quarks are charged particles, each carrying a positive or negative charge equal to one-third or two-thirds the charge of a single electron. Structures called nucleons consist of three quarks bound together by the strong force. Positively charged nucleons (called protons) are made of two quarks with charge !2/3 and one with charge "1/3. Neutral nucleons (called neutrons) have one quark with charge !2/3 and two with charge "1/3, adding together to yield zero net charge. Nucleons are so small that it would take one million million (or 1012) lined up next to each other to reach across the head of a pin. (We will use the notation 1012 [spoken “ten to the twelfth”], because it is an easy way to keep track of the zeros in large or small numbers. By 1012 we mean that we start with 1.0 and move the decimal 12 spaces to the right, resulting in the number 1,000,000,000,000. On the other hand, 10–12 would mean that the decimal point is moved 12 spaces to the Figure 2.1. Models of atomic nuclei: (a) helium, (b) oxygen, (c) uranium. 12 left, resulting in the number 0.000,000,000,001.) Nucleons are so dense, however, that a pinhead-size ball made of nucleons packed next to each other would weigh about a million tons. No crane could lift it. Nucleons coalesce into incredibly small lumps containing from 1 to 238 nucleons, half or more of which are neutrons and the rest protons. Each of these tiny aggregates is the nucleus of an atom (Fig. 2.1). Larger collections of nucleons have been formed in laboratories, but these always break up quickly into smaller groups. The strong force also holds the protons and neutrons in the nucleus of an atom together. Nucleons attract each other (that is, protons attract other protons as well as neutrons; neutrons do the same) by means of the strong force. This means the strong force must overwhelm the electromagnetic repulsion of the positively charged protons. (The electromagnetic force holds atoms, molecules, and people-sized objects together where the separations exceed the range of the strong force.) The strong force is responsible for the energy released by the sun, nuclear reactors, and nuclear explosives. The weak force is also involved in the nucleus but does not control any of the common structures. Some of the nuclei found in nature are unstable. These spontaneously emit high-speed particles. Such nuclei are called radioactive. There is normally one electron in the atom for each proton in a nucleus, so that the atom is electrically neutral. Neutrons are in atomic nuclei as well, but the number may vary for atoms that are otherwise identical. Compared with its nucleus, an atom is enormous. If you imagine the nucleus to have a diameter the size of a ballpoint pen tip, the atom would have a diameter equal to the length of a football field (Fig. 2.2). An atom is mostly empty space. In some ways the nucleus is like a small gnat in the center of a large building. The walls and ceiling of the building and all the space inside are patrolled by the electrons, which move rapidly about like a swarm of bees protecting the atom from intruders. Atoms are 100,000 times as large as their nuclei, but they are still so small that 5 million are needed to form a line across the smallest dot. The electrons have little mass (about 1/1,836 that of nucleons), so atoms have about the same mass as their nuclei. A pinheadsize ball of atoms has about 1021 atoms and weighs about as much as a pinhead. Although individual atoms are much too small to see, you are undoubtedly familiar with objects composed of large groups of essentially identical atoms. For instance, a copper penny is made of approximately 30 billion trillion (3 # 1022) copper atoms. A material like copper composed of only one type of atom is called an element. Additional examples are iron, helium, and uranium. Atoms Molecules and Crystals Each atomic nucleus carries a positive electric charge and attracts a certain number of negatively charged electrons. The nucleus and electrons together form an atom. Atoms, in a variety of combinations, make up matter as we know it. The tiniest speck of dust visible to the unaided eye contains about 1018 atoms. A sample of air the size of a sugar cube has about the same number. Certain atoms join together in small groups by sharing electrons in a way that takes advantage of electromagnetic interactions. Such a group of atoms is called a molecule. Molecules are the basis of many of the common materials you see around you. Sugar is composed of molecules, each containing 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms. Many molecules contain fewer than 50 atoms, although polymers like nylon are long chains that may contain a million or more. A molecule of the common fuel butane is shown in Figure 2.3. Figure 2.3. A butane molecule (carbon atoms are shown in black, hydrogen in white). Figure 2.2. An atom is mostly empty space. On this scale, the nucleus is still only the size of a ballpoint pen tip. 13 Molecules do not deteriorate easily, as your experience with sugar will tell you. Sugar does not spontaneously change into some other material. Yet many common processes can tear molecules apart and reassemble them in different ways. For instance, sugar can be burned. It can also be digested to release its stored energy for use in muscles. This stored chemical energy (based on electromagnetic forces) has been mankind’s most common source of energy. Atoms attract each other, because the protons in each atom and the electrons in its neighbors are attracted to each other by the electromagnetic interaction. Adjacent atoms do not get too close, however, because the positively charged protons in each atom repel the protons in the other. The strong force is inoperative at these distances. Electrons also repel each other. The net result of these electrical attractions and repulsions is the force that holds atoms together. We feel this force when, for example, we tear a piece of paper (separating some of its atoms from each other), bend a piece of metal, strike our head against a solid object, or walk across a room. In fact, these interatomic electric forces are involved in almost everything we do and are responsible for almost all the forces we experience directly. Most common materials contain several kinds of molecules. Milk has over a hundred kinds of molecules and the human body has somewhere near 50,000. The task of identifying important molecules and studying their properties has been one of the great challenges of modern chemistry and biology. Some materials are just large numbers of identical atoms or molecules piled on top of one another. In liquids these slide around each other much like small ball bearings or buckshot in an open can. In solids the atoms sometimes arrange themselves in an orderly array called a crystal. For example, common table salt is a collection of equal numbers of sodium and chlorine atoms in a cubical arrangement. Many solid materials are collections of small crystals held together by the electrical force. The type of atomic organization in crystals generally determines the properties of the bulk material. Carbon atoms, for example, can be arranged in two different ways—one forms diamond; the other, graphite (the “lead” in a pencil). Diamond is clear, colorless, and hard; graphite is opaque, black, and soft. Yet both are composed of the same kind of atoms. Color Plate 1 (located in the color photo section near the end of the book) shows regularly ordered carbon atoms in graphite as imaged with a scanning tunneling microscope. Color Plate 2 shows regularly spaced sulfur atoms in molybdenum disulfide as imaged with a scanning tunneling microscope. es of molecules. Our bodies are composed of various bony and tissue structures which are very large integrated collections of complex molecules. The living plants and animals around us share similar molecular complexes in their structure. The fuel we burn may be composed of homogeneous collections of molecules, as in natural gas, or heterogeneous collections of molecules, as in wood. Buildings are made of steel and concrete and glass; vehicles of metal and plastic. Each in turn is composed of molecular complexes. The Earth The earth on which we live is a huge ball with a radius of almost 6400 kilometers (4000 miles). It is so large that we generally perceive it to be flat from our perch upon its surface. We do not generally notice that the surface of a lake curves downward so that it is about 30 feet higher at our feet than it is 5 miles away. Nevertheless, pictures taken from space reveal the overall spherical shape, a shape which has been known indirectly for centuries. The outer layer, or crust, is a comparatively thin skin composed of a variety of rocks and materials. The mountains, which seem so magnificent and overpowering to us, are no more than the smallest wrinkles when compared with the earth as a whole—thinner, by comparison, than the skin on an apple. We may think of the whole earth as being the same as the crust we experience. But the crust is not at all representative of the interior (Fig. 2.4). The core of the earth is thought to be a hot (3500 °C or more) ball of iron and nickel under tremendous pressure. The core seems to have two parts: a solid inner core and an outer core. The latter has many properties normally associated with liquids. The core constitutes about 30 percent of the earth’s volume and one-half its mass. Inner Core Mantle Outer Core Complexes of Molecules Some physical objects that we have firsthand experience with are composed of one or more complex- Figure 2.4. The internal structure of the earth. 14 Surrounding the core is the mantle, a 2900-kilometer-thick layer of solid rock that constitutes most of the earth. The mantle is composed almost entirely of rocks made of the elements silicon, oxygen, magnesium, and iron. Evidence indicates that its temperature ranges from 2700 °C just outside the core to 1000 °C just inside the crust. The rigid outer layer of the earth is divided into several sections, or plates, upon which the continents rest. These plates move slowly over the surface of the earth, sometimes colliding with each other with enormous force and sometimes separating to leave a rift through which molten rock from lower levels may escape onto the ocean floor. Many of the phenomena we observe (e.g., earthquakes, volcanic activity, and mountain building) can be understood in terms of the motion of these plates. Their discovery and study, a field of inquiry known as plate tectonics, has been one of the major triumphs of modern geology. The gravitational and electromagnetic forces combine to govern the size of the earth. Each piece of the earth is attracted to every other piece by gravity, the result being a net force directed toward the center of the earth. As the atoms that make up the earth are pulled close together by gravity, their interatomic (electromagnetic) forces begin to resist. Otherwise, the earth would collapse into a much smaller ball. The nuclear force also plays an important role in the earth’s dynamics, releasing energy from radioactive nuclei that keeps the interior of the earth hot. Neptune, and Pluto. Pluto, usually the outermost, travels in an elliptical orbit that varies from 4 to 5.5 billion kilometers from the sun and sometimes carries the planet inside the orbit of Neptune. Again, the scale is hard to comprehend. If we were to start today and travel with a constant speed of 40,000 kilometers/hour, about as fast as the fastest rocket, it would take about 14 years to reach Pluto. The planets differ in their speeds as they travel around the sun. Mercury, the fastest at a speed of 170,000 kilometers/hour (110,000 miles/hour), completes its orbit in just 88 days. Pluto, the slowest, travels only one-tenth as fast and takes almost 250 years to complete its orbit. The earth’s orbital speed is a moderate 107,000 kilometers/hour (67,000 miles/hour). The sun governs these motions through the gravitational force that reaches out through the immensity of space to hold the planets in their orbits. The sun itself is a vast collection of atomic nuclei, mostly hydrogen, and electrons. These charged particles are free to move about independently of one another in a kind of gaseous state called a plasma. (Over 99 percent of all visible matter in the universe is in the plasma state.) The temperature of the sun is quite high, ranging from about 15 million degrees Celsius at the center to about 5500 °C near its surface. The nuclear furnaces of the sun provide the light that illuminates its satellites. This light is the principal source of terrestrial energy, providing the energy for atmospheric motion, for plant and animal growth, and for virtually every process that occurs on the planetary surface. The Solar System The Milky Way Galaxy and Beyond Circling the sun with the earth are eight other planets (with their moons), several comets, and a variety of smaller objects called asteroids. Together these bodies form the solar system (Fig. 2.5). The nine planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, The sun is just one of the billions of stars, a few of which can be seen on any clear night, particularly if interference from artificial lighting is not too great. Those closest to us form the Milky Way galaxy (Color Plate 3, see color photo section near the end of the book), an immense collection of 100 billion stars held together by their mutual gravitational attractions. The stars of the Milky Way are, on the average, about 30 trillion miles apart, a distance so great that it takes light six years to traverse it. The distance that light can travel in a year is called a light-year. The galaxy itself is 600,000 trillion miles across; it requires 100,000 years for light to go from one side to the other, so the diameter of the galaxy is about 100,000 light-years. If the universe were to shrink so that the sun was reduced to the size of an orange, the stars in the galaxy would be about 1,000 miles from their nearest neighbors and the galaxy as a whole would be 20 million miles across. The picture is still not complete. Millions of galaxies have been seen through our most powerful telescopes. Each contains billions of stars. Some galaxies are grouped together in clusters, with individual clusters Figure 2.5. The solar system. 15 26 10 containing as many as 10,000 galaxies. Our Milky Way is part of a smaller cluster, called the Local Group, which contains one other spiral galaxy and several fainter objects. The typical distance between galaxies in a cluster is a million light years. With all this, keep in mind that the universe is mostly empty space. The stars and galaxies, although immense from our perspective, are mere specks when compared to the immensity of the universe in which they move. The space between them is emptier than the most perfect vacuum attainable on the earth. ? Universe 24 10 Clusters of Galaxies 22 10 20 10 Galaxies 18 10 10 Distance to nearest stars 14 Summary Gravity 16 By now you might feel a little unstable. Think of the range of things we have described—from nucleons so tiny that a quadrillion of them could fit in a line across a small pinhead, to clusters of galaxies so vast that even light takes many millions of years to go from one side to the other. As the structure is built up level by level, perhaps you can see that each level of organization is a logical combination of simpler ones. Try not to be overwhelmed by all the numbers and names. The important names will recur in subsequent chapters so that you will become familiar with them as we proceed. The short exercises at the end of this chapter will help you to put things into proper perspective. The purpose of this chapter is to help you develop an accurate framework into which you can fit the more complete and precise information that follows (Fig. 2.6). 10 12 Solar System 10 10 10 8 10 6 10 4 10 2 10 0 Planets Continents Mountains Historical Perspectives Plants, Animals, People -2 10 -4 10 10 One-celled organisms, bacteria -6 -8 10 10 -10 10 -12 -16 10 Molecules Atoms Nucleus Protons, Neutrons Quarks ? Strong Force -14 10 Viruses Science as practiced today has evolved over five or so millennia. Some early roots of science may have appeared as early as 3000 B.C. in observations of the heavens. The Babylonians developed the “art” of astrology from their observations and charting of lunar cycles and the apparent motions of the sun and planets. The Egyptians had a rather sophisticated understanding of the seasonal cycles, probably motivated by their need to predict the yearly overflow of the Nile. At Stonehenge in England stones were arranged so as to predict the eclipses. In these civilizations the apparent motion of the sun and the planets played an important role. The Greek civilization produced many philosophers who pondered nature and described its workings. As we have already noted, Pythagoras (ca. 550 B.C.) introduced the notion of a spherical earth and a spherical universe. Democritus (ca. 450 B.C.) introduced the notion of the atom as the smallest particle into which matter could be divided. Aristotle (ca. 350 B.C.) envisioned a universe consisting of a spherical earth surrounded by spherical shells containing the planets and stars. Aristotle taught the young Alexander who became Alexander the Great and who established a city Electromagnetic Force Size [in meters] 10 Stars Figure 2.6. The sizes of things. How much larger than 10n is 10n+1? 16 and center of learning at Alexandria, Egypt. Archimedes (ca. 250 B.C.) and Ptolemy (ca. A.D. 150) were two of many important pupils of the Alexandrian Academy. The Ptolemaic model of the universe had a spherical earth at rest at its center. The planetary motions were explained in terms of epicycles—one circular motion about a point which in turn moved in a circular motion about some other point. When Islamic forces conquered Alexandria (ca. A.D. 500) there was a flow of scientific information to the East. Baghdad became a center for the exchange of knowledge, and many works were translated into Arabic. Much of the body of scientific knowledge was preserved and enlarged in nations under Islamic influence. Many Greek ideas were preserved during this period at Constantinople, which was not conquered by Islamic forces until the 15th century. The Dark Ages encompassed Europe until about the 15th century, when the Renaissance developed. As the Greeks lost Constantinople they fled into Europe and carried with them their scientific and cultural treasures. At this time the Moorish influence in southern Spain also provided an infusion into Europe of the science preserved by the Islamic culture. In England, Francis Bacon (1561-1626) introduced the inductive method, in which observations of many specific cases are generalized as the laws of nature. In contrast, the deductive method employs general assumptions (which may or may not be true) from which specific conclusions are logically deduced. C. GLOSSARY 1. Atom: A structure made up of a nucleus (containing protons and neutrons) and surrounding electrons. The electrons are bound to the nucleus by the electromagnetic force. 2. Core: The spherical center of the earth. The solid inner core consists of iron and nickel while the liquid outer core surrounds the inner core and consists of molten iron and nickel. 3. Crust: The relatively thin outer layer of rock that forms the surface of the earth. 4. Crystal: A form of solid in which atoms or molecules arrange themselves in orderly arrays to create distinctive geometric shapes. Common table salt exists as crystals. 5. Electric Charge: A characteristic of objects that determines the strength of their electromagnetic interaction (force) with matter, specifically with other charged objects. 6. Electron: A particular kind of elementary particle that carries a negative charge, has an electromagnetic interaction with matter, and is a constituent part of atoms. Electrons are best represented as a point without spatial extent. 7. Element: A substance made up of atoms, all of which contain the same number of protons. Hydrogen, helium, silver and gold are elements. 8. Light-Year: The distance light can travel in one year, i.e., about 6 trillion miles. 9. Macroscopic: A descriptive adjective referring to the sizes of objects large enough to see with the unaided eye. Automobiles and basketballs are macroscopic objects. 10. Mantle: The spherical shell of rock that lies under the crust of the earth but overlies its core. 11. Mass: A characteristic of objects that determines the degree to which they can be accelerated by applied forces. Mass is also a characteristic of objects that determines the strength of their gravitational interaction with matter, specifically with other objects with mass. 12. Microscopic: A descriptive adjective referring to the sizes of objects at the limit of visibility with the unaided eye or smaller. Molecules and atoms are described as microscopic objects. 13. Molecule: A microscopic structure usually made up of more than one atom. 14. Neutrino: A particular kind of elementary particle that carries no electrical charge, is best represented by a point without spatial extent, and is particularly notable for having neither a strong nor an electromagnetic interaction with matter. The neutrino interacts with matter through the fundamental force called the “weak force.” 15. Neutron: A composite, strongly-interacting particle made up of three quarks, but which carries no STUDY GUIDE Chapter 2: Science and the Universe A. FUNDAMENTAL PRINCIPLES 1. The Strong Interaction: The interaction between objects that gives rise to one of four fundamental forces in nature, called the “strong force.” The strong force is a short-range, nuclear force which is responsible for the binding of the nucleus together as a structure. 2. The Electromagnetic Interaction: The interaction between objects that gives rise to the electrical (or, better, the electromagnetic) force. The electromagnetic force is also fundamental and is responsible for binding atoms and molecules as structures. 3. The Gravitational Interaction: The interaction between objects that gives rise to the weakest of the fundamental forces, the gravitational force. The gravitational force is responsible for binding structures such as the solar system and galaxies. B. MODELS, IDEAS, QUESTIONS, OR APPLICATIONS None 17 16. 17. 18. 19. 20. 21. 22. 23. 24. net electrical charge. Neutrons are a constituent part of the nucleus of atoms. Nucleon: A generic name for either a proton or a neutron. Nucleus: The very small core structure at the center of an atom. The nucleus is a structure of protons and neutrons held together by the strong force. Plasma: A physical state of matter (such as solids, liquids, and gases) that is characterized by fluid properties, but in which particles with positive and negative electric charges move independently. Plates: Pieces or sections of the fractured rigid outer layer of the earth on which the continents and ocean basins sit. Proton: A composite, strongly interacting particle made up of three quarks. The proton carries a positive electrical charge and is a constituent part of the nucleus of atoms. Quarks: The elementary particles of which protons and neutrons consist. A proton and a neutron each consist of three quarks. Reductionism: A strategy of science to understand complex structures by reducing them to their smaller and simpler parts. Solar System: A star with its associated revolving planets, moons, asteroids, comets, etc. Weak Force: One of four fundamental forces of nature (strong, electromagnetic, weak and gravity). Unlike the other three, the weak force is not directly associated with binding together the common structures of the universe. 2.2. By analogy or number, contrast the size of the nucleus and the size of the atom. 2.3. By analogy or number, contrast the distances between stars, the size of the galaxy, and the distance between galaxies. 2.4. Describe the organization of the universe. Show how clusters of galaxies are ultimately composed of the simplest entities we know about. 2.5. Is it true that matter is “mostly empty space”? Explain what this statement means by describing the real structure of (a) an atom (b) a steel ball bearing (c) a galaxy 2.6. Of the five levels of organization listed here, which is second in order of increasing size and complexity? (a) quark (b) apple (c) moon (d) gold nucleus (e) protein molecule 2.7. Which of the following forces is electrical? (a) weight of a book (b) force exerted by book on table (c) gravitational force of earth (d) force keeping the moon in orbit (e) force keeping the solar system together D. FOCUS QUESTIONS 1. Identify at least five levels of organization observed in the universe. Describe these levels of organization in order, beginning with the smallest, and explain how each structure is held together. Identify the fundamental force which dominates in each structure. E. EXERCISES 2.1. For each of the following structures, identify their primary constituent parts and their sizes and the fundamental force(s) which maintain the integrity of the structure. cluster of galaxies galaxy solar system star earth crystal molecule atom nucleus nucleon quark electron 18