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CHA PTER Energy CONCEPTS — -In A c tio n — How do science concepts apply to your world? Here are some questions you'll be able to answer after you read this chapter. m W hy does a basketball bounce higher than a slice of bread? (Section 15.1) m How can an object from space damage a car? (Section 15.2) m How do gulls use energy conversion to obtain food? (Section 15.2) ■ How do pole vaulters propel themselves high into the air? (Section 15.2) m How can the wind be used to light a bulb? (Section 15.3) Dl^ouerv " “ ™ Ennxnj loot Video Field Trip .......... Physics of Fun How are roller coasters designed to be both safe and thrilling? (page 460) PHYSICS Chapter Preview 15.1 Energ y and Its Form s 15.2 En erg y C on version and C o n servatio n 15.3 En erg y Resources How Can Energy Change Form? Procedure Think About It 1. Examine a flashlight, a solar calculator, and a wind-up toy. Determine how each object operates. 1. Classifying W hat was the source of energy 2. Record the name of each object, the source of the energy that it requires, and the kind of energy it produces. For exam ple, does the object require or produce electricity or light? for each object? 2. Inferring W hat form of energy did each object produce? 3. Drawing Conclusions How did energy change form in each object? For exam ple, did any of the objects convert light into another form of energy? Energy 445 Reading Focus Key Concepts © How are energy and work related? © What factors does the kinetic energy of an object depend on? © How is gravitational potential energy determined? © What are the major forms of energy? Reading Strategy ♦ energy ♦ kinetic energy ♦ potential energy ♦ gravitational potential energy ♦ elastic potential energy ♦ mechanical energy ♦ thermal energy ♦ chemical energy ♦ electrical energy ♦ electromagnetic energy ♦ nuclear energy Building Vocabulary Copy the partially completed concept map below. Then, as you read, complete it with vocabulary terms and definitions from this section. A he road that winds through the m ountain valley was closed. Skiers were banned from the area. The sound of artillery fire echoed from the mountains. It seemed out of place in this picturesque scene of snow-covered peaks surrounding the valley. A m om ent of quiet followed the blast. Suddenly the snow broke loose with a menacing roar. Tumbling down the mountainside, the snow buried everything in its path. You can see in Figure 1 the enormous mass of accumulated snow that had hung above the valley. After the avalanche, the valley was quiet again. It is safe for skiers and hikers to return to the area. This is the work of researchers at the Avalanche Control Section in Glacier National Park, Canada. These scientists m onitor the snow as it builds up layer by layer on the park’s upper peaks. They can predict when an avalanche is about to happen. W ith well-timed artillery shots, they make the avalanche happen at a time when the released energy cannot harm anyone. Figure 1 In an avalanche, a mass of loose snow, soil, or rock suddenly gives way and slides down the side of a mountain. 446 C hap ter- i 5 * * Energy and Work Where did the energy of the avalanche come from? Where did it go? Energy is known by the changes it causes. You can hear the roar of an avalanche and see the movement of the snow. Sound and m otion are examples of energy in action. In order to define energy, you need to return to the definition of a related topic, work. Recall that work is done when a force moves an object through a distance. Energy is the ability to do work. In other words, energy is transferred by a force moving an object through a distance. Work and energy are closely related. W hen work is done on an object, energy is transferred to that object. Work is a transfer of energy. Both work and energy are typically measured in joules (J). Recall that 1 joule equals 1 newton-meter, the work done when an object is moved 1 meter by a 1-newton force. Although energy can take many different forms, it can always be measured in joules. Think about the work and energy involved in doing something as simple as carrying your backpack up a flight of stairs. You do work on the backpack by lifting it against the force of gravity. This work requires energy. The energy to do the work comes from your muscles. Your muscles receive energy from the food you eat. The energy contained in your food comes from plants that have used the energy of sunlight, or animals that have eaten such plants. Figure 2 shows some of the many forms of energy. Figure 2 Energy has many different forms. A The sun gives off energy in the form of heat and light. B Plants convert sunlight into food that we can process and eat. C People convert food energy into muscle movement. Applying Concepts How did the skiers in the photo obtain the energy to climb the mountain slope? Kinetic Energy Many forms of energy can be classified into two general types: kinetic energy and potential energy. The energy of motion is called kinetic energy. The word kinetic comes from the Greek word kinetos, meaning “moving.” The kinetic energy of any moving object depends upon its mass and speed. To calculate the kinetic energy of an object in joules, multiply y by the object’s mass (m) in kilograms and the square of its speed (v) in meters per second. r- Kinetic E n e rg y ----------------------------------------------Kinetic energy (KE) = y m v 2 Notice that doubling the mass in the formula would double the kinetic energy. However, doubling the speed would quadruple the kinetic energy, since kinetic energy is proportional to the square of an object’s speed. Energy 447 Calculating Kinetic Energy A 0 .1 0-kilogram bird is flying at a constant speed of 8.0 m/s. What is the bird's kinetic energy? P fun ilex Read and Understand What information are you given? 1. A 70.0-kilogram man is walking at a speed of 2.0 m/s. W hat is his kinetic energy? 2. A 1400-kilogram car is moving at a speed of 25 m/s. How much kinetic energy does the car have? 3. A 50.0-kilogram cheetah has a kinetic energy of 18,000 J. How fast is the cheetah running? (Hint: Rearrange the equation to solve for v.) Mass, m = 0.10 kg Speed, v = 8.0 m/s What unknown are you trying to calculate? Kinetic energy of the bird, KE ^ Plan and Solve What equation contains the given quantities and the unknown? KE = yfflV2 Substitute the known values in the formula for KE. KE = y(0.10 kg)(8.0 m/s)2 = 3.2 kg*m2/s2 = 3.2 J Look Back and Check Is your answer reasonable? It seems reasonable because the bird has a low mass, so it would not have much kinetic energy. Potential Energy Figure 3 When this musician pulls the string of her cello to one side, the string is stretched and gains potential energy. 448 Chapter 15 P otential energy is energy that is stored as a result of position or shape. The musician in Figure 3 adds energy to the cello string by plucking it. The energy is stored in the stretched string when the musician pulls it to one side. Then she releases the string and allows it to vibrate. The stored energy is converted into kinetic energy. You can also store energy just by picking up a book and holding it in the air. Let go of the book and that stored energy will turn into the kinetic energy of m otion as the book falls to the floor. Plucking a string and lifting a book are two examples of stored energy— energy with the potential to do work. Two forms of potential energy are gravitational potential energy and elastic potential energy. Gravitational Potential Energy When you lift your gym bag up to the top seat of the bleachers, you do work to increase the potential energy of your bag. Potential energy that depends upon an object’s height is called gravitational potential energy. This type of potential energy increases when an object is raised to a higher level. The diver shown in Figure 4 is standing motionless at the end of a diving board, so she has no kinetic energy. But she does have energy— gravitational potential energy. She gained this potential energy by doing work— by climbing up the steps to the diving board. As always, the work done equals force (her weight) m ultiplied by distance (the height she climbs). You can use this work to calculate how much potential energy is gained. © An object’s gravitational potential energy depends on its mass, its height, and the acceleration due to gravity. The gravitational potential energy an object gains is equal to its weight (mg) multiplied by its height (h). — Gravitational Potential Energy Potential energy (PE) = mgh To calculate gravitational potential energy in joules, the mass of the object is expressed in kilogram s and the height o f the object is expressed in meters. The acceleration due to gravity, g, has a value in SI units of 9.8 m /s2 on Earth. Note that height is m easured from the ground or floor or some other reference level. Therefore, gravitational p o ten tial energy is m easured relative to th a t sam e reference level. Gravitational potential energy is directly related to the mass of the object and its height relative to a reference level. Thus, doubling either the mass of the object or its height doubles its gravitational potential energy. Suppose the diver at the top of a 10.0-meter-high diving platform has a mass of 50.0 kilograms. You can calculate her potential energy relative to the ground as follows. PE = mgh = (50.0 kg)(9.8 m /s2)(10.0 m) = 4900 kg.m2/s2 = 4900 J If, instead, the diver was standing on the ground, her height above the ground would be zero. Therefore, her gravitational potential energy relative to the ground would also be zero. C h e ck p o in t What is gravitational potential energy? Figure 4 This diver has gravitational potential energy as she stands at the end of a diving board. Predicting How would the diver's potential energy change i f she stood on a platform twice as high as the one shown in the photo? rGo in lin e SCllN KS For: Links on potential and kinetic energy Visit: www.SciLinks.org Web Code: ccn-2151 Energy 449 Figure 5 A compressed bicycle shock absorber and a wound-up toy robot both have elastic potential energy. Elastic Potential Energy The potential energy of an object that Investigating Elastic Potential Energy M aterials basketball, tennis ball, meter stick Procedure ^ 1. Drop the basketball from a height of 1 m. Use a meter stick to measure how high it bounces. Record your result. 2. Repeat Step 1 with the tennis ball. 3. Place the tennis ball on top of the basketball and drop both together from a height of 1 m. Record your observations. 1. Observing How high did the tennis ball bounce in Steps 2 and 3? 2. Applying Concepts From your observations of Step 2, how can you prove that the kinetic energy of the tennis ball just after a bounce is less than it was just before the bounce? Mechanical Energy The energy associated with the motion and position of everyday objects is mechanical energy. Don’t be confused by the name, however. Mechanical energy is not limited to machines. Mechanical energy is the sum of an object’s potential energy and kinetic energy. Speeding trains, bouncing balls, and sprinting athletes all have mechanical energy. 3. Drawing Conclusions Use the concepts of kinetic and potential energy to explain your observations of the tennis ball in Step 3. 450 J Chapter 15 Forms of Energy All energy can be considered to be kinetic energy, potential energy, or the energy in fields such as those produced by electromagnetic waves. Some familiar examples are the chemical energy in fireworks, electrical energy in lightning bolts, and nuclear energy within the sun. ( ^ T h e major forms of energy are mechanical energy, thermal energy, chemical energy, electrical energy, electromagnetic energy, and nuclear energy. Each of these forms of energy can be converted into other forms of energy. Analyze and Conclude v is stretched or compressed is known as elastic potential energy. Something is said to be elastic if it springs back to its original shape after it is stretched or compressed. Think back to the last time you stretched a rubber band between your fingers. By stretching the rubber band, you did work on it. Just like the musician did with her cello string, the energy you added was stored in the rubber band as potential energy. If you’ve ever broken a stretched rubber band, you may have felt a painful snap on your hand. The rubber band’s elastic potential energy was converted into kinetic energy. Elastic potential energy can also be stored in objects that are compressed, such as springs. Drop a slice of bread on the floor and it does not bounce noticeably. Why doesn’t it? The bread is not very elastic. Drop a basketball on the floor and the basketball bounces back up. The compressed air in the ball forces the ball to spring back into shape after hitting the ground, propelling the ball back up. Other examples of elastic potential energy are shown in Figure 5. Mechanical energy does not include thermal energy, chemical energy, or other forms of energy associated with the motion or the arrangement of atoms or molecules. Most of these other forms of energy do involve kinetic or potential energy, but on an atomic scale. However, the mechanical energy of a speeding train and a sprinting athlete comes from the chemical energy of the train’s fuel and the sprinter’s body cells. Thermal Energy Almost all of the matter around you contains atoms. These particles are always in random m otion and thus have kinetic energy. The total potential and kinetic energy of all the microscopic particles in an object make up its th erm al energy. When an object’s atoms move faster, its thermal energy increases and the object becomes warmer. As Figure 6 shows, when objects are hot enough they can emit visible light. Chemical Energy The campers in Figure 7 are toasting m arshmallows over a campfire. The source of energy for the fire is the energy stored in wood. When the wood is burned, energy is released and heats the marshmallows as well as the area around the campfire. The energy stored in wood is chemical energy. Chem ical energy is the energy stored in chemical bonds. When bonds are broken, the released energy can do work. All chemical compounds, including fuels such as coal and gasoline, store energy. For example, cars can use the chemical energy stored in gasoline to move about. The gasoline is burned in the car’s engine and some of its chemical energy is converted into mechanical energy to move the car. ^ Checkpoint j W hat i$ chem ical energy? Figure 6 Energy occurs in many forms. This molten metal is extremely hot. It contains a great deal of thermal energy. Observing What other kinds o f energy are evident in this photograph? Figure 7 This family is using the chemical energy of burning wood to produce thermal energy for heating marshmallows. Energy 451 Electrical Energy Many devices you use every day use electricity, or electrical energy. Electrical energy is the energy associated with electric charges. Electric charges can exert forces that do work. Batteries, which convert chemical energy to electrical energy, are used to operate portable CD players, flashlights, and calculators. Electrical energy also occurs in nature. The powerful bolts of lightning shown in Figure 8A are produced by electrical energy. Electromagnetic Energy The sun radiates electromagnetic B Figure 8 Two major forms of energy are electrical energy and electromagnetic energy. A Lightning bolts transfer electric charge. B Galaxies are giant structures in space that typically contain billions of stars. The stars give off enormous amounts of electromagnetic energy. energy into space and is the source, either directly or indirectly, of most of the world’s energy supplies. Electromagnetic energy is a form of energy that travels through space in the form of waves. Visible light and X-rays are examples of electromagnetic energy. Because electromagnetic waves can travel long distances through air and space, they are often used for communication. The glowing galaxy in Figure 8B is emitting electromagnetic energy of many kinds. Nuclear Energy The nucleus of an atom is held together by strong and weak nuclear forces, which can store an enormous amount of potential energy. The energy stored in atomic nuclei is known as nuclear energy. A nuclear power plant uses nuclear fission reactions to generate electricity. Nuclear fission is a process that releases energy by splitting nuclei apart. A second type of nuclear reaction, nuclear fusion, releases energy when less massive nuclei combine to form a more massive nucleus. The heat and light of the sun are produced by the fusion of hydrogen nuclei into helium nuclei. Section 15.1 Assessment Reviewing Concepts 1. ® Describe the relationship between work and energy. 7. Applying Concepts Can an object have both kinetic energy and potential energy at the same time? Explain. 2. O How is the kinetic energy of an object determined? 3. ® W hat factors determine the gravitational potential energy of an object? 4. ® Give an example of each of the major forms of energy. 5. When you heat a pot of water over a flame, what form of energy is added to the water? Critical Thinking 6. Applying Concepts W hat kind of energy is represented by an archer stretching a bow string? 452 Chapter 15 8. A 60.0-kg person walks from the ground to the roof of a 74.8-m-tall building. How much gravitational potential energy does she have at the top of the building? 9. A pitcher throws a 0.145-kg baseball at a velocity of 30.0 m/s. How much kinetic energy does the ball have? 15.2 Energy Conversion and Conservation Key Concepts Vocabulary Reading Strategy © Can energy be converted from one form into another? ♦ energy conversion © What is the law of conservation of energy? Relating Cause and Effect Copy the flow chart below. As you read, complete the chart to explain an energy conversion. Make two similar charts for pendulums and pole vaults. © What energy conversion takes place as an object falls toward Earth? © How are energy and mass related? O n October 9,1992, people from Kentucky to New York reported seeing a bright streak of white light shooting across the night sky. Most observers, having seen “shooting stars” before, expected this one to quickly b urn out and disappear. However, that did not happen. The shooting star, or meteor, continued streaking across the sky. After a few seconds, pieces of the meteor broke off, creating a series of smaller streaks of light. Eventually, the streaks disappeared from view. Although the event was interesting, most witnesses probably soon forgot about it. However, the meteor was not soon forgotten by the owners of a red automobile in Peekskill, New York. Unfortunately for them, a large chunk of the meteor made it through Earth’s atmosphere and struck their parked car. The car was badly damaged, as shown in Figure 9. Luckily, no one was in the car at the time, and so no one was hurt. As the Peekskill meteor traveled through the atmosphere, some of its kinetic energy was converted into light and heat. The light made the meteor visible in the sky. The heat caused a large portion of the meteor to vaporize in the atmosphere. Upon impact, much of the m eteor’s remaining kinetic energy went into smashing the metal body of the car. Figure 9 A meteor crashed into the rear of this car, causing considerable damage. Energy 453 Energy Conversion Figure 10 Energy is converted from one form to another as this match is lit. A p p lyin g Concepts What energy conversions take place when you turn on a batterypow ered portable radio? The Peekskill m eteor clearly shows that energy can change forms. Energy can be converted from one form to another. The process of changing energy from one form to another is energy conversion. Not all energy conversions are as dramatic as the Peekskill meteor. Energy conversions are constantly talcing place all around you, often without you noticing. W ind-up toys store elastic potential energy in a compressed spring. When the spring unwinds, potential energy is converted into the kinetic energy of the toy’s moving parts. Light bulbs convert electrical energy into thermal energy and electromagnetic energy. In some cases, energy is converted from one form into another in a series of steps. The striking of the match shown in Figure 10 is a good example. In lighting a match, your muscles use chemical energy moving your hand to strike the match against a rough area on the matchbox. Friction between the match and the matchbox converts some of the match’s kinetic energy into thermal energy. The thermal energy triggers a chemical reaction on the match tip, releasing some of the match’s stored chemical energy. The stored chemical energy is then converted into thermal energy and electromagnetic energy in the flame. Reading Checkpoint J What energy conversions occur in lighting a match? Exploring Energy Conversion M aterials 5. small steel ball of known mass, box lined with soft modeling clay, meter stick, graph paper Procedure r a Analyze and Conclude 1. Construct a data table with 3 blank rows and 5 columns labeled Mass, Height, Diameter, Potential Energy, and Kinetic Energy. 1. Using Graphs According to your graph, 2. Drop the ball into the box of clay from a height of 30 cm . Record this height. 3. Measure and record the diameter of the crater that the ball formed. 4. Repeat Steps 2 and 3, dropping the ball from 60 cm and 90 cm . V 454 Graph your data. Plot the crater diameter on the vertical axis and height on the horizontal axis. how are crater diam eter and the height of the ball related? 2. Calculating For each height, calculate and record the initial potential energy of the ball. 3. Drawing Conclusions How are kinetic energy and crater diameter related? (Hint: The ball's kinetic energy when it hits the clay equals the potential energy it started with, mgh.) _________________________________ Chapter 15 Figure 11 Although speed skaters slide quickly over smooth ice, they are still slowed down by friction with the air and the surface of the ice. Inferring What are tw o ways that skaters can reduce friction? Conservation of Energy When energy changes from one form to another, the total energy remains unchanged even though many energy conversions may occur. This is one of the most im portant concepts in science, the law of conservation of energy. (^ The law of conservation of energy states that energy cannot be created or destroyed. According to the law of conservation of energy, energy can be converted from one form to another. However, in a closed system, the am ount of energy present at the beginning of a process is the same as the am ount of energy at the end. (In a closed system, nothing can enter or leave.) You know that if you stop pedaling when you’re riding a bike on a flat path, the bike will eventually come to a stop. The moving bike had kinetic energy. Where did the bike’s kinetic energy go? The bike slowed down and stopped because of frictional forces acting over a distance. The work done by friction changes kinetic energy into thermal energy. As a bicycle moves, it encounters friction with the ground and the air. Such friction causes a continual conversion of kinetic energy into thermal energy. Thus, as the bicycle slows, it gains thermal energy. So do the ground and the air. W hen the energy lost to frictional forces is accounted for, energy is conserved overall. Recall that friction within machinery reduces efficiency. Friction is a major cause of energy consumption in cars and factories. All moving parts are subject to friction. You can reduce friction but you can’t avoid it. Friction is everywhere. Even the skaters in Figure 11 are subject to friction. Recall that objects moving through the air are slowed by air resistance. In many cases, most of a falling object’s potential energy is converted into thermal energy because of air resistance. You can ignore the effects of friction in many everyday situations. A marble dropped from one meter above the ground, for example, encounters so little air resistance that it can be ignored. c Go i n l i n e — • « » > ,/ For: Links on energy Visit: www.SciLinks.org Web Code: ccn-2152 Energy 455 Figure 12 Some gulls use energy conversion to obtain food by dropping oysters onto rocks. A p p lyin g Concepts Where is the oyster's potential energy greatest? Where is its kinetic energy greatest? Energy Conversions One of the m ost com m on energy conversions is between potential energy and kinetic energy. The gravitational potential energy of an object is converted to the kinetic energy of motion as the object falls. That’s what happens when an avalanche brings tons of snow from the top of a m ountain to the valley floor. Similarly, when you release a compressed spring, the elastic potential energy of the spring is converted into kinetic energy as the spring expands. Conversions between kinetic and potential energy can happen in both directions, that is, from kinetic to potential energy, or from potential to kinetic energy. Some gulls use energy conversion to obtain food. Oysters are a food source for gulls. However, it is difficult for a gull to break open an oyster’s hard shell with its beak. Unfortunately for the oyster, gulls have learned a clever way to use gravitational potential energy, as shown in Figure 12. A hungry gull picks up an oyster and flies high into the air directly over some rocks. The gull does work on the oyster by raising it against the force of gravity, thereby increasing the oyster’s potential energy. While over the rocks, the gull lets the oyster fall. As the oyster falls, its gravitational potential energy is converted into kinetic energy. The oyster picks up speed until it hits the rocks. The impact breaks open the shell. The gull then swoops down to enjoy its meal. Energy Conversion in Pendulums At the lake, you grab a Figure 13 Pendulum clocks use pendulums to maintain accurate time. 456 Chapter 15 rope hanging from a tree branch. With a yell, you swing down toward the lake, and pick up speed until you land with a giant splash. A rope swing is an example of a pendulum. A pendulum consists of a weight swinging back and forth from a rope or string. Pendulums were used in the first truly accurate clocks. The Dutch scientist Christiaan Huygens (1629-1695) made the first pendulum clock in 1656. Pendulum clocks such as the one shown in Figure 13 make use of the fact that the time it takes for a pendulum to swing back and forth once is precisely related to its length. Kinetic energy and potential energy undergo constant conversion as a pendulum swings. At the highest point in its swing, the pendulum is momentarily motionless as it changes direction. At this point, the weight at the end of the pendulum has zero kinetic energy and maxim um potential energy. As the pendulum swings downward, potential energy is converted to kinetic energy. At the bottom of the swing, the pendulum has maximum kinetic energy and zero potential energy. The pendulum then moves upward again, repeating the process. Eventually, frictional forces slow down the pendulum. In a clock, a spring mechanism or hanging weights provide energy to keep the pendulum swinging despite the effects of friction. Energy Conversion in the Pole Vault The pole vault is a difficult track and field event that requires a combination of speed, strength, timing, and energy conversion. In the pole vault, an athlete uses a flexible pole to propel himself over a high bar. Look at the polevaulter’s jum p in Figure 14 and think about how energy changes during the jump. In order to start the jum p with as much kinetic energy as possible, the pole-vaulter sprints down the runway as fast as he can. At the end of his sprint, he plants the end of a long pole at the base of the high bar and propels himself into the air. The pole-vaulter’s kinetic energy is partially converted into elastic potential energy as the pole bends. The pole springs back into shape, propelling the pole-vaulter upward, hopefully high enough to clear the bar. As the pole-vaulter soars, his kinetic energy decreases while he gains gravitational potential energy. Once the highest point has been reached, his gravitational potential energy begins to convert back to kinetic energy. The pole-vaulter picks up speed as he falls back to the ground. / Reading | | / Checkpoint What energy changes occur in a pole vault? Energy Conversion Calculations W hen friction is small enough to be ignored, and no mechanical energy is added to a system, then the system’s mechanical energy does not change. Recall that mechanical energy is the total kinetic and potential energy of an object. Figure 14 A pole-vaulter converts kinetic energy into potential energy in order to propel himself high into the air. Applying Concepts A t which point does a pole-vaulter have the m ost gravitational potential energy? Mechanical energy = KE + PE You can apply the law of conservation of energy to any mechanical process. A mechanical process can be any action, such as the m otion of a pendulum, water falling in a waterfall, or a diver propelled by a diving board. In all of these processes, if friction can be neglected, the mechanical energy at the beginning equals the mechanical energy at the end. That is, total mechanical energy remains constant. This equality can be stated as follows. Conservation of Mechanical Energy (KE + PEjbeghming (KE + PE)en(j The Math Skills box on the following page shows how the conservation of mechanical energy equation can be used. Energy 457 Conservation of Mechanical Energy At a construction site, a 1.50-kg brick is dropped from rest and hits the ground at a speed of 26.0 m/s. Assuming air resistance can be ignored, calculate the gravitational potential energy of the brick before it was dropped. Read and Understand 1. A 10-kg rock is dropped and hits the ground below at a speed of 60 m/s. Calculate the gravi tational potential energy of the rock before it was dropped. You can ignore the effects of friction. 2. A diver with a mass of 70.0 kg stands motionless at the top of a 3.0-m-high diving platform. Calculate his potential energy relative to the water surface while standing on the platform, and his speed when he enters the pool. (Hint: Assume the diver's initial vertical speed after diving is zero.) 3. A pendulum with a 1.0-kg weight is set in motion from a position 0.04 m above the lowest point on the path of the weight. What is the kinetic energy of the pendulum at the lowest point? (Hint: Assume there is no friction.) What information are you given? Mass, m — 1.50 kg Final speed, v = 26.0 m/s What unknown are you trying to calculate? Gravitational potential energy of the brick before it was dropped, PE Plan and Solve What equations or formulas contain the given quantities and the unknow n? Because the brick falls without air resistance, the conservation of mechanical energy equation can be used. (KE + PE) beginning — (KE + PE)en(J You will also need to use the formula for kinetic energy (KE). KE = y my2 N ote that the KE at the beginning is zero because the brick has not yet begun to fall. Also, when the brick hits the ground, its potential energy is zero. Substitute these values into the conservation of energy formula. (PE)beginning ~~ (KE)end Substitute the formula for KE. PE = KE = y m v 2 Substitute the known values and calculate the PE. PE = y (1.50 kg) (26.0 m/s)2 = 507 kg*m2/s2 = 507 J Look Back and Check Is your answ er reasonable? Check the answer by finding the initial height of the brick, using PE = 507 J = mgh. Substituting in m and g gives h = 34.5 m. This is a reasonable height for an object in free fall to reach a speed of 26.0 m/s. 458 Chapter 15 Energy and Mass Physicist Albert Einstein (1879-1955), shown in Figure 15, developed his special theory of relativity in 1905. This theory included the now-famous equation E = me2. In Einstein’s equation, E is energy, m is mass, and c is the speed of light. This seemingly ordinary equation has surprising consequences. ® Einstein’s equation, E = me2, says that energy and mass are equivalent and can be converted into each other. In other words, energy is released as m atter is destroyed, and matter can be created from energy. Notice that the speed of light is squared in Einstein’s equation. The speed of light is an extremely large number, 3.0 X 108 meters per second. Thus, a tiny am ount of matter can produce an enorm ous am ount of energy. Suppose 1 gram of matter were entirely converted into energy. Figure 15 Albert Einstein made important contributions to many areas of physics. His theory of special relativity showed that energy and mass are equivalent. E = me2 = (10“3 kg) X (3 X 108 m/s) X (3 X 108 m/s) = 9 X 1013 kg*mr/s2 = 9 X 1013 J In comparison, 1 gram of TNT produces only 2931 joules of energy. In nuclear fission and fusion reactions, however, large amounts of energy are released by the destruction of very small am ounts of matter. Therefore, the law of conservation of energy has been modified to say that mass and energy together are always conserved. Section 15.2 Assessment Reviewing Concepts 1. ® Describe how energy can be converted from one form to another in a wind-up toy. 2. ® W hat does the law of conservation of energy state? 8. Applying Concepts To begin a dive, a diver jumps into the air and then lands on the diving board, causing it to bend. W hat type of energy conversions occur as the board springs back and propels the diver up into the air? 3. ® As an object falls in free fall, w hat energy change is taking place? 4 . ® W hat did Einstein conclude about the relationship between energy and mass? 5. W hat type of energy change results when friction slows down an object? 6. Describe the energy of a playground swing at its highest position. Critical Thinking 7. Inferring W hy does a bouncing ball rise to a lower height with each bounce? W hat energy conversion is taking place? 9. A 0.15-kg ball is thrown into the air and rises to a height of 20.0 m. How much kinetic energy did the ball initially have? 10. A 125-g steel ball with a kinetic energy of 0.25 J rolls along a horizontal track. How high up an inclined track will the ball roll if friction can be ignored? Energy 459 Roller Coasters A roller coaster is a train powered by gravity. An initial store of potential energy is released dramatically during the ride. A roller coaster’s first hill, called the lift hill, is the highest point of the ride. As the coaster plunges down the first drop, its potential energy is converted into kinetic energy, the energy of motion. Later, some of this kinetic energy is converted back into potential energy as the coaster’s speed decreases during a climb. Over the course of the ride, the coaster’s total mechanical energy (potential plus kinetic energy) gradually decreases. This change occurs due to friction from the rails and the air, which causes the coaster’s mechanical energy to be converted into heat. By the end of the ride, all the potential energy created on the lift hill has been lost as heat. 460 Chapter 15 As it falls, the coaster converts potential energy into kinetic energy. Releasing energy A roller coaster goes through a series of exchanges between potential and kinetic energy. A cable pulls cars Potential energy rapidly turns into kinetic energy during the first plunge. Kinetic energy reaches a maximum as the coaster reaches the bottom o f the first hill. The second loop has a lower peak than the first because the coaster's energy is reduced friction. to the top o f the lift hill. During the climb, the coaster builds up potential energy. Design and safety The job of a roller coaster designer is to make sure that a ride is terrifying but safe. Some people are concerned that designers are providing riders with greater thrills at the expense of their safety. Safety experts say that the risk of injury is small, but it is important to pay attention to restrictions regarding height and medical conditions. Coaster forces During a ride, you are kept in your seat by the safety bar and by the force of the seat pushing on you. The coaster also exerts large forces on the track, which must be inspected daily for structural damage. Going Further Acceleration An early loop ride at Coney Island subjected riders to about 12 g's (12 times the acceleration due to gravity). For safety reasons, most modern rides do not exceed 5 g's. Select an am usem ent p ark ride th at you enjoy. Use lib rary or In tern et resources to discover th e principles o f physics th a t a llo w th is ride to w o rk . W rite a p aragraph or d raw a diagram exp lain in g w h a t you have learned. Take a Discovery Channel Video Field Trip by w atch in g "Physics o f Fu n ." f t D ouerv 0 0 1 : Video Field Trip 15.3 Energy Resources Reading Focus Key Concepts Vocabulary Reading Strategy © ♦ n on ren ewable en ergy resources ♦ fossil fuels ♦ ren ewable en ergy resources ♦ hydroelectric en ergy ♦ solar en ergy ♦ geothermal en ergy ♦ biomass en ergy ♦ hydrogen fuel cell ♦ en ergy con servation Identifying Main Ideas Copy the table © What are the major nonrenewable and renewable sources of energy? How can energy resources be conserved? Figure 16 Crude oil is pumped out of the groun d or ocean floor. It is then refin ed an d turn ed in to gasolin e, fuel oil, an d other oil products. below. As you read, write the main idea for each headin g. Headin g Main Idea Non ren ewable en ergy resources a. ? Ren ew able en ergy resources b. ? Con servin g en ergy resources c. ? rom the alarm clock that wakes you up each morning to the light that you turn off before you sleep, you depend on energy resources to operate many different devices to get you through each day. Energy resources can be classified as either renewable or nonrenewable. Nonrenewable Energy Resources Nonrenewable energy resources exist in limited quantities and, once used, cannot be replaced except over the course of millions of years. ® Nonrenewable energy resources include oil, natural gas, coal, and uranium. Such resources are currently being used much faster than they can be replaced, creating concern about how long they will last. Oil, natural gas, and coal are known as fossil fuels because they were formed underground from the remains of once-living organisms. Currently, fossil fuels account for the great majority of the world’s energy use. These fuels are not distributed evenly throughout the world. For example, about 60 percent of known oil supplies are located in a small area in the Middle East. The United States has just 2 percent of the world’s oil supplies but about 25 percent of the world’s coal supplies. Fossil fuels are relatively inexpensive and are usually readily available, but their use creates pollution. 462 Chapter 15 Renewable Energy Resources Renewable energy resources are resources that can be replaced in a relatively short period of time. Most renewable energy resources originate either directly or indirectly from the sun. The sun and Earth are constantly releasing large am ounts of energy. This energy could be used for generating electric power, heating buildings or other purposes. Renewable energy resources include hydroelectric, solar, geothermal, wind, biomass, and, possibly in the future, nuclear fusion. The challenge for engineers and scientists is to find efficient ways to make these energy resources inexpensive and convenient. Hydroelectric Energy Energy obtained from flowing water is known as hydroelectric energy. As water flows downhill, its gravitational potential energy is converted into kinetic energy. This kinetic energy can be used to turn turbines that are connected to electric generators. Some hydroelectric power plants simply depend upon the natural flow of water in a river. But most modern plants, such as the one shown in Figure 17, rely on dams built across rivers. A dam blocks the flow of water, storing potential energy that is converted into kinetic energy when the water is released. The m ajor advantages of hydroelectric energy include its low cost to produce and lack of pollution. Dams, however, cause a variety of environm ental problems. For example, dams hamper the run of fish upriver for spawning. Also, in the United States many of the most suitable sites for hydroelectric plants are already in use. Figure 17 Hoover Dam was built across the Colorado River on the Arizona-Nevada border. This 221meter-tall structure can generate over 2 million kilowatts of power. Applying Concepts What type o f energy conversion is involved in a hydroelectric plant? W o r ld E n e r g y U s e ( x 1 0 15 B t u ) 1991 2000 136.47 154.28 Coal 88.35 94.22 Natural gas 76.03 90.15 Hydroelectric power 23.13 27.80 Nuclear fission 21.29 25.66 1.82 2.99 Source Petroleum Which energy resources are most com m only used around the world? How is energy use changing over time? The table shows total world energy use in 1991 and 2000. Energy use is measured in British thermal units, or Btu (1 Btu = 1055 J). Note that petroleum includes oil and related fuels. Other 1. Using Tables W hat was the world's largest source of energy in 1991 ? In 2000? 4. Analyzing Data W hat percentage of world energy use in 2000 was accounted for by fossil fuels? 2 . Analyzing Data In general, how did usage change from 1991 to 2000? 3. Graphing Make a circle graph of world energy use by source for the year 2000. 5. Predicting How m ight total world energy use be different in 2020? Explain. Energy 463 Solar Energy Sunlight that is converted into usable energy is called solar energy. Passive solar designs use sunlight to heat a building w ithout using machinery. For example, sunlight passing through the windows of a house may be absorbed by thick walls that then radiate thermal energy to warm the house. In an active solar energy system, sunlight heats flat collection plates through which water flows. The heated water may be used directly for the buildings hot water needs, or it maybe used to heat the house. Sunlight can also be converted directly into electrical energy by means of solar cells, also known as photovoltaic cells. Many small devices such as calculators now run on solar cells. A few large solar electric plants, like the one shown in Figure 18A, use mirrors that concentrate sunlight to produce electricity. The benefits of solar energy depend on the climate. Solar energy is nonpolluting, but for areas where cloudy days are frequent, solar energy is less practical. Geothermal Energy Geothermal energy is thermal energy beneath Earth’s surface. In some regions, especially near volcanoes, geothermal energy is used to generate electicity. The geothermal power plant in Figure 18B pumps water into the ground, where it turns into steam. The steam is then used to drive electric generators. Geothermal energy is nonpolluting, but is not widely available. Figure 18 Solar and geothermal energy plants use renewable resources to generate electricity. A A solar electric plant uses solar cells to convert sunlight into electricity. B A geothermal plant in California uses Earth's thermal energy to generate electricity. Comparing and Contrasting What are some similarities and differences betw een solar energy and geothermal energy? rGo i n l i n e — For: Articles on energy and energy resources Visit: PHSchool.com Web Code: cce-2153 464 Chapter 15 Other Renewable Resources Sunlight causes plants to grow, converting electromagnetic energy into chemical energy. The chemical energy stored in living things is called biomass energy. Biomass can be converted directly into therm al energy. For example, many people around the world burn wood or peat to heat their homes or for cooking. Also, agricultural wastes such as corn stalks can be converted into a high-energy alcohol fuel that can be added to gasoline for cars. A hydrogen fuel cell generates electricity by reacting hydrogen with oxygen. Hydrogen fuel cells can be used to convert energy from renewable resources. For example, hydrogen fuel can be extracted from water using electricity from solar cells. The end product of fuel cells is water, so they offer a nonpolluting means for transporting energy. A form of hydrogen is also the most likely raw material for another future source of energy, nuclear fusion. The process of fusion will probably produce little pollution or radioactive waste. Scientists have been working on sustained fusion for years, but many challenges remain. Wind Turbine W ind turbines convert the kinetic energy in the horizontal movement o f w ind into rotational energy of the turbine's rotor shaft. Rotational energy is then converted into electrical energy using an electric generator. The output of a w ind turbine depends on the turbine's size and the wind's W ind farm Enormous wind farms like this one in California can generate as much power as a large power plant (without any air pollution). speed. In terp retin g D ia g ra m s What is the purpose o f the gearbox? Blades The shape and angle of the blades are designed to extract as much energy as possible from the horizontal movement of the wind. Gearbox The rotor shaft itself rotates fairly slowly at about 20-30 revolutions per minute. The gearbox converts this to a rotation some 50 times faster—about 1500 revolutions per minute. Rotor shaft Generator The shaft from the gearbox enters I the generator, which converts the kinetic energy of the shaft into electrical energy. The anemometer measures wind speed. Wind vane Direction o f rotation Wind direction A yaw ring changes the direction o f the turbine. Electrical energy passes to the local grid. I Yaw drive mechanism Control unit This computer-operated unit controls the yaw drive mechanism, ensuring that the turbine always faces directly into the wind. Energy 465 Conserving Energy Resources Figure 19 Mass transportation systems include buses, trains, and streetcars such as the one shown here. In fe rrin g How can the use o f mass transportation save energy? Fossil fuel supplies may become increasingly scarce and expensive in the future. An im portant way to make these energy resources last longer is to use them more slowly. Energy resources can be conserved by reducing energy needs and by increasing the efficiency of energy use. Finding ways to use less energy or to use energy more efficiently is known as energy conservation. It’s easy to forget that the energy you use often comes at the expense of resources that are being used up forever. People can reduce the use of these resources by making energy-saving decisions, for example turning off lights when they are not being used. Because conventional cars consume an enormous am ount of energy, the decisions that people make about transportation are very important. Walking or biking on short trips and carpooling can save considerable energy. Using mass transportation, such as the streetcar shown in Figure 19, can also reduce energy use. Making appliances, cars, and even light bulbs more energy efficient is a way of reducing energy use while still enjoying its benefits. Much has already been done to make appliances more energy efficient. Light bulbs have been developed that provide superior lighting at a lower energy cost. The technology for further improvement, including more fuel-efficient cars, is already known in many cases. However, the initial cost of energy efficiency can be an obstacle for manufacturers and for consumers. Energy-efficient purchases often cost more initially, but can save money in fuel costs over time. Section 15.3 Assessment Reviewing Concepts 1. ® List the major nonrenewable and renewable sources of energy. 5. Comparing and Contrasting How are passive and active solar energy systems different? 2. ® W hat could be done to make present energy resources last longer? 6. Applying Concepts Describe three ways that you used energy resources today. 3. W hy are coal, oil, and natural gas called fossil fuels? Science )------------- Critical Thinking 4. Applying Concepts You are looking for the best place to build a hydroelectric plant along a river. Would you locate the plant along a steep or flat section of the river? Explain. 466 Chapter 15 W riting to Persuade Suppose you are an energy planner who is concerned about the possibility of future shortages of electricity. Write a paragraph describing one or two pro posals that you think would help to avoid this potential problem. Investigating a Spring Clip There are many ways to use potential energy. A spring clip is a device used to hold weights on a barbell. The spring clip stores energy when you compress it. In this lab, you will determine how the distance you compress a spring clip is related to the force you apply and to the spring's potential energy. Problem How does the force you apply to a spring clip affect its elastic potential energy? Materials • • • • clamp spring clip masking tape metric ruler 50-newton spring scale graph paper For the probeware version of this lab, see the Probeware Lab Manual, Lab 6. Skills Measuring, Using Tables and Graphs Procedure 1. Make a data table with three columns. Label the columns Force (N ), Position of Handle (cm ), and Total Distance Moved (cm ). 2. Using the clam p, firm ly attach one handle of the spring clip to a tabletop, with the other handle facing up and away from the table as shown. CA U TIO N Be careful not to pinch your fingers with the clamp or spring clip. 3. Remove the plastic cover from the upper handle of the spring clip. Hook the spring scale to the spring clip handle as shown and use masking tape to secure it. Have your teacher check your setup for safety before proceeding. 4. Have a classmate hold the ruler next to the spring clip as shown. Record the starting position of the handle. (The reading on the spring scale should be zero.) 5. Slowly pull the spring scale down at a right angle to the upper handle until the upper handle moves 0.1 cm . Record the force and the position of the upper handle. Slowly release the scale back to the starting position. 6. Repeat Step 5, this time pulling the handle 0.2 cm from the starting position. 7. Repeat Step 5 a few more times, pulling the handle 0.1 cm farther each time. Continue until the spring scale reaches its maximum force. 8. Calculate the total distance the handle moved each time you pulled it and record these values in your data table. Graph your data. Place distance on the horizontal axis and force on the vertical axis. A n a ly z e a n d C o n c lu d e 1. Using Graphs W hat is the approximate relationship between the total distance you compressed the spring clip and the force you applied to it? 2. Classifying W hat type of energy transfer did you use to compress the spring clip? What type of energy did the spring clip gain when it was compressed? 3. Drawing Conclusions W hat relationship exists between the distance the spring clip was compressed and its potential energy? (Hint: The elastic potential energy of the spring clip equals the work done on it.) c Go ^Inline — PHSchool.com For: Data sharing V isit: PHSchool.com Web Code: ccd-2150 Energy 467 CHAPTER 15 ■ Study Guide 15.1 Energy and Its Forms 15.3 Energy Resources (^ K e y Concepts ( ^ K e y Concepts • Work is the transfer of energy. • Nonrenewable energy resources include oil, natural gas, coal, and uranium. • The kinetic energy of any moving object depends on its mass and speed. • An object's gravitational potential energy depends on its mass, its height, and the acceleration due to gravity. • The major forms of energy are mechanical energy, thermal energy, chemical energy, electrical energy, electromagnetic energy, and nuclear energy. Vocabulary energy, • Energy resources can be conserved by reducing our energy needs and by increasing the efficiency of energy use. Vocabulary nonrenewable energy resources, p. 447 fossil fuels, p. 447 potential energy, p. 448 kinetic energy, elastic potential energy, hydroelectric energy, p. 449 p. 450 p. 450 mechanical energy, p. 451 chemical energy, p. 451 electrical energy, p. 452 thermal energy, electromagnetic energy, p. 452 p. 452 p. 462 p. 462 renewable energy resources, gravitational potential energy, nuclear energy, • Renewable energy resources include hydroelectric, solar, geothermal, w ind, biomass, and, possibly in the future, nuclear fusion. solar energy, p. 463 p. 463 p. 464 p. 464 p. 464 hydrogen fuel cell, p. 464 energy conservation, p. 466 geothermal energy, biomass energy, Th in kin g Visually Comparing and Contrasting Copy the partially 15.2 Energy Conversion and Conservation completed table below. Complete it to compare the major forms of energy. C ^ K ey Concepts Form of Energy Definition Example • Energy can be converted from one form to another. M echanical Energy o f motion and position o f everyday objects Bouncing ball Lightning • The law of conservation of energy states that energy cannot be created or destroyed. a. ? b. • The gravitational potential energy of an object is converted to the kinetic energy of motion as the object falls. c. ? Energy stored in chemical bonds d. ? • Einstein's equation shows that energy and mass are equivalent, and can be converted into each other. Electrom agnetic e. ? f. ? Nuclear g. ? Fission Therm al h. ? i. Vocabulary energy conversion, 468 Chapter 15 p. 454 ? ? Assessment Reviewing Content Interactive textbook with assessment at PHSchool.com 10. Most renewable energy originates from a. fossil fuels. b. the ground, c. the sun. d. uranium. Choose the letter that best answers the question or completes the statement. 1. The energy of a moving object is a. kinetic energy. b. potential energy, c. chemical energy. d. nuclear energy. 2. If the speed of an object doubles, its kinetic energy a. doubles. b. quadruples, d. is halved. c. stays the same. 3. An example of an object having elastic potential energy is a. a stretched spring, b. books on a shelf, c. a moving arrow. d. a falling oyster. 4. An example of electromagnetic energy is a. a falling rock. b. a stretched spring, c. a speeding train. d. sunlight. 5. Energy stored in the bonds between atoms is called a. kinetic energy. b. mechanical energy, c. chemical energy. d. thermal energy. 6. Mechanical energy is a. found in machinery only. b. usually measured at the atomic level. c. the sum of the chemical and thermal energy of an object. d. the sum of the kinetic and potential energy of an object. 7. An example of the conversion of gravitational potential energy into kinetic energy is a. a falling raindrop. b. a gasoline-powered engine. c. striking a match. d. a hockey puck sliding on ice. 8. The law of conservation of energy states that a. energy cannot be converted from one form to another. b. energy cannot be created or destroyed. c. energy resources must be used efficiently. d. energy is constantly being lost to friction. 9. Which of the following energy resources accounts for most of the world's present energy use? a. uranium b. solar energy d. wind energy c. fossil fuels U nderstanding Concepts 11. W hat is energy? 12. How are kinetic energy and potential energy different? 13. How does the potential energy of an object change when its height is tripled? 14. Look at the graphs below. One of the graphs is a plot of kinetic energy vs. mass for a set of objects with different masses, all moving at the same speed. The other graph is a plot of kinetic energy vs. speed for a set of objects with the same mass, all moving at different speeds. Identify each graph and explain how can you tell which graph is w hich. 15. W hat happens to the atoms of an object if its thermal energy is increased? 16. Explain the energy conversions that occur as a basketball falls, hits the ground, and bounces back up. Ignore frictional forces. 17. W hat force is acting when the kinetic energy of a moving object is converted into thermal energy? 18. Explain how energy is converted as a pendulum swings. 19. Is mechanical energy always conserved? Explain w h y or w h y not. 20. Explain the meaning and the importance of the formula E = me2. 21. W hy are coal, oil, and natural gas considered to be nonrenewable energy resources? 22. W hat is the source of geothermal energy? Energy 469 CHAPTER 15 Assessment C ritical Thinkin g (continued) Concepts in A ction 23. Calculating A small meteoroid is approaching Earth. It has a mass of 100.0 kg and a speed of 10.0 km/s. How much kinetic energy does the meteoroid have? (Hint: 1 km = 1000 m) 24. Designing an Experiment Design an experiment to find out how much elastic potential energy is in a compressed spring. In addition to the spring, you may use a small metal ball, a ruler, and a scale for measuring mass. 25. Calculating A 0.15-kg ball is dropped from the top of a 150-m building. W hat is the kinetic energy of the ball when it passes the sixteenth floor at a height of 63 m? (Ignore air resistance.) 30. Using Graphs A soccer ball is kicked from the ground into the air. Describe two graphs that show how the potential energy and kinetic energy change between the time the ball is kicked and when it lands. (Hint: Make time the x-axis.) 31. inferring When a falling object reaches a speed called terminal velocity, its speed no longer increases. The object is losing gravitational potential energy but not gaining kinetic energy. Since energy must be conserved, where must the gravitational potential energy be going? 32. Calculating Suppose a 200.0-kilogram dolphin is lifted in the air to be placed into an aquarium tank. How much energy is needed to lift the dolphin 3.00 meters into the air? M ath Skills 26. Converting Units Using mass in kg, velocity in m/s, and height in m, show that the formulas for kinetic energy and gravitational potential energy result in energy values with the same units. W hat is the energy unit called? Questions 2 7 -2 9 refer to the data in the table below. Three balls are thrown vertically into the air from different heights above the ground. The data for each ball are shown in the table below. D ata Table Object Mass (kg) Initial Upward Speed (m/s) Initial Height Above Ground (m) Ball 1 1.00 8.00 15.00 Ball 2 2.00 1.00 10.00 Ball 3 3.00 4.00 5.00 27. Calculating How much kinetic energy does each ball have when it is thrown? 33. Applying Concepts How is energy converted as a car moves between the top and the bottom of a roller coaster loop? 34. Making Judgments What are some advantages and disadvantages of solar energy? 35. Applying Concepts How can electricity be obtained from the wind? 36. Writing in Science Using your own words, write a brief summ ary of the law of conservation of energy. Include an example from your everyday life. Perform ance-Based Assessm ent Applying Concepts Identify one practical w ay that your fam ily or school could reduce energy use. Find out how much energy is currently being used for the purpose you choose, and estimate how much energy your suggestion could save. Write a brief summary of your recommendations. 28. Analyzing Data Which ball has the greatest gravitational potential energy when it reaches its maximum height? (Hint: Find the total energy for each ball.) 29. Predicting Which ball hits the ground with the most kinetic energy? 470 Chapter 15 cGo i n l i n e — PHSchool.com For: Self-grading assessment V isit: PHSchool.com Web Code: cca-2150 Standardized Test Prep Test-Taking Tip Avoiding Careless Mistakes Students often make mistakes when they fail to examine a test question and possible answers thoroughly. Read the question carefully and underline key words that may change the mean ing of the question, such as not, except, exclud ing, and so on. After choosing an answer, reread the question to check your selection. Which statement about work and energy is N OT correct? (A) (B) (C ) (D) Both can be expressed in joules. Energy is the ability to do work. Work done is always equal to energy input. Energy input is always greater than work output. (E ) Energy is transferred in order to accomplish work. 4. Friction causes kinetic energy to be converted into (A) potential energy. (B) thermal energy. (C ) mechanical energy. (D) electrical energy. (E ) chemical energy. 5. Which of the following is N OT considered a renewable energy resource? (A) geothermal energy (B) biomass (C) hydroelectric energy (D) uranium (E ) wind energy Use the figure below to answer Questions 6 and 7. (Answer: C) Choose the letter that best answers the question or completes the statement. 1. Which form of energy does a plant store when light is transformed during photosynthesis? (A) chemical energy (B) thermal energy (C) mechanical energy (D) electrical energy (E ) nuclear energy 2. Which of the following is N OT an example of kinetic energy being converted to potential energy? (A) a basketball player jumping for a rebound (B) releasing a compressed spring (C) squeezing a rubber ball (D) pulling a sled up a hill (E) a swing moving upward 3. A 50.0-kg wolf is running at 10.0 m/sec. W hat is the wolf's kinetic energy? (A) 5 j (B) 500 j (C) 5000 j (D) 2 5 0 ] (E ) 2 5 0 0 ] A pendulum weight is released in position A. The weight moves through positions B, C, D, and E. 6. At which location(s) does the pendulum have m axim um potential energy? (A) A and B (B) B and D (C) C only (D) D and E (E) A and E 7. At w hich location(s) does the pendulum have m axim um kinetic energy? (A) A and B (B) B and D (C) C only (D) D and E (E) A and E Energy 471