CHAPTER 2 Energy and Energy Transformations Forms of Energy What do you think? Read the three statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 1. A fast-moving baseball has more kinetic energy than a slow-moving baseball. LESSON 1 Key Concepts What is energy? What are potential and kinetic energy? How is energy related to work? What are different forms of energy? 2. A large truck and a small car moving at the same speed have the same kinetic energy. 3. A book sitting on a shelf has no energy. What is energy? Think of the last time you saw a fireworks display. When fireworks explode, you can see bursts of color in the night sky. Fireworks release energy when they explode. Energy is the ability to cause change. The energy in the fireworks causes the changes that you see as flashes of light and that you hear as loud booms. Energy also causes other changes. Plants use energy from the Sun to make food for growth and other processes. Energy can cause changes in the motions or positions of objects. When a hammer hits a nail, energy from the hammer moves the nail. The explosion of fireworks, the growth of a flower, and the motion of a hammer involve energy. Kinetic Energy Energy of Motion Have you ever been to a bowling alley? When you rolled the ball and it hit the pins, a change occurred the pins fell over. This change occurred because the ball had a form of energy called kinetic (kuh NEH tik) energy. Kinetic energy is energy due to motion. All moving objects have kinetic energy. The kinetic energy of a moving object depends on two factors: the object s speed and its mass. Sticky Notes As you read, use sticky notes to mark information that you do not understand. Ask your teacher to explain. WORD ORIGIN energy from Greek energeia, means activity 1. Define What is energy? Reading Essentials Energy and Energy Transformations 13
Visual Check 2. Interpret Which car in the figure has more kinetic energy? Why? KE Speed, Mass, and Kinetic Energy Speed = 15 m/s Mass = 8,000 kg KE Speed = 15 m/s Mass = 1,500 kg KE Speed = 25 m/s Mass = 1,500 kg Make a two-pocket book. Organize information about the forms of energy on quarter sheets of paper and put them in the pockets. Kinetic Energy Potential Energy 3. Define What is kinetic energy? Kinetic Energy and Speed Speed is one factor that affects kinetic energy. The faster an object moves, the more kinetic energy it has. The figure above shows two cars and a truck moving along a highway. All the vehicles have kinetic energy (KE) because they are moving. However, each vehicle s speed helps determine the amount of kinetic energy the vehicle has. The vertical bars show the kinetic energy of each vehicle. Kinetic Energy and Mass The kinetic energy of a moving object also depends on its mass. If two objects are moving at the same speed, the object with more mass has more kinetic energy. Notice in the figure that the two cars have the same mass. The car in front has more kinetic energy because it is moving faster. The car in the back and the truck are moving at the same speed. The truck has more kinetic energy than the car in the back because the truck has more mass than that car. Potential Energy Stored Energy An object can have energy even when it is not moving. If you hold a ball in your hand and then let it go, gravity will cause the ball to fall to Earth. The gravitational interaction between the ball and Earth causes a change to occur. Before you dropped the ball, the ball had energy. This form of energy is called potential (puh TEN chul) energy. Potential energy is stored energy due to the interactions between objects or particles. Potential energy has different forms: gravitational potential energy, elastic potential energy, and chemical potential energy. 14 Energy and Energy Transformations Reading Essentials
Gravitational Potential Energy When you are holding a book, energy is stored between the book and Earth. This type of energy is called gravitational potential energy. If you lift the book higher, the gravitational potential energy between the book and Earth increases. The gravitational potential energy stored between any object and Earth depends on the object s mass and its height above the ground. Dropping a bowling ball from a height of 1 m causes greater change than dropping a tennis ball from the same height. When two objects are at the same height, the one with more mass has more gravitational potential energy. The two vases on the bookcase are identical; however, they have different potential energies because they are at different positions above the ground. The vase on the top shelf of the bookcase has more gravitational potential energy than the vase on the bottom shelf. An object that falls from a greater height can cause a greater change than an identical object that falls from a lower height. Gravitational Potential Energy Elastic Potential Energy Another form of potential energy is elastic (ih LAS tik) potential energy. Elastic potential energy is energy that is stored when an object is compressed or stretched. When you jump on a pogo stick, you compress the spring. This gives the spring elastic potential energy. When the spring decompresses, it pushes you into the air. Stretching an object also stores elastic potential energy. When you stretch a rubber band, elastic potential energy is stored in the rubber band. When you release the rubber band, the stored elastic potential energy changes into kinetic energy. The kinetic energy causes the rubber band to snap back to its original shape. Plucking the strings of a guitar, jumping on a trampoline, and pulling back on the string of a bow give these objects elastic potential energy by stretching. When the strings and the trampoline return to their original positions, they cause change. Reading Check 4. Relate Gravitational potential energy depends on which two factors? (Circle the correct answer.) a. speed and distance b. mass and height above the ground c. mass and speed Visual Check 5. Identify The two vases on the bookcase have the same mass. Circle the vase that has the greater gravitational potential energy. 6. Consider Which is not an example of how an object gains elastic potential energy by stretching? (Circle the correct answer.) a. jumping on a pogo stick b. pulling on a rubber band c. jumping on a trampoline Reading Essentials Energy and Energy Transformations 15
Chemical Potential Energy When you eat, you take in another form of potential energy. Food and other substances are made of atoms joined together by chemical bonds. Chemical potential energy is energy stored in the bonds between atoms. 7. Compare In what way are all forms of potential energy the same? Look at the figure below. The small balls in the figure represent atoms that make up a glucose molecule. The lines between the atoms represent chemical bonds. Chemical potential energy is stored in these bonds. When you eat food, chemical reactions within your body release chemical potential energy stored in the food. Your body uses chemical potential energy in foods for all its activities, such as moving, thinking, and growing. Bonds between atoms in other substances, such as gasoline, also store chemical potential energy. People use the chemical potential energy in gasoline to drive cars. Energy is stored in the chemical bonds between atoms. Glucose Molecule Visual Check 8. Identify Highlight some of the chemical bonds in the figure. Chemical bond Glucose molecule Energy and Work A force is a push or a pull. When a force is applied to an object, the object s kinetic and potential energy can change. You can transfer energy by doing work. Work is the transfer of energy that occurs when a force makes an object move in the direction of the force while the force is acting on the object. Look at the figure at the top of the next page. The girl does work on the box as she lifts it. As she lifts the box onto the shelf, the energy of the box changes. The work she does transfers energy to the box. The energy of the box increases because of the gravitational interaction between the box and Earth. The box s potential energy increases as she lifts the box higher. The vertical bars in the figure show the work that the girl does (W) and the box s potential energy (PE). Work depends on force and distance. You do work on an object only if the object moves. Suppose the girl shown in the figure tries to lift the box but cannot lift it off the floor. She transfers no energy, so she does no work on the box. 16 Energy and Energy Transformations Reading Essentials
Work PE W PE W PE W An object that has energy can also do work. What will happen if the girl drops the box as she is moving it onto the shelf? When the box hits the floor, it does work on the floor. Some of the box s kinetic energy is transferred to the floor. The girl will hear some of the energy as a loud crash and feel some of the energy near her feet as the energy travels through the floor. Because energy and work are connected, energy is sometimes described as the ability to do work. Other Forms of Energy You have just learned about two forms of energy kinetic energy and potential energy. Kinetic energy is energy due to motion. Potential energy is stored energy. There are other forms of energy as well. All forms of energy are measured in units called joules (J). A softball dropped from a height of about 0.5 m has about 1 J of kinetic energy just before it hits the floor. Mechanical Energy The sum of potential energy and kinetic energy in a system of objects is mechanical energy. When you do work on an object, you change the object s mechanical energy. Think again about the girl moving the box shown above. At what point did the mechanical energy of the box change? The mechanical energy of the box increased when the girl lifted it off the ground. Now think about a basketball game. The mechanical energy of a basketball increases when a player shoots the ball. Sound Energy Musical instruments are just a few of the many things that produce sound. When you pluck a guitar string, the string vibrates and creates sound. You hear a sound when sound waves produced by the vibrating guitar string reach your ears. The energy that sound carries is sound energy. Sound energy is produced by objects that vibrate. Sound energy cannot travel through a vacuum such as the space between Earth and the Sun. Visual Check 9. Determine When did the transfer of energy take place between the girl and the box? 10. Analyze How is energy related to work? 11. Apply Imagine that you push on a large rock. At what point does your effort change the rock s mechanical energy? Reading Essentials Energy and Energy Transformations 17
Visual Check 12. Apply Circle the object with the least thermal energy. What is causing the ice block to melt? Thermal Energy All objects and materials are made of particles that are always moving. Because these particles move, they have energy. Thermal energy is the sum of kinetic energy and potential energy of the particles that make up an object. Thermal energy moves from warmer objects to colder objects. In the figure below, the hot chocolate has more thermal energy than the bottle of cold water. The bottle of cold water has more thermal energy than the block of ice that has the same mass. Thermal Energy Reading Check 13. Define What are electromagnetic waves? (Circle the correct answer.) a. waves that form on a beach b. waves that can travel through a vacuum c. waves that can be heard 14. Describe three forms of energy. Electric Energy An electrical fan uses another form of energy electric energy. When you turn on a fan, an electric current flows through the fan s motor. Electric energy is the energy an electric current carries. Electrical appliances, such as fans and dishwashers, change electric energy into other forms of energy. Radiant Energy Light Energy The Sun gives off energy that travels to Earth as electromagnetic waves. Unlike sound waves, electromagnetic waves can travel through a vacuum. Light waves, microwaves, and radio waves are electromagnetic waves. The energy that electromagnetic waves carry is radiant energy. Sometimes radiant energy is called light energy. Nuclear Energy A nucleus is at the center of every atom. Nuclear energy is energy that is stored and released in the nucleus of an atom. In the Sun, nuclear energy is released when nuclei join together. In a nuclear power plant, nuclear energy is released when the nuclei of uranium atoms are split apart. 18 Energy and Energy Transformations Reading Essentials
Mini Glossary electric energy: energy that an electric current carries energy: the ability to cause change kinetic (kuh NEH tik) energy: energy due to motion mechanical energy: the sum of potential energy and kinetic energy in a system of objects nuclear energy: energy stored and released in the nucleus of an atom radiant energy: the energy that electromagnetic waves carry sound energy: the energy that sound carries thermal energy: the sum of kinetic energy and potential energy of the particles that make up an object work: the transfer of energy that occurs when a force makes an object move in the direction of the force while the force is acting on the object potential (puh TEN chul) energy: stored energy due to the interactions between objects or particles 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes how energy is related to work. 2. Use what you have learned about energy to complete the table. Form of Energy Definition Form of Energy Definition Kinetic Electric Mechanical Nuclear What do you think Potential Thermal Radiant Sound Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON Reading Essentials Energy and Energy Transformations 19