Energy, Work, and Simple Machines Energy Transformations and Work Key Concepts • What is the law of conservation of energy? What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column • In what ways can energy if you agree with the statement or a D if you disagree. After you’ve read be transformed? this lesson, reread the statements to see if you have changed your mind. • How are energy and work Before Statement After related? 3. Energy cannot be created or destroyed, but it can be transformed. 4. Work describes how much energy it takes for a force to push or to pull an object. Sticky Notes As you read, use sticky notes to mark Energy Transformations information that you do not As you read in Lesson 1, different types of electric energy understand. Read the text plants supply the energy you use in your home and school. carefully a second time. If Energy transformation is the conversion of one form of energy to Companies, Inc. The McGraw-Hill of a division © Glencoe/McGraw-Hill, Copyright you still need help, write a another. For example, suppose a zoo uses a heat lamp to list of questions to ask your warm a newborn zebra. The electric energy in the wiring teacher. of the heat lamp is transformed into thermal energy. ACADEMIC VOCABULARY Energy also is transferred when it moves from one object transform to another. When energy is transferred, the form of energy (verb) to change form or does not have to change. For example, the thermal energy in structure the heat lamp is transferred to the air and then to the zebra. Energy Conservation Suppose you turn on a lamp switch. The radiant energy coming from the bulb had many other forms before it shined Key Concept Check in your eyes. The radiant energy was electric energy in the 1. Define What is the law lamp’s wiring, and it was chemical energy in the fuel at the of conservation of energy? electric energy plant. The law of conservation of energy says that energy can be transformed from one form to another, but it cannot be created or destroyed. Even though energy can change forms, the total amount of energy in the universe does not change. It just changes form. 46 Energy, Work, and Simple Machines Reading Essentials CC353_008_012_RE_L2_889408.indd353_008_012_RE_L2_889408.indd 8 44/3/10/3/10 11:12:05:12:05 PPMM Roller Coasters Have you ever thought about the energy transformations that occur on a roller coaster? Most roller coasters first pull your car to the top of a big hill. At this point, the distance between you and Earth increases. Your potential energy also increases. Next, your car races down the hill. You move faster and faster. The gravitational potential energy is transformed to kinetic energy. At the bottom of the hill, your gravitational potential energy is small, but you have a lot of kinetic energy. This kinetic energy is transformed back to gravitational potential energy as your car moves up the next hill. Reading Check Plants and the Body 2. Apply At what point in a roller coaster ride is When a plant, such as broccoli, carries on photosynthesis, gravitational potential it transforms radiant energy from the Sun into chemical energy the greatest? energy. The chemical energy is stored in the bonds of the plant’s molecules. When you eat the broccoli, your body breaks apart the chemical bonds in the molecules that make up the broccoli. This releases chemical energy. Your body transforms the chemical energy to energy your body needs for movement, temperature control, and other life processes. Electric Energy Power Plants Plants that lived millions of years ago carried on Create a vertical half-book to photosynthesis just like the plants that are alive today do. summarize, in your own These ancient plants stored radiant energy from the Sun as words, the relationship chemical energy in their molecular bonds. After the plants between work and energy. died, they were buried under sediment. After much time, pressure from the sediments that covered the plant remains turned the plants into fossil fuels. How is work When fossil fuels burn, the chemical energy, which was related to stored in the molecules of plants that lived millions of years energy? ago, transforms. At electric energy power plants, that chemical energy is transformed to the electric energy that you use in your home and school. As you read in Lesson 1, other forms of energy also can be transformed at electric energy plants. At these facilities, forms of energy—solar, wind, geothermal, and hydroelectric energy—are Key Concept Check transformed to electric energy. 3. Identify three energy transformations that occur to Energy and Work make electric energy. Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. The McGraw-Hill of a division © Glencoe/McGraw-Hill, Copyright When you study for a test, it might feel like a lot of work. But science does not define studying as 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 acts on the object. Reading Essentials Energy, Work, and Simple Machines 47 CC353_008_012_RE_L2_889408.indd353_008_012_RE_L2_889408.indd 9 44/3/10/3/10 11:12:45:12:45 PPMM Work Upward force Upward force Drums’ weight Drums’ weight The drummer does work on the drums as The drummer is no longer doing work on he lifts them. The drums’ kinetic energy and the drums because the drums are not gravitational potential energy increase. moving in the direction of the applied force. Recall that forces are pushes or pulls. When you lift an Visual Check object, you transfer energy from your body to the object. As What 4. Recognize the boy lifts the drums in the figure above, the drums move energy transformations occur as the drums are lifted? and have kinetic energy. As the drums move higher above Earth’s surface, they gain gravitational potential energy. The Companies, Inc. The McGraw-Hill of a division © Glencoe/McGraw-Hill, Copyright boy has done work on the drums. On the right in the figure above, the boy is standing still with his drums lifted in place. Because he is not moving the drums, he is not doing work. To do work on an object, an object must move in the direction of the force. Work is done Key Concept Check only while the force is moving the object. 5. Describe If you do work on an object, how will its Doing Work energy change? How much work do you do when you lift your backpack off the ground? The amount of work depends on the force you apply. If you lift a backpack with a force of 20 N, you do less work than if you lift with a force of 40 N. Work also depends on the distance the object moves during the time the force is applied. If you lift a backpack 1 m, you do less work than if you lift it 2 m. Suppose you toss a backpack in the air. When you release it, it continues moving upward. Even though the backpack is still moving when you let go, no work is being done. This is because you are no longer applying a force to the backpack while it is in the air. 48 Energy, Work, and Simple Machines Reading Essentials CC353_008_012_RE_L2_889408.indd353_008_012_RE_L2_889408.indd 1100 44/3/10/3/10 11:13:38:13:38 PPMM Calculating Work Math Skills The equation for work is shown below. Force is the force applied to the object. Distance is the distance the object A student lifts a bag from the moves in the direction of the force while the force is acting floor to his or her shoulder 1.2 m above the floor, using on it. a force of 50 N. How much Work Equation work does the student do on work (in joules) = force (in newtons) × distance (in meters) the bag? W = Fd a. This is what you know: The force in the equation is in newtons (N), and distance force: F = 50 N is in meters (m). The product of newtons and meters is distance: d = 1.2 m newton-meter (N·m). A newton-meter is a joule (J). b. You need to find: Energy and Heat work: W c. Use this formula: Have you ever heard the phrase burning rubber? The tires W = Fd of race cars are made of rubber. The tires and the road are in Substitute the values for contact, and they move past each other quickly. Recall that d. F and d into the formula friction is a force between two surfaces in contact with each and multiply: other. The direction of friction is in the opposite direction of W = (50 N) × (1.2 m) the motion. = 60 N·m = 60 J Friction between a car’s tires and the road causes some of Answer: The amount of work the kinetic energy of the tires to transform into thermal done is 60 J. energy. If race cars are moving very quickly, thermal energy 6. Solve for Work in the tires causes the rubber to give off a burnt odor. A student pulls out his or her chair in order to sit down. As a race car moves, energy is transformed and transferred. The student pulls the chair For example, a car’s engine transforms energy from fuel and 0.75 m with a force of 20 N.