Energy, Work, and Simple Machines

Key Concepts

? What is the law of conservation of energy?

? In what ways can energy be transformed?

? How are energy and work related?

Energy, Work, and Simple Machines

Energy Transformations and Work

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 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

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.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Sticky Notes As you read, use sticky notes to mark information that you do not understand. Read the text carefully a second time. If you still need help, write a list of questions to ask your teacher.

ACADEMIC VOCABULARY

transform (verb) to change form or structure

Key Concept Check 1. Define What is the law of conservation of energy?

Energy Transformations

As you read in Lesson 1, different types of electric energy plants supply the energy you use in your home and school. Energy transformation is the conversion of one form of energy to another. For example, suppose a zoo uses a heat lamp to warm a newborn zebra. The electric energy in the wiring of the heat lamp is transformed into thermal energy.

Energy also is transferred when it moves from one object to another. When energy is transferred, the form of energy does not have to change. For example, the thermal energy in 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 in your eyes. The radiant energy was electric energy in the lamp's wiring, and it was chemical energy in the fuel at the 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

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.

Plants and the Body

When a plant, such as broccoli, carries on photosynthesis, it transforms radiant energy from the Sun into chemical 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 photosynthesis just like the plants that are alive today do. These ancient plants stored radiant energy from the Sun as chemical energy in their molecular bonds. After the plants died, they were buried under sediment. After much time, pressure from the sediments that covered the plant remains turned the plants into fossil fuels.

When fossil fuels burn, the chemical energy, which was stored in the molecules of plants that lived millions of years 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 transformed to electric energy.

Energy and Work

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 Check 2. Apply At what point in a roller coaster ride is gravitational potential energy the greatest?

Create a vertical half-book to summarize, in your own words, the relationship between work and energy.

How is work related

to energy?

Key Concept Check 3. Identify three energy transformations that occur to make electric energy.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Energy, Work, and Simple Machines 47

Work

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Upward force Drums' weight

Upward force Drums' weight

The drummer does work on the drums as he lifts them. The drums' kinetic energy and gravitational potential energy increase.

The drummer is no longer doing work on the drums because the drums are not moving in the direction of the applied force.

Visual Check 4. Recognize What energy transformations occur as the drums are lifted?

Key Concept Check 5. Describe If you do work on an object, how will its energy change?

Recall that forces are pushes or pulls. When you lift an object, you transfer energy from your body to the object. As the boy lifts the drums in the figure above, the drums move and have kinetic energy. As the drums move higher above Earth's surface, they gain gravitational potential energy. The 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 only while the force is moving the object.

Doing Work

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

Calculating Work

The equation for work is shown below. Force is the force applied to the object. Distance is the distance the object moves in the direction of the force while the force is acting on it.

Work Equation work (in joules) = force (in newtons) ? distance (in meters)

W = Fd The force in the equation is in newtons (N), and distance is in meters (m). The product of newtons and meters is newton-meter (N?m). A newton-meter is a joule (J).

Energy and Heat

Have you ever heard the phrase burning rubber? The tires of race cars are made of rubber. The tires and the road are in contact, and they move past each other quickly. Recall that friction is a force between two surfaces in contact with each other. The direction of friction is in the opposite direction of the motion.

Friction between a car's tires and the road causes some of the kinetic energy of the tires to transform into thermal energy. If race cars are moving very quickly, thermal energy in the tires causes the rubber to give off a burnt odor.

As a race car moves, energy is transformed and transferred. For example, a car's engine transforms energy from fuel and transfers it to the wheel axle. In every energy transformation and every energy transfer, some energy is transformed into thermal energy. This thermal energy is transferred to the surroundings. Thermal energy moving from a region of higher temperature to a region of lower temperature is called heat. Scientists sometimes call this heat waste energy because it is not easily used to do useful work.

Math Skills

A student lifts a bag from the floor to his or her shoulder 1.2 m above the floor, using a force of 50 N. How much work does the student do on the bag?

a. This is what you know:

force: F = 50 N

distance: d = 1.2 m

b. You need to find:

work: W

c. Use this formula:

W = Fd

d. Substitute the values for

F and d into the formula and multiply:

W = (50 N) ? (1.2 m)

= 60 N?m = 60 J

Answer: The amount of work done is 60 J.

6. Solve for Work

A student pulls out his or her chair in order to sit down. The student pulls the chair 0.75 m with a force of 20 N. How much work does he or she do on the chair?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Energy, Work, and Simple Machines 49

Mini Glossary

energy transformation: the conversion of one form of energy to another

law of conservation of energy: a principle that says that energy can be transformed from one form to another, but it cannot be created or destroyed

work: 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

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes an energy transformation that occurs during photosynthesis.

2. The figure below depicts a hill in the middle of a roller coaster ride. On each line, identify the energy transformation that is occurring at that point in the ride.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

3. Write a question about something you marked with a sticky note. Then answer the question from what you learned by rereading the lesson or asking your teacher.

What do you think

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?

50 Energy, Work, and Simple Machines

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