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Lesson
ESSENTIAL QUESTION
What is energy?By the end of this lesson, you should be able to describe how energy is conserved through transformation between different forms.
Introduction to Energy
The chemical energy contained in fireworks is transformed into sound, light, and energy as heat when the fireworks shells explode.
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Get Energized!5 Identify As you read this page
and the next, underline the factors that affect an object’s kinetic and potential energy.
Active Reading
As the skater moves up the ramp, he gains height but loses speed. Some of his kinetic energy is converted back to potential energy. The rest of it is transferred as heat due to friction.
At the bottom of the ramp, the skater’s kinetic energy is at its peak because he is going the fastest. His potential energy is at its lowest because he is closer to the ground than at any other point on the ramp.
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What are two types of energy?In science, energy is the ability to cause change. Energy takes many different forms and has many different effects. There are two general types of energy: kinetic energy and potential energy.
Kinetic EnergyKinetic energy
its speed. Kinetic energy increases as mass increases. Imagine that a bowling ball and a soccer ball roll across the floor at the same speed. The bowling ball has more kinetic energy than the soccer ball has because the bowling ball has a greater mass.
Kinetic energy also increases as speed increases. If two bowling balls with the same mass roll across the floor at different speeds, the faster ball will have the greater kinetic energy.
6 Unit 1 Energy and Energy Transfer
Think Outside the Book Inquiry
Potential EnergyPotential energy
Can objects have potential and kinetic energy at the same time?
6 Diagram Think of another situation that shows kinetic and potential energy. Draw a sketch of and write a description explaining the situation in terms of potential and kinetic energy.
7 Analyze Do you think that the skater has any gravitational potential energy at point C? Why?
At the top of the ramp, the skater has potential energy because gravity can pull him downward. He has no speed, so he has no kinetic energy.
As the skater moves closer to the ground, the decrease in potential energy is equal to the increase in his kinetic energy. As he rolls down the ramp, his potential energy decreases because his distance from the ground decreases. His kinetic energy increases because his speed increases.
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Lesson 1 Introduction to Energy 7
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8 Identify Label the three forms of energy represented in this image.
Visualize It!
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In Perfect FormWhat forms can energy take?Kinetic energy and potential energy are two types of energy that can come in many different forms. Some common forms of energy include mechanical, sound, radiant, electrical, chemical, thermal, and nuclear energy. Energy is expressed in joules (J) (JOOLZ).
Mechanical EnergyMechanical energy is the sum of an object’s kinetic energy and potential energy. Remember that kinetic energy is the energy of motion, and potential energy is the energy of position. So mechanical energy is the energy of position and motion. A moving car has mechanical energy. An object’s mechanical energy can be all potential energy, all kinetic energy, or a combination of potential and kinetic energy.
Sound EnergySound energy is kinetic energy caused by the vibration of particles in a medium such as steel, water, or air. As the particles vibrate, they transfer the sound energy to other particles. The sound a guitar makes is caused by the vibrations of its strings transferring energy to the air around it. You hear the sound because special structures in your ears detect the vibrations of the particles in the air.
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Radiant EnergyRadiant energy is the energy of electromagnetic waves. Electromagnetic waves are transmitted through space. These waves are caused by the vibration of electrically charged particles. Electromagnetic waves include visible light, x-rays, and microwaves. X-rays are high-energy waves used by doctors and dentists to look at your bones. Microwaves can be used to cook food or to transmit cellular telephone calls. The sun releases a large amount of electromagnetic energy, some of which reaches Earth as radiant energy. This radiant energy heats and lights Earth’s surface, making life possible.
Electrical EnergyElectrical energy is the energy that results from the position or motion of charged particles. The electrical energy that powers the lights overhead is associated with negatively charged particles moving in a wire. The wire also has positively charged particles that do not move. The negatively charged particles move within the wire and create an electric current. Anything you plug in or use batteries to operate, from lamps to computers to flashlights, uses electrical energy.
9 Compare How does electrical energy differ from radiant energy? How does electrical energy differ from mechanical energy?
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10 Infer Would you expect to detect electrical energy if you played the pinball game shown in the picture? Explain your answer.
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Chemical EnergyChemical energy is a form of potential energy. The amount of chemical energy in a molecule depends on the kinds of atoms and their arrangement. During a chemical change, bonds between these atoms break, and new bonds form. The food you eat, batteries, and matches are sources of chemical energy.
Thermal EnergyThe thermal energy of an object is the kinetic energy of its particles. Particles move faster at higher temperatures than at lower temperatures. The faster the molecules in an object move, the more thermal energy the object has. Also, the more particles an object has, the more thermal energy it has. Heat is the energy transferred from an object at a higher temperature to an object at a lower temperature.
Nuclear EnergyThe nucleus of an atom is the source of nuclear energy. When an atom’s nucleus breaks apart, or when the nuclei of two small atoms join together, energy is released. The energy given off by the sun comes from nuclear energy. In the sun, hydrogen nuclei join to make a helium nucleus. This reaction gives off a huge amount of energy. The sun’s light and heat come from these reactions. Without nuclear energy from the sun, life would not exist on Earth.
11 Identify As you read, underline sources of chemical and thermal energy.
Active Reading
12 Synthesize Why is the chemical energy of a battery potential energy and not kinetic energy?
Solar flares are explosions of hot gases on the sun. They can release radiant energy that reaches all the way to Earth.
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Batteries
What is an energy transformation?An energy transformation
The chemical energy from the batteries is transformed into electrical energy.
Some examples of energy transformation are illustrated in this flashlight. Follow the captions to learn how energy is transformed into the light energy that you rely on when you turn on a flashlight.
16 Describe Give two examples of other devices in which the chemical energy in a battery is transformed into electrical energy.
Visualize It!
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Electric wire Lightbulb
Is energy conserved?A closed system is a group of objects that transfers energy only to one another. For example, a roller coaster can be considered a closed system if it includes everything involved, such as the track, the cars, and the air around them. Energy is conserved in all closed systems. The law of conservation of energy states that energy cannot be created or destroyed. It can only change forms. All of the different forms of energy in a closed system always add up to the same total amount of energy. It does not matter how many energy transformations take place.
For example, on a roller coaster some mechanical energy gets transformed into sound and heat energy as the roller coaster goes down a hill. The total of the coaster’s mechanical energy at the bottom of the hill, the extra heat energy, and the sound energy is the same total amount of energy as the original amount of mechanical energy. In other words, total energy is conserved.
Active Reading 17 Relate How are energy transformations related to the law of conservation of energy?
The electrical energy in the wire is transformed into light in the light bulb. Some of the electrical energy is also transformed into heat energy.
19 Describe Give another example of electrical energy being transformed into light.
18 Apply Have you ever thought about how cell phones work? What form of energy is used to power a cell phone? What form of energy do you use from a cell phone? Can you think of any other forms of energy that may be used inside of a cell phone?
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Think Outside the Book
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Potential energy results from an object’s position, composition, or condition, and kinetic energy results from an object’s motion.
Energy is the ability to cause change; it cannot be created or destroyed.
Answers: 20 remains the same; 21 potential energy; 22 kinetic energy; 23 heat
Visual SummaryIntroduction
to Energy
24 Apply Identify and give examples of at least three types of energy you see being used as you look around your classroom.
To complete this summary, circle the correct word. Then use the key below to check your answers. You can use this page to review the main concepts of the lesson.
21 A basketball that is balanced on the rim of a basketball hoop has potential energy / kinetic energy.
22 A basketball that is rolling across a floor has potential energy / kinetic energy.
20 The total energy in a closed system remains the same / changes as energy changes forms.
Energy transformation takes place when energy changes from one form to another.
23 When a candle is burned, some chemical energy is transformed into nuclear / heat energy.
Unit 1 Energy and Energy Transfer14
Lesson
ESSENTIAL QUESTION
How can we calculate kinetic and potential energy?By the end of this lesson, you should be able to calculate kinetic and potential energy and know how these two types of energy are related.
ESSENTIAL QUESTION
Kinetic and Potential Energy
Climbing a hill requires a lot of energy but it makes the ride back down fun.
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Visualize It!
On the Move
What is kinetic energy?Energy is the ability to do work. There are different forms of energy. One form that you can find all around you is kinetic energy. Kinetic energy is the energy of motion. Every moving object has kinetic energy. For example, a hammer has kinetic energy as it moves toward a nail. When the hammer hits the nail, energy is transferred. Work is done when movement occurs in the direction of the force, and the nail is driven into a board.
The Energy of MotionActive Reading 5 Identify As you read, underline
two factors that affect an object’s kinetic energy.
What determines the amount of kinetic energy that an object has? The faster an object moves, the more kinetic energy it has. So kinetic energy depends, in part, on speed. Kinetic energy also depends on mass. If two objects move at the same speed, then the one that has more mass will have more kinetic energy. Imagine a bike and a car that are moving at the same speed. The car has more kinetic energy than the bike has because the car has more mass.
The bike at the top of the hill is not moving. It does not have kinetic energy. The bike that is going down the hill has kinetic energy. As the bike moves faster, its kinetic energy increases.
6 Apply How does the rider’s ability to stop the bike change as the bike moves down a steep hill?
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How is the kinetic energy of an object calculated?An object’s kinetic energy is related to its mass and speed. The speed of an object is the distance that it travels in a unit of time. The following equation shows how kinetic energy is calculated.
Sample ProblemThe foal has a mass of 100 kg and is moving at 8 m/s along the beach. What is the kinetic energy (KE) of the foal?
Identify
A. What do you know? The mass, m, is 100 kg. The speed, v, is 8 m/s.
B. What do you want to find? kinetic energy
Plan
C. Write the formula: KE = 1 __ 2 mv2
D. Substitute into the formula: KE = 1 __ 2 (100 kg)(8 m/s)2
Solve
E. Multiply: KE = 1 __ 2 (100 kg)(64 m2/s 2) = 3,200 kg · m2/s2 = 3,200 J
Answer: 3,200 J
Do the Math
kinetic energy = 1 _ 2 mv2
The letter m is the object’s mass, and the letter v is the object’s speed. When the mass is expressed in kilograms and the speed in meters per second, kinetic energy is expressed in joules (J).
You Try It
7 Calculate Complete the table at the right to calculate the kinetic energy of the horses.
Horse m v v2 KE
foal 100 kg 10 m/s
mare 800 kg 10 m/s
mare 800 kg 15 m/s
Lesson 2 Kinetic and Potential Energy 21
Think Outside the Book Inquiry
9 Identify Fill in the type of potential energy that is illustrated in each image.
Fruit can provide energy to your body.
The archer pulls back on the string and bends the bow’s limbs.
The boulder is high above the ground.
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8 Classify With a partner, create a poster that shows examples of potential energy from everyday life. Label each example as gravitational, elastic, or chemical potential energy.
It Could Change
Visualize It!
What is potential energy?Some energy is stored energy, or potential energy. Potential energy is the energy an object has because of its position, condition, or chemical composition. Like kinetic energy, potential energy is the ability to do work. For example, an object has elastic potential energy when it has been stretched or compressed. Elastic potential energy is stored in a stretched spring or rubber band. An object has gravitational potential energy due to its position above the ground. An object held above the ground has the potential to fall. The higher the object is above the ground, the greater its gravitational potential energy. Potential energy that depends on an object’s position is referred to as mechanical potential energy. But there are other types of potential energy that do not depend on an object’s position. For example, a substance stores chemical potential energy as a result of its chemical bonds. Some of that energy can be released during chemical reactions.
Unit 1 Energy and Energy Transfer22
How is the gravitational potential energy of an object calculated?The following equation describes an object’s gravitational potential energy.
Sample ProblemThe cat has a mass of 4 kg and is 1.5 m above the ground. What is the gravitational potential energy of the cat?
Identify
A. What do you know? mass = 4 kg, height = 1.5 m, acceleration due to gravity = 9.8 m/s2
B. What do you want to find? gravitational potential energy
Plan
C. Write the formula: GPE = mghD. Substitute the given values into the formula:
GPE = (4 kg)(9.8 m/s2)(1.5 m)
Solve
E. Multiply: PE = (4 kg)(9.8 m/s2)(1.5 m) = 58.8 kg · m2/s2 = 58.8 J
Answer: 58.8 J
Do the Math
gravitational potential energy = mgh
The letter m represents the object’s mass expressed in kilograms. The letter g represents the acceleration due to Earth’s gravity, which is 9.8 m/s2. The letter h is the object’s height from the ground in meters. The height is a measure of how far the object can fall. Like kinetic energy, potential energy is expressed in units of joules.
You Try It
10 Calculate Three books are on different shelves. Calculate the gravitational potential energy of each book based on its mass and its height above the floor.
Object m h PEpicture book 0.2 kg 2 mpicture book 0.2 kg 3 m
textbook 1.5 kg 4 m
Lesson 2 Kinetic and Potential Energy 23
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It All Adds Up!
The mechanical energy of the ball is the sum of its kinetic energy and potential energy.
11 Identify As you read, underline the two components of mechanical energy.
Active Reading
12 Compare Circle the position of the ball when its gravitational potential energy is greatest.
13 Analyze When does the ball have zero gravitational potential energy?
mechanical energy = KE + PE
ME = 1 _ 2 mv2 + mgh
If the object’s only potential energy is gravitational potential energy, you can use the following equation to find mechanical energy.
Visualize It!
How is the mechanical energy of an object calculated?A moving object can have both kinetic and potential energy. Mechanical energy is the energy possessed by an object due to its motion and position. For example, a thrown baseball has kinetic energy. It also has potential energy because it is above the ground. The sum of the ball’s kinetic energy and mechanical potential energy is its mechanical energy. You can use the following equation to find mechanical energy.
Unit 1 Energy and Energy Transfer24
3m
2m
1m
0mEnergy of the Basketball
Ener
gy (J
)
Height above floor (m)
You Try It
14 Calculate When the basketball is at its maximum height of 3 meters, it is not moving. The table below lists the KE and GPE for the basketball at heights of 3 m, 2 m, 1 m, and 0 m. Write the missing values for KE and GPE in the table. Then find the mechanical energy for each height.
Height KE GPE ME
3.0 m 18 J
2.0 m 6 J 12 J
1.0 m 12 J 6 J
0 m 18 J
Do the Math
15 Graph Use the data above to plot and label two lines representing the kinetic energy and the gravitational potential energy of the basketball.
16 Analyze What is the relationship between the kinetic energy and the gravitational potential energy of the basketball?
The rim height is 3 meters.
Lesson 2 Kinetic and Potential Energy 25
Visual Summary
20 Synthesize A skydiver jumps out of a plane. Describe how gravitational potential energy changes as the skydiver falls. Describe how the skydiver’s kinetic energy changes when the parachute opens.
Answers: 17 speed; 18 chemical; 19 gravitational
To complete this summary, fill in the blanks with the correct word. Then, use the key below to check your answers. You can use this page to review the main concepts of the lesson.
Kinetic and Potential Energy
All moving objects have kinetic energy.
Potential energy is stored energy.
17 Kinetic energy depends on an object’s mass and
19 The formula ME = 1 __ 2 mv2 + mgh can be used to calculate mechanical energy if the only potential energy is potential energy. 18 Potential energy can be gravitational,
, or elastic.
kinetic energy = 1 _ 2 mv2
gravitational potential energy = mgh
Mechanical energy is kinetic energy plus potential energy due to position.
mechanical energy = KE + PE
Unit 1 Energy and Energy Transfer26
