Describing Energy

LESSON 2 DESCRIBING ENERGY FOCUS QUESTION How do you classify and calculate different forms of energy? Change Requires Energy When something is able to change its surroundings or itself, it has energy. Energy is the ability to cause change. The avail- ability of energy limits what can occur in any system. Without nothing would ever change. The moving tennis racket energy in Figure 6 has energy. That racket causes change when it deforms the tennis ball and changes the tennis ball's motion. Work transfers energy The tennis racket in Figure 6 also does work on the tennis ball, applying a force to that ball through a distance. When this happens, the racket transfers energy to the ball. Therefore, energy can also be described as the ability to do work. Because energy can be described as the ability to do work, energy can be measured with the same units as work. Energy, like work, can be measured in joules. Imagine that the tennis racket in Figure 6 does 250 J of work on the tennis ball. Then, 250 J of energy is transferred from the racket to the ball. Get it? Figure 6 The tennis racket causes changes to occur when it hits the tennis ball. Describe the changes that are occurring. Identify What limits the speed at which the ball will leave the racket? Systems The tennis racket and the tennis ball in Figure 6 are systems. A system is anything around which you can imagine a boundary. A system can be a single object, such as a tennis ball, or a group of objects, such as the solar system. Energy is a quanti- tative property of a system. When one system does work on a second system, energy is transferred from the first system to the second system. COLLECT EVIDENCE Use your Science Journal to record the evidence you collect as you complete the readings and activities in this lesson. INVESTIGATE GO ONLINE to find these activities and more resources. SEP Quick Investigation: Interpret Data from a Slingshot Carry out an investigation to observe and compare the forces in a system needed to move an object using a slingshot with varying degrees of applied force. ? Revisit the Encounter the Phenomenon Question What information from this lesson can help you answer the Module question? Different Forms of Energy Lesson 2. Describing Energy 95 Turn on an electric light, and a dark room becomes bright. Turn on a portable music player, and sound comes through your headphones. In both situations, a change occurs. These changes differ from each other and from the tennis racket hitting the tennis ball in Figure 6. This is because energy has many different forms. These forms include mechani- cal energy, electrical energy, chemical energy, and radiant energy. Figure 7 shows some everyday situations in which you might notice energy. Automobiles make use of the chemical energy of gasoline. Many household appliances require electri- cal energy to function. Radiant energy from the Sun warms Earth. In short, energy plays a role in every activity that you do. Get It?pono energy. Identify three different forms of energy. An energy analogy Is the chemical energy of food the same as the energy that comes from the Sun or the energy that comes from gasoline? Money can be used in an analogy to help you under- stand energy. Money exists in a variety of forms, such as coins, dollar bills, and twen- ty-dollar bills. You can convert money from one form to another. For example, you could obtain four quarters for a dollar bill. Regardless of its form, money is money. The same is true for energy. Energy from the Sun that warms you and energy from the food that you eat are only different forms of the same thing. Chemical potential energy Electrical energy Figure 7 Energy can be stored and transferred from one place to another. For example, the energy due to the chemical bonds in gasoline is easy to transport in cars. Electrical energy is transferred from a power plant to appliances in your home. Radiant energy is transferred from the Sun to Earth. energy Science usage: the ability to cause change You transfer energy when you do work. Common usage: the capacity of acting or being active That soccer player had a lot of energy on the field today. Kinetic energy When you think of energy, you might think of objects in motion. Objects in motion can collide with other objects and cause change. Therefore, objects in motion have energy. Kinetic energy is energy due to motion. A car moving along a highway and a ballet dancer leaping through the air have kinetic energy. The kinetic energy from an object's motion depends on that object's mass and speed. Kinetic Energy Equation kinetic energy (in joules) == Radiant energy KE mass (in kg) X [speed (in m/s)]? mv2 If mass is measured in kilograms (kg) and speed is measured in meters per second (m/s), then kinetic energy is measured in joules (J). If you drop a softball from just above your knee, the kinetic energy from that ball's falling motion is about 1 J just before the ball reaches the floor. EXAMPLE Problem 4 SOLVE FOR KINETIC ENERGY A jogger with a mass of 60.0 kg is moving forward at a speed of 3.0 m/s. What is the jogger's kinetic energy from this forward motion? Identify the Unknown: List the Knowns: Set Up the Problem: Solve the Problem: Check the Answer: kinetic energy: KE mass; m 60.0 kg KE = 1⁄2 mv2 speed: v 3.0 m/s KE = 1⁄2 (60.0 kg)(3.0 m/s)2 KE = 1 (60,0 kg)(9.0 m2/s2) KE=270 J Check the last step by estimating. Round 9.0 m2/s2 up to 10 m2/s2. Then, ÷ (60,0 kg)(10 m2/s2) = 300 J. This is close to 270 J, so the final calculation was reasonable. PRACTICE Problems 16. A baseball with a mass of 0.15 kg is moving at a speed of 40.0 m/s. What is the baseball's kinetic energy from this motion? 17 CHALLENGE A 1500-kg car doubles its speed from 50 km/h to 100 km/h. By how many times does the kinetic energy from the car's forward motion increase? Matter and Energy Compare the kinetic energies of different objects that travel at the same speed but have different masses. Then compare objects that have the same mass but travel at different speeds. Use your results to explain whether doubling mass or doubling speed has a greater effect on kinetic energy. ADDITIONAL PRACTICE Lesson 2. Describing Energy Potential energy Energy does not always involve motion. Even motionless objects can have energy. 97 Potential energy is energy that is stored due to the interactions between objects. One example is the energy stored between an apple hanging on a tree and Earth. Energy is stored between the apple and Earth because of the gravitational force between the apple and Earth. Another example is the energy stored between objects that are connected by a compressed spring or a stretched rubber band. Get It? Explain how a book can have energy even if it is not moving. Elastic potential energy If you stretch a rubber band and let it go, it sails across the room. As it flies through the air, it has kinetic energy due to its motion. Where did this kinetic energy come from? Just as there is potential energy due to gravitational forces, there is also potential energy due to the elastic forces between the particles that make up a stretched rubber band. The energy of a stretched rubber band or a compressed spring is called elastic potential energy. Elastic potential energy is energy that is stored by compressing, stretching, or bending an object. Get It? Describe how the elastic potential energy of a trampoline changes as a person jumps on it. Chemical potential energy The food that you eat and the gasoline in cars also have stored energy. This stored energy is due to the chemical bonds between atoms. Chemical potential energy is energy that is due to chemical bonds. You might notice chemical potential energy when you burn a substance. When an object is burned, chemical potential energy becomes thermal energy and radiant energy. Figure 8 shows the process for burning methane. ↑ + Methane gas Oxyge n gas Carbon dioxide 935 + Water vapor Figure 8 When methane burns, it combines with oxygen to form carbon dioxide and water vapor. In this chemical reaction, chemical potential energy is converted to other forms of energy. WORD ORIGINS potential comes from the Latin word potens, form of posse, which means to be able The rock on the cliff has potential energy because it is able to cause change if it falls. 98 Module 4. Work and Energy I 1 7 F 9 E Gravitational potential energy Consider the blue vase in Figure 9. Together, the blue vase and Earth have potential energy. Gravitational potential energy is energy that is due to the gravitational forces between objects. Gravitational potential energy is often shortened to GPE. Any system that has objects that are attracted to each other through gravity has ti gravitational poten- tial energy. An apple and Earth have gravitational potential energy. The solar system also has gravita- tional potential energy. The gravitational potential energy of a system containing just Earth and another object depends on the object's mass, Earth's gravity, and the object's height. Recall that near Earth's surface, g is equal to 9.8 N/kg. Gravitational Potential Energy Equation gravitational potential energy (J) mass (kg) X gravity (N/kg) x height (m) GPE migh Height and gravitational potential energy Look at the bookcase in Figure 9. Moving a vase from one shelf to another changes its GPE because the relative position of the object in Earth's gravitational field changes. Moving the vase to a higher shelf increases GPE as more energy is stored in the vase-Earth system, and moving it to a lower shelf decreases GPE as less energy is stored in the vase-Earth system. Now imagine that this bookcase is on the second floor of a building and that this building is at the top of a large hill. How should you measure the heights of the objects on the shelves? You could measure the heights from the floor. You could also measure the heights from the ceiling, the ground outside, the bottom of the hill, or Earth's center. Figure 9 The gravitational potential energy of any system containing only an object on the bookcase and Earth depends on the object's mass, the strength of Earth's gravity, and the object's height. The object's height is measured relative to a reference level. The floor, the ground, the ceiling, and Earth's center are possible reference levels. To calculate gravitational potential energy, height is measured from a reference level. This means that gravitational potential energy varies depending on the chosen reference level. Relative to the floor, the GPE of a system containing just the blue vase and Earth is about 90J. Relative to the ceiling, the GPE of this same system might be about -40 J. Relative to Earth's center, this system's GPE is about 300 million J. All of these statements are correct. In addition, the GPE of the blue vase-Earth system is greater than the GPE of the green vase-Earth system for every reference level. However, statements such as "The gravita- tional potential energy is 100 J" are meaningless unless a reference level is given. Lesson 2 + Describing Energy 99 EXAMPLE Proble SOLVE FOR GRAVITATIONAL POTENTIAL ENERGY A 4.0-kg ceiling fan is placed 2.5 m above the floor. What is the gravitational potential energy of the Earth-ceiling fan system relative to the floor? Identify the Unknown: List the Knowns: Set Up the Problen Solve the Problem: Check the Answer: gravitational potential energy: GPE mass: m 4.0 kg gravity: g = 9.8 N/kg height: h = 2.5 m GPE = (4.0 kg)(9.8 N/kg)(2.5 m) = 98 Nm = 98 J Round 9.8 N/kg to 10 N/kg. Then, GPE = (4.0 kg)(10 N/kg)(2.5 m) == 100 J. This is very close to the answer above. Therefore, that answer is reasonable. PRACTICE Problems 18. An 8,0-kg history textbook is placed on a 1.25-m high desk. What is the gravitational potential energy of the textbook-Earth system relative to the floor? 19. CHALLENGE What is the GPE of the textbook-Earth system in problem 18, relative to the desktop? * / Check Your Progress Summary * Forms of energy include mechanical, electrical, chemical, thermal, and radiant energy. Kinetic energy is the energy that a moving object has because of its motion. • Potential energy is stored energy due to the interactions between objects. Different forms of potential energy include elastic potential energy, chemical potential energy, and gravitational potential energy. Demonstrate Understanding 20. Describe a change caused by kinetic energy as well as a change that involves potential energy. 21. Infer whether a system can have kinetic energy and potential energy at the same time. 22. Differentiate elastic potential energy and chemical potential energy. Explain Your Thinking 23. Compare The different molecules that make up the air in a room have, on average, the same kinetic energy. How does the speed of the different molecules that make up the air depend on their masses? 24. MATH >Connection A 0.06-kg ball is moving at 5.0 m/s. What is the k inetic energy from this motion? 25. MATH Connection A 0.50-kg apple is 2.0 m above the reference level. What is the GPE of the apple-Earth system? LEARNSMART Go online to follow your pe Energy rsonalized learning path to review, practice, and reinforce your understanding.