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Chapter 10 Energy and Work Chapter 10 Energy and Work Lecture Presentation Chapter 10 Energy and Work Chapter Goal: To introduce the concept of energy and to learn a new problem-solving strategy based on conservation of energy. © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Slide 10-2 Chapter 10 Preview Chapter 10 Preview Looking Ahead: Forms of Energy Looking Ahead: Work and Energy • This dolphin has lots of kinetic energy as it leaves the • As the band is stretched, energy is transferred to it as water. At its highest point its energy is mostly potential work . This energy is then transformed into kinetic energy energy . of the rock. • You’ll learn about several of the most important forms of • You’ll learn how to calculate the work done by a force, energy—kinetic, potential, and thermal. and how this work is related to the change in a system’s energy. © 2015 Pearson Education, Inc. Slide 10-3 © 2015 Pearson Education, Inc. Slide 10-4 Chapter 10 Preview Chapter 10 Preview Looking Ahead: Conservation of Energy Looking Ahead • As they slide, their potential energy decreases and their kinetic energy increases, but their total energy is unchanged: It is conserved . • How fast will they be moving when they reach the bottom? Text: p. 283 You’ll use a new before-and-after analysis to find out. © 2015 Pearson Education, Inc. Slide 10-5 © 2015 Pearson Education, Inc. Slide 10-6 Chapter 10 Preview Chapter 10 Preview Looking Back: Motion with Constant Acceleration Stop to Think In Chapter 2 you learned how to describe the motion of a A car pulls away from a stop sign with a constant particle that has a constant acceleration. In this chapter, acceleration. After traveling 10 m, its speed is 5 m/s. What you’ll use the constant-acceleration equations to connect will its speed be after traveling 40 m? work and energy. A. 10 m/s B. 20 m/s C. 30 m/s D. 40 m/s A particle’s final velocity is related to its initial velocity, its acceleration, and its displacement by v2= v 2 +2 ax ∆ ( x)f( x) i x © 2015 Pearson Education, Inc. Slide 10-7 © 2015 Pearson Education, Inc. Slide 10-8 Reading Question 10.1 Reading Question 10.1 If a system is isolated , the total energy of the system If a system is isolated , the total energy of the system A. Increases constantly. A. Increases constantly. B. Decreases constantly. B. Decreases constantly. C. Is constant. C. Is constant. D. Depends on the work into the system. D. Depends on the work into the system. E. Depends on the work out of the system. E. Depends on the work out of the system. © 2015 Pearson Education, Inc. Slide 10-9 © 2015 Pearson Education, Inc. Slide 10-10 Reading Question 10.2 Reading Question 10.2 Which of the following is an energy transfer? Which of the following is an energy transfer? A. Kinetic energy A. Kinetic energy B. Work B. Work C. Potential energy C. Potential energy D. Chemical energy D. Chemical energy E. Thermal energy E. Thermal energy © 2015 Pearson Education, Inc. Slide 10-11 © 2015 Pearson Education, Inc. Slide 10-12 Reading Question 10.3 Reading Question 10.3 If you raise an object to a greater height, you are increasing If you raise an object to a greater height, you are increasing A. Kinetic energy. A. Kinetic energy. B. Heat. B. Heat. C. Potential energy. C. Potential energy. D. Chemical energy. D. Chemical energy. E. Thermal energy. E. Thermal energy. © 2015 Pearson Education, Inc. Slide 10-13 © 2015 Pearson Education, Inc. Slide 10-14 Reading Question 10.4 Reading Question 10.4 If you hold a heavy weight over your head, the work you do If you hold a heavy weight over your head, the work you do A. Is greater than zero. A. Is greater than zero. B. Is zero. B. Is zero. C. Is less than zero. C. Is less than zero. D. Is converted into chemical energy. D. Is converted into chemical energy. E. Is converted into potential energy. E. Is converted into potential energy. © 2015 Pearson Education, Inc. Slide 10-15 © 2015 Pearson Education, Inc. Slide 10-16 Reading Question 10.5 Reading Question 10.5 The unit of work is The unit of work is A. The watt. A. The watt. B. The poise. B. The poise. C. The hertz. C. The hertz. D. The pascal. D. The pascal. E. The joule. E. The joule. © 2015 Pearson Education, Inc. Slide 10-17 © 2015 Pearson Education, Inc. Slide 10-18 The Basic Energy Model Every system in nature has a quantity we call its total energy E. Section 10.1 The Basic Energy Model © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Slide 10-20 Forms of Energy Energy Transformations Some important forms of energy are Energy of one kind can be transformed into energy of • Kinetic energy K: energy of motion. another kind within a system. • Gravitational potential energy Ug: stored energy associated with an object’s height above the ground. • Elastic or spring potential energy Us: energy stored when a spring or other elastic object is stretched. • Thermal energy Eth : the sum of the kinetic and potential energies of all the molecules in an object. • Chemical energy Echem : energy stored in the bonds between molecules. • Nuclear energy Enuclear : energy stored in the mass of the nucleus of an atom. © 2015 Pearson Education, Inc. Slide 10-21 © 2015 Pearson Education, Inc. Slide 10-22 Energy Transformations QuickCheck 10.1 A child is on a playground swing, motionless at the highest point of his arc. What energy transformation takes place as he swings back down to the lowest point of his motion? A. K → Ug The weightlifter converts Elastic potential energy of the B. Ug → K chemical energy in her body springboard is converted into C. Eth → K into gravitational potential kinetic energy. As the diver D. Ug → Eth energy of the barbell. rises into the air, this kinetic E. K → Eth energy is transformed into gravitational potential energy. © 2015 Pearson Education, Inc. Slide 10-23 © 2015 Pearson Education, Inc. Slide 10-24 QuickCheck 10.1 QuickCheck 10.2 A child is on a playground swing, motionless at the highest A skier is gliding down a gentle slope at a constant speed. point of his arc. What energy transformation takes place as What energy transformation is taking place? he swings back down to the lowest point of his motion? A. K → Ug A. K → Ug B. Ug → K B. Ug → K C. Eth → K C. Eth → K D. Ug → Eth D. Ug → Eth E. K → Eth E. K → Eth © 2015 Pearson Education, Inc. Slide 10-25 © 2015 Pearson Education, Inc. Slide 10-26 QuickCheck 10.2 Energy Transfers and Work • Energy can be transferred between a system and its A skier is gliding down a gentle slope at a constant speed. environment through work and heat. What energy transformation is taking place? • Work is the mechanical transfer of energy to or from A. K → Ug a system by pushing or B. Ug → K pulling on it. C. Eth → K • Heat is the nonmechanical D. Ug → Eth transfer of energy between E. K → Eth a system and the environment due to a temperature difference between the two. © 2015 Pearson Education, Inc. Slide 10-27 © 2015 Pearson Education, Inc. Slide 10-28 Energy Transfers and Work QuickCheck 10.3 A tow rope pulls a skier up the slope at constant speed. What energy transfer (or transfers) is taking place? A. W → Ug B. W → K The athlete does The hand does The boy does C. W → Eth work on the work on the work on the D. Both A and B. shot, giving it match, giving it slingshot, E. Both A and C. kinetic energy, thermal energy, giving it elastic K. Eth . potential energy, Us. © 2015 Pearson Education, Inc. Slide 10-29 © 2015 Pearson Education, Inc. Slide 10-30 QuickCheck 10.3 The Work-Energy Equation • Work represents energy that is transferred into or out of a A tow rope pulls a skier up the slope at constant speed. system. What energy transfer (or transfers) is taking place? • The total energy of a system changes by the amount of work done on it. A. W → Ug B. W → K C. W → Eth • Work can increase or decrease the energy of a system. D. Both A and B. • If no energy is transferred into or out of a system, that is E. Both A and C. an isolated system . © 2015 Pearson Education, Inc. Slide 10-31 © 2015 Pearson Education, Inc. Slide 10-32 QuickCheck 10.4 QuickCheck 10.4 A crane lowers a girder into place at constant speed. A crane lowers a girder into place at constant speed. Consider the work Wg done by gravity and the work WT Consider the work Wg done by gravity and the work WT done by the tension in the cable. Which is true? done by the tension in the cable. Which is true? A. Wg > 0 and WT > 0 A. Wg > 0 and WT > 0 B. Wg > 0 and WT < 0 B. Wg > 0 and WT < 0 The downward force of gravity is in the direction of motion ⇒ positive work. C. Wg < 0 and WT > 0 C. Wg < 0 and WT > 0 D. Wg < 0 and WT < 0 D. Wg < 0 and WT < 0 The upward tension is in the direction opposite the motion ⇒ negative work. E. Wg = 0 and WT = 0 E. Wg = 0 and WT = 0 © 2015 Pearson Education, Inc.
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