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Chapter 10 Energy Topics: • • Important forms of energy • • Definition of work • • The law of conservation of energy How energy can be transformed and transferred Concepts of kinetic, potential, and thermal energy Elastic collisions Sample question: When flexible poles became available for pole vaulting, athletes were able to clear much higher bars. How can we explain this using energy concepts? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-1 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-3 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-4 Forms of Energy Mechanical Energy Ug K Thermal Energy Us Other forms include E th Echem Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Enuclear Slide 10-12 The Basic Energy Model Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-13 Energy Transformations Kinetic energy K = energy of motion Potential energy U = energy of position Thermal energy Eth = energy associated with temperature System energy E = K + U + Eth + Echem + ... Energy can be transformed within the system without loss. Energy is a property of a system. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-14 Some Energy Transformations Echem Ug K Eth Echem Ug Us K U g Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-15 Energy Transfers These change the energy of the system. Interactions with the environment. Work is the mechanical transfer of energy to or from a system via pushes and pulls. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-20 Energy Transfers: Work W K W Eth W Us Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-21 The Law of Conservation of Energy Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-23 The Basic Equation Kf Uf Eth Ki Ui + Esys A few things to note: • Esys can be positive (Energy in) or negative (Energy out) • We are, for now, ignoring heat. • Thermal energy is…special. When energy changes to thermal energy, this change is irreversible. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-24 Energy Model Types of Energy Kinetic Energy KE = 1/2 mv2 Potential Energy: Gravitational Potential Energy Spring Potential Energy PEg = mgy Pes = 1/2 kL2 Conservation of Energy (Closed System) Before After KEi PEgi PEsi Esys KE f PEgf PEsf Eth Visualizations: • Energy Bar Charts Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding A skier is moving down a slope at a constant speed. What energy transformation is taking place? A. EK => Eg B. Eg => Eth C. Es => Eg D. Eg => EK E. EK => Eth Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-12 Answer A skier is moving down a slope at a constant speed. What energy transformation is taking place? B. Ug Eth Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-13 Demonstration: Smash the Professor - Part 2 Energy Bar Charts for a swinging pendulum Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-7 Examples of Energy Bar Charts & solving Energy Problems Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-7 Energy Bar Charts and Energy Transformations For the following questions, start by drawing energy bar graphs and identifying energy transformations. Then answer the question. 1. A block slides down a frictionless ramp of height h. It reaches velocity v at the bottom. To reach a velocity of 2v, the block would need to slide down a ramp of height A. 1.41 h h B. 2 h Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. C. 3 h D. 4 h E. 6 Slide 10-23 Energy Bar Charts and Energy Transformations For the following questions, start by drawing energy bar graphs and identifying energy transformations. Then answer the question. 2. A block is shot up a frictionless 40 degree slope with initial veloctiy v. It reaches height h before sliding back down. The same block is shot with the same velocity up a frictionless 20 degree slope. On this slope, the block reaches height A. 2 h B. h C. 1/2 h D. Greater than h, but I connot predict an exact value. E. Less than h, but I can't predict an exact value Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-23 Energy Bar Charts and Energy Transformations For the following questions, start by drawing energy bar graphs and identifying energy transformations. Then answer the question. 3. Two balls, one twice as heavy as the other, are dropped from the roof of a building. Just before hitting the ground, the heavier ball has _________ the kinetic energy of the lighter ball. A. one-half B. the same amount as C. twice D. four times Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-23 Conceptual Example Problem A car sits at rest at the top of a hill. A small push sends it rolling down a hill. After its height has dropped by 5.0 m, it is moving at a good clip. Write down the equation for conservation of energy, noting the choice of system, the initial and final states, and what energy transformation has taken place. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-25 Example: Roller Coaster Demonstration: which ball reaches the end of the track first A. The one in front B. The one in back C. Neither, they both reach the end of the track at the same time Using Conservation of Energy to find the speed of a roller coaster Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-23