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Objectives

Section 4 Conservation of Energy. Chapter 12. Objectives. Identify and describe transformations of energy. Explain the law of conservation of energy. Discuss where energy goes when it seems to disappear. Analyze the efficiency of machines. Section 4 Conservation of Energy. Chapter 12.

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Objectives

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  1. Section 4 Conservation of Energy Chapter 12 Objectives • Identifyand describe transformations of energy. • Explainthe law of conservation of energy. • Discusswhere energy goes when it seems to disappear. • Analyzethe efficiency of machines.

  2. Section 4 Conservation of Energy Chapter 12 Bellringer You give yourself and your sled gravitational potential energy as you pull your sled to the top of a snowy hill. You get on board your sled and slide to the bottom of the hill, speeding up as you go.

  3. Section 4 Conservation of Energy Chapter 12 Bellringer, continued 1. When does the sled have the most potential energy? When does it have the least potential energy? 2. Where does the sled have the most kinetic energy? the least kinetic energy? 3. What happens to the relative amounts of potential and kinetic energy as the sled slides down the hill? What happens to the total energy? 4. After the sled reaches the bottom of the hill, it coasts across level ground and eventually stops. What happened to the energy the sled had?

  4. Section 4 Conservation of Energy Chapter 12 Energy Transformations • Energy readily changes from one form to another. • Potential energy can become kinetic energy. • As a car goes down a hill on a roller coaster, potential energy changes to kinetic energy. • Kinetic energy can become potential energy. • The kinetic energy a car has at the bottom of a hill can do work to carry the car up another hill.

  5. Section 4 Conservation of Energy Chapter 12 Energy Graphs

  6. Section 4 Conservation of Energy Chapter 12 Energy Transformations, continued • Energy transformations explain the flight of a ball. • Mechanical energy can change to other forms of energy. • Mechanical energy can change to nonmechanical energy as a result of friction, air resistance, or other means.

  7. Section 4 Conservation of Energy Chapter 12 Kinetic and Potential Energy Graph

  8. Section 4 Conservation of Energy Chapter 12 Energy Conversion in Automobile Engines

  9. Section 4 Conservation of Energy Chapter 12 The Law of Conservation of Energy • The law of conservation of energy states that energy cannot be created or destroyed. • Energy doesn’t appear out of nowhere. • Whenever the total energy in a system increases, it must be due to energy that enters the system from an external source. • Energy doesn’t disappear,but it can be changed to another form.

  10. Section 4 Conservation of Energy Chapter 12 The Law of Conservation of Energy, continued • Scientist study energy systems. • Boundaries define a system. • Systems may be open or closed. • When the flow of energy into and out of a system is small enough that it can be ignored, the system is called aclosed system. • Most systems areopen systems,which exchange energy with the space that surrounds them.

  11. Section 4 Conservation of Energy Chapter 12 Conservation of Mechanical Energy

  12. Section 4 Conservation of Energy Chapter 12 Efficiency of Machines • Not all of the work done by a machine is useful work. • A machine cannot do more work than the work required to operate the machine. • Because of friction, the work output of a machine is always somewhat less than the work input. • Efficiencyis the ratio of useful work out to work in. • Efficiency is usually expressed as a percentage. • The efficiency of a machine is a measure of how much useful work it can do.

  13. Section 4 Conservation of Energy Chapter 12 Efficiency of Machines, continued • Efficiency Equation • Perpetual motion machines are impossible. • Energy is always lost to friction or air resistance. • Machines need energy input. • Because energy always leaks out of a system, every machine needs at least a small amount of energy input to keep going.

  14. Section 4 Conservation of Energy Chapter 12 Math Skills EfficiencyA sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? Express your answer as a percentage. 1. List the given and unknown values. Given: work input = 180 J useful work output = 140 J Unknown: efficiency = ? %

  15. Section 4 Conservation of Energy Chapter 12 Math Skills, continued 2. Write the equation for efficiency. 3. Insert the known values into the equation, and solve.

  16. Section 4 Conservation of Energy Chapter 12 Mechanical Efficiency

  17. Section 4 Conservation of Energy Chapter 12 Concept Map

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