Transcript PPT
Chapter 7 Energy Topics: • • • • • Important forms of energy How energy can be transformed and transferred Definition of work Concepts of kinetic, potential, and thermal energy The law of conservation of energy 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 Clicker Question 1 1. Which of the following is an energy transfer? A. Kinetic energy B. Heat C. Potential energy D. Chemical energy E. Thermal energy Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-4 Answer 1. Which of the following is an energy transfer? B. Heat Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-5 A “Natural Money” Called Energy Income System Liquid Asset: Cash Saved Asset: Stocks Transfers into and out of system Transformations within system Key concepts: Expenses • Definition of the system. • Transformations within the system. • Transfers between the system and the environment. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-9 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 10-10 Class Energy Question What do we mean by conservation of energy? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-3 Types of Energy in a system • Kinetic Energy => KE = 1/2 mv2 • Gravitational Potential Energy => PEg = mgh • Spring Potential Energy => PEs = 1/2 k(L)2 (k is the stiffness of the spring and L is the change in length) • Thermal Energy => Eth (measure of how hot something is => related to speed of atoms) • Chemical Energy => Echem (Stored in chemical bonds - released in chemical reactions) • Mechanical Energy = Kinetic Energy + Potential Energy Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-4 The Basic Energy Model Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-11 Gravitational Potential Energy • PE g = mgh Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Transferring Energy into of out of a system • Heat => Q • Work => W = F|| x Energy that changes form within the system is said to be transformed from one form to another Energy that enters or leaves the system is transferred from the system to the environment or vice versa. Need to distinguish what is the system and what is the environment. Forces from the environment can act on the system or objects in the system (external forces) -- Can also add heat from the environment Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-4 Conservation of Energy • Full form KE + PEg + PEs + Eth + Echem + … = Wext + Q • If there is no heat transferred in or out of the System and we are limited to mechanical energy KE + PEg + PEs + Eth = Wext • This becomes KEi + Sum PEi + Wext = KEf + Sum PEf + Eth • Note thatEth can come from friction, drag, collisions, etc. as well as Q Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-4 Clicker Question 2 2. If you raise an object to a greater height, you are increasing A. kinetic energy. B. heat. C. potential energy. D. chemical energy. E. thermal energy. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-6 Answer 3. If you raise an object to a greater height, you are increasing C. potential energy. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-7 Clicker Question 3 A skier is moving down a slope at a constant speed. What energy transformation is taking place? A. K Ug B. Ug Eth C. Us Ug D. Ug K E. K 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 Clicker Question 4 A child is on a playground swing, motionless at the highest point of his arc. As he swings back down to the lowest point of his motion, what energy transformation is taking place? A. K Ug B. Ug Eth C. Us Ug D. Ug K E. K Eth Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-14 Answer A child is on a playground swing, motionless at the highest point of his arc. As he swings back down to the lowest point of his motion, what energy transformation is taking place? D. Ug K Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-15 Conservation of Mechanical Energy • KE 1 + PEg1 = KE2 + PEg2 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Question • Does an automobile consume more fuel when it’s air conditioner is turned on? • When it’s lights are turned on? • When it’s radio is turned on while it is sitting in the parking lot? Note that fuel economy improves when tires are inflated to maximum pressure. Why? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Choosing the System Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-16 Solving Problems Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-22 How can we check to see if the Sum of KE + PE is conserved? •Energy Bar Charts •Equation •Example - pendulum •Ball thrown up in the air Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-13 Conceptual Example 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 10-17 Work and Work-Energy Theorem • Work = Force x displacement (parallel) We either need the component of force parallel to the displacement or the component of displacement parallel to the force • Work is the energy equivalent of Impulse Impulse = momentum * Delta t Work = Force x displacement (parallel) • Work by net force = Delta KE • Impulse of net force = Delta P • Machines => Work in = Work out Pulley Example Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-23 Clicker Question 5 Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the largest change in kinetic energy? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-18 Answer Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the largest change in kinetic energy? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-19 Checking Understanding Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the smallest change in kinetic energy? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-20 Answer Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the smallest change in kinetic energy? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-21 Additional Clicker Questions Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-37 Answer Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-38 Example A 200 g block on a frictionless surface is pushed against a spring with spring constant 500 N/m, compressing the spring by 2.0 cm. When the block is released, at what speed does it shoot away from the spring? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-23 Example A 2.0 g desert locust can achieve a takeoff speed of 3.6 m/s (comparable to the best human jumpers) by using energy stored in an internal “spring” near the knee joint. A. When the locust jumps, what energy transformation takes place? B. What is the minimum amount of energy stored in the internal spring? C. If the locust were to make a vertical leap, how high could it jump? Ignore air resistance and use conservation of energy concepts to solve this problem. D. If 50% of the initial kinetic energy is transformed to thermal energy because of air resistance, how high will the locust jump? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-24 Power and Efficiency • Power = work done (or energy change) / time interval • Two cars accelerate from rest to highway speed • One does it in five seconds (Porsche) • One does it in ten seconds (4 cylinder SUV) • Which uses more power? • Efficiency = useful energy output / Total Energy Input • At best 30-35% of the Chemical Energy in gas => Moving a car Car engines are at best 30-35% Efficient Electric motors can have efficiencies approaching 99% Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-23 Energy And Real Life • Tour de France A Tour de France bicycle racer needs to eat 8000 calories per day to maintain their weight. About 6570% of this energy is used to maintain body temperature. • Bow and Arrows About 60-75% of the PE in a drawn bow goes into the KE of an Arrow. The rest of the energy heats the bow. • Guns and Bullets Only about 30% of the energy in a firearms discharge is transferred to the projectiles they fire Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-23