Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 Chapter 7 Energy Topics: 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? Slide 10-1

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 Slide 10-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 1.Which of the following is an energy transfer? B.Heat Slide 10-5 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A “Natural Money” Called Energy Key concepts: Definition of the system. Transformations within the system. Transfers between the system and the environment. Liquid Asset: Cash Saved Asset: Stocks Income Expenses Transformations within system System Transfers into and out of system Slide 10-9

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Forms of Energy Mechanical Energy Thermal Energy Other forms include Slide 10-10

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. What do we mean by conservation of energy? Slide 10-3 Class Energy Question

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Types of Energy in a system Kinetic Energy => KE = 1/2 mv 2 Gravitational Potential Energy => PE g = mgh Spring Potential Energy => PE s = 1/2 k(  L) 2 (k is the stiffness of the spring and  L is the change in length) Thermal Energy => E th (measure of how hot something is => related to speed of atoms) Chemical Energy => E chem (Stored in chemical bonds - released in chemical reactions) Mechanical Energy = Kinetic Energy +  Potential Energy Slide 10-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Basic Energy Model Slide 10-11

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 Slide 10-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Conservation of Energy Full form  KE +  PE g +  PE s +  E th +  E chem + … = W ext + Q If there is no heat transferred in or out of the System and we are limited to mechanical energy  KE +  PE g +  PE s +  E th = W ext This becomes KE i + Sum PE i + W ext = KE f + Sum PE f +  E th Note that  E th can come from friction, drag, collisions, etc. as well as Q Slide 10-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. Slide 10-6

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 3.If you raise an object to a greater height, you are increasing C.potential energy. Slide 10-7 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Clicker Question 3 A skier is moving down a slope at a constant speed. What energy transformation is taking place? A. B. C. D. E. Slide 10-12

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A skier is moving down a slope at a constant speed. What energy transformation is taking place? B. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. B. C. D. E. Slide 10-14

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Conservation of Mechanical Energy KE 1 + PE g1 = KE 2 + PE g2

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 Slide 10-16

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Solving Problems Slide 10-22

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide How can we check to see if the Sum of KE + PE is conserved? Energy Bar Charts Equation Example - pendulumpendulum Ball thrown up in the air

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. Slide 10-17

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 Slide 10-23

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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? Slide Clicker Question 5

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 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? Additional Clicker Questions

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 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? Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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? Slide 10-23

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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? Slide Example

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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% Slide 10-23

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 % 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 Slide 10-23