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Teaching energy before work but after linear momentum. Michael C. Faleski Delta College Michigan AAPT Meeting March 28, 2009 Lansing, MI. Rationale for change. Students: Struggle with ideas of work / energy Have conceptions about energy Textbooks:
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Teaching energy before work but after linear momentum Michael C. Faleski Delta College Michigan AAPT Meeting March 28, 2009 Lansing, MI
Rationale for change • Students: • Struggle with ideas of work / energy • Have conceptions about energy • Textbooks: • Progression: Vector to Scalar to Vector (F, E, p) • Often treat only point systems • Treatment of work done by friction as W=-fd (Pseudowork) • Conservation of energy an afterthought (later section) • Potential Energy seemingly attributed to an object • Practical: • Improve understanding in same or less class time • More general cases with minimal number of equations • Cover more topics in class
Previous Energy Approach (PHY 111 - Serway)~20 hours… 3 expt + Lab Exam (latent heat expt) • Ch. 5 (~12 hrs with 1 expt + collisions expt): • work, its calculation, and examples • Kinetic energy & potential energy, examples • Conservation of Mechanical Energy & Examples • Friction & Examples with work-energy theorem • Ch. 11 (~6 hrs with 1 expt + Lab Exam): • ~4 class weeks after Ch. 5 • Heat and energy transfer processes • Chapter 12 (thermodynamics - entirely skipped) • Ch. 13(~ 2 hours) - Spring Energy
New Energy Approach (PHY 111 – Serway)~18 hourswith 3 expt + Lab Exam (latent heat) • Ch. 5 (~ 8hrs with 1 expt + collisions expt): (AFTER Ch. 8) • Internal, kinetic, potential, and thermal energy • Discuss Conservation of Energy • Conservation of Mech. Energy using 1st Law (Q,W=0) , , • Examples with W≠0, but Q=0,… closed systems only • Ch. 11 (~6hrs with 1 expt + Lab Exam): • ~2 class weeks after Ch 5 • Heat and energy transfer processes • Chapter 12 (~2hrs + introduction of 1st Law previously) • Ideal gas processes on PV diagram • Ch. 13(~2 hrs) - Spring Energy
OLD WAY Work ↓ Kinetic Energy + Work – Energy Theorem ↓ Potential Energy ↓ Mechanical Energy Conservation ↓ Energy Conservation (non-conservative Forces) NEW WAY Types of energy (kinetic, potential, thermal) ↓ Energy Conservation ↓ Mechanical Energy Conservation ↓ Thermal Energy ↓ Work Approaches
Test Results I • A 2.0-kg box is pulled by a rope across a frictionless horizontal floor. What’s the work done on the box by the rope that speeds it up from to ? (A) 0.00 J (B) 4.00 J (C) 8.00 J (D) 12.0 J (E) 24.0 J PHY 111: Old Way (42 stud.): 71.4% New Way (78 stud.): 69.2%
Test Results II • PHY 111 exam problem - roller coaster Old Way: 58.5% (42 students) New Way: 72.3% (78 students)
Conclusions • Results from new vs. old approach on exams: • At least no worse using new approach • Scores were comparable on similar problems • More topics covered Force Concept Inventory (PHY 111)
Challenges with new approach • Textbooks: • Problems ask for work by friction, total ME of a system • Don’t use 1st Law of Thermo until after mechanics • PE seemingly assigned to object rather than system • starting to change in newer editions of traditional texts • Source material: • I had to provide new text for this approach • Texts use various definitions of quantities presented here • Chapters out of order – course outcomes require Ch 1-14 covered • Students didn’t seem to mind… no mention in course evals • Unlike other sections of same course (PHY 111) being taught • Tutors not familiar with it, but students seemed to adapt • Contact with students in subsequent classes - also adapted easily • Instructors: • Not how we generally learned material • Takes time to get comfortable with change in thinking
Advantages with new approach • Physics • Proper treatment of friction effects, PE of system • Problems of real objects possible • By considering large “systems”, WORK is from either • 1. People forces or 2. Strings and ropes (tension) • Approach based on what students “know” • Anecdotally, asked why we didn’t start with this. • Conservation of Energy is at forefront of all problems • Internal energy is seen directly as thermal energy for ideal gas • One basic equation (1st Law of Thermo) applies for all cases • Practical: • Covered more topics in less time • Less time between subsequent introduction of energy topics • Topics covered as “vector to scalar” rather than “vector-scalar-vector” • Connections made with later material (Liquid = Solid + energy)
Some References • A.B.Arons, “Developing the Energy Concepts in Introductory Physics,” The Physics Teacher, 506-517, (October 1989). • Arnold B. Arons, “Development of energy concepts in introductory physics courses,” Am. J. Phys., 67, 1063-1067, (1999). • A.B.Arons, Teaching Introductory Physics, (Wiley, New York, 1997), Part I, Chap. 5, pp 135-163. • A. John Mallinckrodt and Harvey S. Leff, “All about work,” Am. J. Phys., 60, 356-365, (1992). • Bruce Arne Sherwood, “Pseudowork and real work,” Am. J. Phys., 51, 597-602, (1983). • Bruce Arne Sherwood and W.H. Bernard, “Work and heat transfer in the presence of sliding friction,” Am. J. Phys., 52, 1001-1007, (1984). • Claude M. Penchina, “Pseudowork-energy principle,” Am. J. Phys., 46, 295-296, (1978). • Harvey S. Leff and A. John Mallinckrodt, “Stopping objects with zero external work: Mechanics meets thermodynamics,” Am. J. Phys., 61, 121-127, (1993). • Herman Erlichson, “Work and kinetic energy for an automobile coming to a stop,” Am. J. Phys., 45, 769, (1977). • H.R. Kemp, “Internal work: A thermodynamic treatment,” Am. J. Phys., 53, 1008, (1985). • John W. Jewett Jr., “Energy and the Confused Student I: Work”, The Physics Teacher, 46, 38-43, (2008) • John W. Jewett Jr., “Energy and the Confused Student II: Systems”, The Physics Teacher, 46, 81-86, (2008) • John W. Jewett Jr., “Energy and the Confused Student III: Language”, The Physics Teacher, 46, 149 – 153, (2008) • John W. Jewett Jr., “Energy and the Confused Student IV: A Global Approach to Energy”, The Physics Teacher, 46, 210 – 217, (2008) • John W. Jewett Jr., “Energy and the Confused Student V: The Energy/Momentum Approach to Problems Involving Rotating and Deformable Systems”, The Physics Teacher, 46, 269 – 274, (2008) • Mark W. Zemansky, “The Use and Misuse of the Word “Heat” in Physics Teaching,” The Physics Teacher, 295-300, (1970). • Nancy Hicks, “Energy is the capacity to do work - or is it?” The Physics Teacher, 529-530, (November 1983). • S.G. Canagaratna, “A Critique of the definitions of heat,” Am. J. Phys., 37, 679-683, (1969). • S.G. Canagaratna, “Critique of the treatment of work,” Am. J. Phys., 46, 1241-1244, (1978). • W.H. Bernard, “Internal work: A misinterpretation,” Am. J. Phys., 52, 253, (1984).