Teaching energy before work but after linear momentum

1. Teaching energy before work but after linear momentum Michael C. Faleski Delta College Michigan AAPT Meeting March 28, 2009 Lansing, MI

2. 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

3. 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

4. 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

5. 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

6. 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%

7. Test Results II • PHY 111 exam problem - roller coaster Old Way: 58.5% (42 students) New Way: 72.3% (78 students)

8. 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)

9. 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

10. 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)

11. 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).