from The Textbook Letter, May-June 1996

Reviewing a high-school book in physics

PHYSIC-AL: An Activity Approach to Physics
1994. 587 pages. ISBN of the student's edition: 0-920008-30-5.
J.M. LeBel Enterprises, 6420 Meadowcreek Drive, Dallas, Texas 75240.

This Book Lays Emphasis
on the Testing of Concepts

Sumner P. Davis

PHYSIC-AL: An Activity Approach to Physics is a "hands on" instructional package in both its philosophy and its presentation. The ideas are not new, but they have been shaped into an explicit expression of the premise that "learning is most efficient when it is rooted in the student's experience." That premise is stated on page 4 of the PHYSIC-AL teacher's manual.

In the PHYSIC-AL textbook, each new concept is introduced by an activity or experiment, then is developed in a more formal, textual presentation. In terms of content and writing style, PHYSIC-AL follows traditional high-school physics texts, although it seems to be a good deal more rigid than the ones that we are used to. In PHYSIC-AL there is only one way to think about things.

The book is divided into nineteen units, beginning with several that consider the mechanics of motion (and that constitute some 35% of the book's content). It continues with units on heat (11%), mechanical waves (6%), light and optics (13%), electricity and magnetism (22%), and atomic structure (7%), and it ends with a single unit titled "New Directions in Physics" (5%). This last unit is useless. It consists of smatterings about many topics -- such as relativistic time dilation and its effects, relativistic mass, the Compton effect (dealing with interactions between photons and electrons), matter waves, polarization, diffraction, the uncertainty principle, and quantum mechanics -- all in 26 pages.

The units are generally self-contained and can be taught selectively. Each begins with a brief statement of a topic, followed by the first of the laboratory experiments that the unit has to offer. Then examples, questions, problems, illustrations, and discussion fill out the presentation until the next experiment is introduced.

Let's take the very first unit as an example of how the writers apply their philosophy of instruction. The unit is called "Simple Motion," and it begins with a section titled "Falling Bodies." The writers present a few historical facts, then they state Aristotle's law: "The rate of the fall of a body depends on its weight: the heavier the body, the faster it falls." Immediately after this comes the first laboratory experiment, whose purpose is to test Aristotle's claim. Under the heading "Prelab," the student is asked to write out the "simple problem," the "manipulated variable/s that you must change to produce meaningful results," and a "tentative answer to the problem based on the best knowledge you have."

(At this point I must comment that the "Prelab" requirements in PHYSIC-AL are generally too sophisticated and demand too much self-discipline. Students want to do something first, then maybe decide what should be done. What incentive is there for sitting, thinking and writing when you can make something happen?)

Next comes a list of materials and a helpful drawing of pennies falling to the floor, with a meter stick nearby to measure their height. Then, under "Procedure," there are explicit and carefully written instructions that guide the students through a logical series of trials. There is also a suggested format for recording data.

Next, "Evaluation." Here the writers ask, "Is the tentative answer to the problem supported? Are new answers and questions suggested?" (Given the experimental skills of students at this level, I doubt that the responses will be anything but "I can't tell.")

Next come questions. Some of these are well chosen and thought-provoking, but questions 2 and 5 display conceptual weaknesses that recur throughout the book. Question 2 asks, "If Aristotle's law was correct, from what height would you have to drop a single penny if it is to strike the ground at the same instant as a bundle of 10 pennies falling from 1.0 m?" I see no way to answer this question. Aristotle's law, as stated, deals with a "rate" and with falling "faster" -- in other words, it deals with speed. But the students have not yet learned about speed or acceleration! Question 5 asks whether the experiment helps us "understand why feathers fall more slowly than rocks." I submit that this is unrelated to the experiment. It would be a good question in a proper context, but not here.

Now the writers start a new section, which should introduce a new concept. The section is titled "Air Resistance," but the subsequent text fails even to mention air resistance. It deals with the definition of scientific law, concluding with this statement: "Observations show that large heavy rocks fall at the same rate as light small rocks . . . . Now we have two laws of falling bodies that are contradictory." This is followed by a one-page exposition of "Aristotle's Physics," which is well done and enlightening. Only after these preliminaries do the writers turn to air resistance, presenting an experiment whose purpose is "To explore the effect of air resistance on the time to fall."

Strengths and Defects

So what are the strengths of this book?

First, the idea of doing experiments is a good one, since science is experimentally based. The experiments in PHYSIC-AL can be conducted with relatively simple and inexpensive equipment.

Ample drawings ensure that the student will learn how to draw diagrams to help clarify problems. Graphs and tables illustrate sound practices. Visualization is encouraged. There are lots of examples and problems. And at every stage, the text draws attention to how things that are learned in the classroom apply to everyday living.

Great emphasis is placed on testing theories and hypotheses by observation. In the unit on light, for example, the writers introduce Newton's corpuscular theory and Huygens's wave theory, and then they use text, class activities, and class discussions to familiarize the student with various observations of how light behaves Each time, they ask the student to consider how the observations substantiate or negate the theories in question. The student ponders the fact that light travels in straight lines; that light beams do not appear to interact with each other when they cross (class activity, page 312); that the angle of incidence is equal to the angle of reflection; that a particle, represented by a steel ball, is refracted at an interface (class activity, page 313); that when light strikes a medium at an angle, it will be partially reflected and partially transmitted; that white light can be separated into a spectrum of colors (class discussion, page 315); and that white light forms a pattern of colors when it strikes a thin film (laboratory exercise, page 316).

What are the book's weaknesses?

As I already have said, the PHYSIC-AL program is a rigid one. (Most students will find it deadly dull unless the teacher is especially adept at making classes interesting.) Many discussions in the text require patterns of thinking that are too sophisticated for high-school students. And the book has many conceptual defects -- mostly minor but nevertheless significant -- as well as some outright errors. Here are a few examples:

  • Page 5: The purpose of the laboratory experiment on this page is given as "to find how far a body travels as it falls." That doesn't make sense, and the experiment actually is intended to show how far a body falls in successive intervals of time.

  • Page 15: "Suppose a world champion track star could cover the 100 metres in 9.79 seconds . . . . Can you calculate how fast he was travelling when he crossed the finish line?" (The implication is that you can. In fact, you can't.)

  • Page 93: As drawn, the forces acting on the sailboat produce a net force that makes the boat sail backward.

  • Page 113: The discussion of circular motion is accurate but is too complex for a student to understand.

  • Page 114 (question 3): If gravity disappeared, "would we continue to travel around the earth every 24 hours?" (We don't do so now.)

  • Page 136: ". . . the earth turns 1/24 of a complete rotation per hour . . . ." (What happened to sidereal time?)

  • Pages 227 and 228: The discussion of microscopic potential energy and phase changes has been written with college-level sophistication.

  • On page 232, a sidebar about "Wave-Driven Power Tower" says: "[When a wave enters the tower] the trapped air is pushed up . . . . When the wave recedes it sucks the air downwards past the turbine." This is obscure at best, because air is compressible. If the water level in the "Power Tower" alternately rises and falls, the "trapped air" will undergo alternate compression and expansion. The PHYSIC-AL writers appear to believe that the entire mass of air moves up and down, behaving as if it were an incompressible fluid.

  • Page 251: "The power of the sun" is said to change as Earth moves.

  • Page 307: Contrary to what the caption says, the dispersion of light rays does not work in such a way that the dispersed rays recombine to produce white light.

  • Page 480: The sidebar titled "Health and Electromagnetic Radiation" is vague and non-scientific, to say the least.

    The PHYSIC-AL teacher's manual speaks to the inexperienced teacher and gives lots of little details for planning, making a course schedule, writing daily lesson plans, ordering laboratory equipment, and not trying to teach everything in the textbook. It also provides a good, low-level guide to each of the textbook's individual units, and it is worth having.

    There Is Good Meat Here,
    but It's Often Hard to Eat

    Lawrence S. Lerner

    Reading PHYSIC-AL is like eating a nicely roasted wild duck that's riddled with birdshot. One is prepared for a few bad chews, but one can't really enjoy the meal if nearly every mouthful contains nasty little pellets. In like manner, one can't really enjoy the good (and sometimes elegant) servings of physics in PHYSIC-AL because one must continually contend with painful little pieces of guesswork and error.

    To show what I mean, let me describe the book's unit about light. As a whole, this is the best exposition of light that I've seen in a high-school book. The writers use a historical approach, presenting classical experimental evidence and tracing the emergence of the competing corpuscular and wave theories, both of which -- for more than a century -- successfully explained most or all of the available observations. The student is encouraged to choose a side in the debate over the two theories, but the writers point out that any such choice is inevitably the result of an unsupported preference. This is a wonderful way to teach physics and to show the student how to think like a physicist.

    Unfortunately, the historical account is riddled with errors that spoil a fine attempt to present physics. The writers incorrectly attribute to Newton, rather than to Descartes, the corpuscular interpretation of the law of refraction. They try to quote from Newton's Opticks, but they turn his phrases "Fits of easy Reflexion" and "Fits of easy Transmission" into "fit for easy transmission or easy reflection." (This grossly distorts Newton's meaning. Newton's word fit is a noun, not an adjective.) They correctly assert that the final nail was driven into the coffin of the classical corpuscular theory when the speed of light in water was found to be less than the speed of light in air (page 340); but they wrongly say that this crucial finding was made by Fizeau in 1849, rather than Foucault in 1850. More importantly, they fail to tell that the evidence which convinced most physicists had been gathered much earlier, in 1820, when Fresnel had repeated Young's two-slit experiment with the addition of polarizers: When two beams were polarized perpendicular to each other, no interference pattern could be obtained.

    The chapter on orbital motion suffers from a similar affliction: The writers try to outline the history of ideas about planetary motion, but their story is full of mistakes. Copernicus was not a monk, and his heliocentric model did not have more cycles and epicycles than contemporary Ptolemaic models had. The actual title of Copernicus's book was De Revolutionibus Orbium Coelestium. The Roman church did indeed put De Revolutionibus onto the Index of Forbidden Books in 1616, but that was not the year in which the church staged the trial of Galileo. The trial took place in 1634. Contrary to what is implied by the upper illustration on page 120, retrograde planetary motion cannot be observed during a single night. And Kepler worked on Tycho's data for roughly eight years, not fifteen, before he demonstrated the ellipticity of planetary orbits.

    There are many other pellets of historical misinformation as well, distributed throughout the book. For example, Marco Polo did not bring gunpowder from China to Europe. Gilbert's De Magnete, true to its title, was written in Latin -- it was not "the first important scientific book written in English"! Galileo, not Huygens, was the first to point out the isochrony of the pendulum. In using the first medical thermometers, physicians measured the temperatures of their patients' hands, not the patients' mouths. Thus there is no reason to imagine that the thermometers did much to "spread disease from one person to another" (page 199). Cavendish used an optical lever, not a microscope, in measuring the gravitational constant, G, with his torsion balance. Count Rumford was not born in poverty, did not die a rich man, and did not "flee to Europe [from America] in 1798" -- he left Boston, with the British army, in 1776. Millikan was not the first to measure the charge on the electron, and he didn't perform his measurements in 1912. That was the year when his definitive publication appeared. And in Marconi's day, "the most sophisticated form of long distance communication" was not the telegraph but the telephone, which had been around since 1876.

    More Mistakes

    Along with the errors involving history, there are various pedagogic mistakes that make PHYSIC-AL hard to enjoy. For instance, a worked problem on page 8 involves accelerated motion, but the idea of acceleration isn't introduced until page 10. The term mass is used on page 9, and again on page 44, but the idea of mass isn't introduced until page 55. The arrow notation for vectors, first used on page 100, is never explained. A computer-based exercise called "The Effect of Air Resistance on a Trajectory" uses a program which exploits the assumption that the air resistance varies with the square of the projectile's speed, but this point isn't explained or even mentioned. (Otherwise, the exercise looks nice.) Perpetual-motion machines appear in unit G, "Balanced Forces and Torques," but the student has not yet learned about the conservation of energy -- the principle that underlies our understanding of why perpetual-motion machines are impossible. (Conservation of energy appears in the next unit, "Conservation Laws.") On page 207 the ideal gas law is misrepresented as a "modified version of Boyle's law," and page 215 offers the meaningless statement that "Pressure and temperature have the same cause."

    The writers also display many lapses in their handling of terms and definitions. For example, such terms as density, absolute zero, absolute pressure and free body are introduced into the text without definition, and free body is used so loosely that I wonder whether the writers are aware of its established meaning. The definitions of potential energy (page 179) and power (page 193) are poor and confusing. The word fulcrum is used, improperly, as if it meant any center of rotation. Later, black body is misdefined, Planck's quantum hypothesis is confused with Planck's radiation law (a very different thing), and Rydberg's formula is misnamed Balmer's formula.

    Finally, there are sundry mistakes that signify careless editing. The text is riddled with typographic errors, erroneous punctuation and dangling participles, and there are irritating inconsistencies in spelling. This book -- published by a company based in Canada -- is obviously an adaptation of a textbook that was created for the Canadian market, and not all of the original Canadian spellings have been changed: For example, metre and meter, or centre and center, sometimes appear on the same page. In some of the chapters, the spelling has hardly been touched. And for some reason that I can't fathom, the purely French dioptre is used, everywhere, in place of the English diopter.

    The Book's Virtues

    Having said all that, I must point out that the vices of PHYSIC-AL are offset by many virtues. For one thing, the laboratory exercises and computer activities are well conceived and well described, so the student should be able to follow the procedures, and get satisfactory results, in nearly all cases. I particularly like the very simple experiment that elucidates the distinction between inertial mass and gravitational mass (page 68), the calorimetry experiment (pages 225 and 226), and the exercise in which the student verifies that the electric force between two charged spheres varies inversely with the square of the distance separating the spheres (pages 383 to 385). This last one is hard to perform because of charge leakage. I hope that it will work for the student.

    A second notable virtue is that most of the homework problems given in PHYSIC-AL are focused and require real thought and calculation on the part of the student. Further, the numbers used in the problems are almost always realistic.

    The figures in PHYSIC-AL are praiseworthy, too, since nearly all of them are useful, correct and properly captioned.

    In general, major ideas are introduced well. The conservation of momentum, for example, is presented with the use of diagrams that obviate the need for complicated calculations. (Unfortunately, the writers do not do a good job of distinguishing between conservation of momentum and the conservation of energy. Many students have trouble with this distinction, and they need all the help they can get.) The material about light and optics is exceptional, as I have already noted. The discussion of diffraction is excellent, while the exposition of geometrical optics in terms of dioptric power strikes me as a nice way to avoid some messy mathematics. And in the unit about electric current, the elegant discussion of drift velocity is exemplary.

    Margin-notes labeled "Science Technology & Society" have been scattered through PHYSIC-AL, with variable effect. Some of them make sense, but too many of them are irrelevant or silly. In a section about an absolute temperature scale, a "Science Technology & Society" note tells about a Nobel laureate in economics. A note about the forces acting on a sailboat ends with an irrelevant quoting of Einstein. A section introducing Newton's second law has an irrelevant note about robots, complete with an invitation to guess whether robots will eliminate jobs. And why does the section on torque and rotation have a note about the sale of an unpublished Einstein manuscript dealing with the special theory of relativity?

    A "Science Technology & Society" note titled "Women Inventors," stuck into a section about the work done by a torque, is not just silly but offensive. Here we read that the cotton gin was actually invented by a woman -- a controversial assertion, to say the least -- and that nameless women secured patents on World War 1 "weaponry" such as "a railway torpedo," "an incendiary ball," "a loading device" and "a resilient missile." (I don't know what the last three objects may be, but the railway torpedo is a signaling device, not a weapon.) Then we jump ahead to Marguerite She-wen Chang, who devised a nuclear trigger, and Barbara S. Askins, who won an award for some undescribed "technique." And finally, we see a warning: "Watch out men." Such ignorant, patronizing, sexist writing is intolerable.

    Overall, however, PHYSIC-AL has much that is good. There is plenty of meat here for a serious student who has a decent command of algebra and a bit of trigonometry. I have high hopes for the next edition, because PHYSIC-AL can become an exceptional product if the publisher will spend some money to correct its factual errors, rectify its pedagogic lapses and editorial inconsistencies, and replace the dumb margin-notes with ones that make sense. In the meantime, I would be glad to learn about the experiences of high-school teachers who have used the present edition of PHYSIC-AL in their classrooms.

    Sumner P. Davis is a professor of physics and a Distinguished Teacher at the University of California at Berkeley. He has taught in Berkeley's "Science for Science Teachers" program, which presents physics to middle-school teachers, and he has served on the Golden State Examination Committee for Coordinated Science, which seeks to identify exceptional achievers among high-school science students.

    Lawrence S. Lerner is a professor in the Department of Physics and Astronomy at California State University, Long Beach. His specialties are condensed-matter physics, the history of science, and science education.


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