Reviewing a high-school book in physics

For High-School Physics,
This Textbook Is the Best

Sumner P. Davis

Why that reply? Because the professors were getting too many students with little feeling for what science was all about, with an unbalanced view of what was important, and with only a minimal understanding of basic concepts in physics.

Now, with Addison-Wesley's splendid Conceptual Physics available, there is no excuse for even asking the original question. If every science student will learn what is in Conceptual Physics, high-school graduates will have a sensible grounding in scientific methods and the scientific attitude, as well as enough factual knowledge to comprehend many of the technological challenges that our society faces. When such students get to college, the blending of formalism and mathematics with a deeper understanding of physics will be easy and smooth because the students will already have gained the necessary qualitative knowledge and the necessary training in scientific thinking. We can't ask for more than that.

The author of Conceptual Physics is Paul G. Hewitt. In an introductory note on page xi, he says that the book treats physics "conceptually rather than mathematically . . . with equations [serving as] 'guides to thinking' rather than recipes for algebraic problem solving." In fact, there are almost no equations, and none of the usual "formulas" for anything. Students will need some algebra to appreciate proportion and ratios, and they will have to be able to read graphs and tables of numbers.

On every page of Conceptual Physics it is easy to see the touch of a master teacher who has probably made every mistake imaginable in teaching, who has learned from his mistakes and from the experiences of others, who his listened to criticism, and who has changed his ways accordingly. Yet he has not sacrificed his own sense of how to teach and how to communicate. This is Hewitt's book, not a consensus book created by a committee.

Conceptual Physics is divided, in a fairly standard way, into six units: "Mechanics" (constituting about 36% of the text pages), "Properties of Matter" (10%), "Heat" (10%), "Sound and Light" (20%), "Electricity and Magnetism" (17%), "Atomic and Nuclear Physics" (8%).

Each unit comprises several chapters, and each chapter ends with a review that has several parts. The "Concept Summary" is a statement of what the student should have learned and should be able to articulate. The "Important Terms" list presents words or phrases that the student should be able to define or explain readily. The "Review Questions" are short-answer questions that focus on definitions and concepts. The "Think and Explain" problems often are open-ended, having no single "right" answer. And some of the reviews also include a hands-on "Activity." The reviews are complete enough to eliminate the need for a study guide.

The text pages are embellished with sketches, cartoons, drawings, photographic reproductions, boxed questions, and other items that can help the student to visualize and understand what the text is saying. These auxiliary items are not overdone and are not distracting. They remind me of how a creative lecturer uses a chalk-board.

What's in it for the teacher? The teacher's edition of Conceptual Physics opens with ten pages of promotional hype and advertising. These are absolutely unnecessary, and having them appear in the book is insulting. Next come 35 pages of "Full Answers to [the] Think and Explain" problems in the chapter reviews. These answers are good, but an experienced teacher will not regard them as "full." In the chapter reviews themselves, the teacher finds short (usually one-line) answers to questions that are posed to the student. These answers are helpful and are especially useful to the teacher who has studied physics only briefly.

Let's now look through the text to see what sticks out at first reading. Chapter 1, "About Science," evidently reflects the philosophy that Hewitt has worked out during his many years of teaching. It is useful, but perhaps more interesting to an adult reader. I can't imagine that many students will do any more than glance at it. The cartoon characters say things such as "Facts are revisable data about the world. Theories interpret facts. . . . Science is about cosmic order. Religion is about cosmic purpose. However, I learned something from this chapter, as I did from every other chapter that I read.

In the unit on mechanics, Hewitt skillfully avoids pitfalls as he discusses motion and elucidates speed, velocity and acceleration. He gets the vectors right, but he also is able to explain that the scientific meanings of the words speed and velocity differ from their meanings in everyday language. He distinguishes between center of gravity and center of mass, yet he does this in a way that leaves the students' common experience intact; and he follows up, somewhat later, by illustrating why the difference is important. The chapters on gravitation and satellites are superb. Want to know about weightlessness, tides, black holes, satellites, relativity? They are all here, with no misconceptions that later will have to be unlearned.

By the time I finished Unit II ("Properties of Matter") and Unit Ill ("Heat"), I could see that Hewitt has combined, in a knowledgeable way, topics that less sophisticated writers usually treat separately. Our surroundings really make sense and are understandable as an integrated whole.

Unit IV ("Sound and Light") is full of answers to everyday questions, and Unit V ("Electricity and Magnetism") is full of physics and engineering. How I wish I'd read material like this during my own days in high school! The pictorial representations are easily translatable to what we see in practice, and there is an unusually smooth transition from the pictorial to the schematic depiction of circuits.

In the final unit ("Atomic and Nuclear Physics"), the material has been chosen and presented well, but the unit is rather brief. It builds on earlier chapters about properties of matter, but it is little more than a summary of quantum and modern physics. There is nothing about superconductivity, lasers, plasma physics, or particle physics. There is no hint that today's "state of the art" physics bears little resemblance to the physics of just a few years ago. Hewitt evidently believes that these topics are best left to college courses. That is a defensible judgment.

Are there any errors? Yes. On page 221, the text states that astronauts traveling at 99% of the speed of light could make a round trip to a distant star in 22.6 years, but that light itself would take 22.8 years to make the same trip. And on pages 375 and 376, in the four figures illustrating bow-wave and shock-wave patterns, the lines representing the fronts should be drawn tangent to the circles, not intersecting them.

No text can satisfy everyone, none can cover every possible topic, and none is so good that it can't be made better in the future. Right now, however, Hewitt's book is a winner, in terms of both content and pedagogy. Of all the high-school physics texts that I have seen (including the conventional, strongly mathematical ones), Conceptual Physics is by far the best.

The Laboratory Manual that accompanies Conceptual Physics looks as good as the book. It can easily stand on its own and can be used in conjunction with a different text, but it is definitely a cookbook. Any intellectual content or understanding must be supplied by the teacher (for whom the manual provides good notes). I believe that even a person who has not had any physics-laboratory experience, but has read Hewitt's book, can complete every one of the experiments that the manual describes.

This Book Is Easy to Read,
but It Has Too Many Errors

Mario Iona

The readers of this review should be aware of some of my biases. I appreciate the emphasis on conceptual physics, both for high-school courses and for introductory college courses. To me, conceptual physics means physics presented in a way that promotes a general understanding of physical phenomena (along with a gut feeling for what is going on), and I would expect any conceptual-physics book to display sound reasoning and clear explanations. I do not feel that the present book lives up to such expectations.

Some of the errors are familiar to me because I have seen them in Hewitt's college text or in the first edition of this high-school book (published in 1987). I have directed Hewitt's attention to many of these errors, and to others as well, both in personal communication and in my published writings. Indeed, I am one of the persons whom Hewitt thanks, on the "Acknowledgments" page of Conceptual Physics, for "suggested improvements." In certain cases he seems to have made changes in response to my suggestions, but the changes have not always been adequate. In other cases there have been no changes at all.

Many teachers favor Conceptual Physics, and often use it as a supplementary text, because students read it and like it. The writing is not demanding, Hewitt's cartoon-like drawings provide comic relief, and his diagrams are usually helpful in illustrating the topics under discussion. (Hewitt was a commercial artist before he began to study physics, at age 27.) Sometimes, however, the artist overshadows the physicist. For example, in illustrating the addition of two forces acting on a horse-drawn cart, it is not helpful to show the horse exerting a force on the cart while a man pulls on the horse (page 67). Similarly confusing are the curve ball" diagrams, which show air moving in one direction while the ball moves in another. This is an artistic attempt to combine, in each diagram, the views from two different reference frames (page 295).

Conceptual Physics covers the usual array of main topics -- from mechanics to nuclear physics -- but it does not strive for the encyclopedic completeness often found in high-school physics books. This seems proper, but Hewitt should have omitted even more material when he could not discuss it adequately. For example, the passage on using a galvanometer to measure voltage (page 560) seems inadequate and can only serve as material for memorizing.

At the same time, Hewitt has usually resisted the temptation to load this book with "modern" topics. The three major exceptions to this rule are his serious attempt to explain relativity (chapters 15 and 16), his section on black holes (page 186), and his inadequate discussion of muon-induced cold fusion (page 631).

There is little use of mathematics in Conceptual Physics, and one wonders whether the book would be better if there were even less of it. Some of the mathematics that Hewitt uses is rather peculiar. In discussing the law of gravitation, for example, he claims that the universal gravitation constant, G, "plays the same role in the gravitational force equation that pi plays in the equation for the circumference of a circle," and he says that the values of both pi and G can be obtained by solving the respective equations in which they appear (page 166). If this is supposed to be a general statement about constants, it seems unnecessarily involved. It certainly blurs an important difference between the constants in question, because pi can be calculated with paper and pencil, but G has to be obtained experimentally. Furthermore, Hewitt correctly states that the numerical value of G (unlike the value of pi) depends on the choice of units, but he seems to forget this when (on page 496) he says, "The small value of G indicates that gravity is a weak force . . . ."

In his introductory sections about history and philosophy and attitudes, Hewitt says that "scientists, like most people, have a vast capacity for fooling themselves." There are examples of this phenomenon in Conceptual Physics itself. For example, on page 339 Hewitt discusses at length the proposition that "under some conditions hot water will freeze faster than warm water." Instead of examining factors that (in a poorly controlled experiment) might produce such a paradoxical result, he presents the result as if it were reasonable, and he tries to relate it to the water's large latent heat of vaporization. On page 295 he seems to imply that there are forces (such as those due to differences in fluid pressure) to which Newton's third law does not apply.

There are some "Review Questions" at the end of each chapter, usually asking for phrases or facts that can be found in the chapter's text. There are also more general questions, which are well described by the title that Hewitt has given to them: "Think and Explain." Unfortunately, some of these are presented in misleading ways. For example, on page 582 a question asks for the turns ratio of an induction coil, acting as a transformer, that produces 24,000-volt sparks from an interrupted, 12-volt direct current. The answer given in the teacher's edition is 2,000, but this overlooks the fact that the induced voltage depends partly on the rate at which the direct current changes with time. The induction-coil question has no easy answer.

For this review I have selected a few typical examples from among the many errors or misleading presentations in Conceptual Physics. I have done this to help my readers decide whether the book's accessible style, which has won a favorable response from a relatively large number of students, can offset the disadvantages arising from the confusion in Hewitt's physics. Some of this confusion becomes apparent only if one studies the text with great care.

In response to my critical comments about Hewitt's earlier books, some of my colleagues have looked closely at those books and have noticed details that they previously had overlooked. Their opinion seems to be that Hewitt sometimes shows a good grasp of physics, but that his books would be much better if he could work with someone who had a deeper understanding of the fundamentals. Their view, I believe, is that Hewitt is a talented teacher who has an inviting technique, and that improved versions of his textbooks could have great educational value.

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

Mario Iona is a professor emeritus in the Department of Physics at the University of Denver. He writes a column for The Physics Teacher, analyzing erroneous and misleading material in science textbooks and journal articles. When the American Association of Physics Teachers gave him its Millikan Lecture Award in 1986, his acceptance speech was entitled "Why Johnny Can't Learn Physics from Textbooks I Have Known."

Addendum

Addison-Wesley issued another version of Conceptual Physics in 1997. Then Addison-Wesley merged with Longman Publishers to form a company called Addison Wesley Longman, and this company produced a "new" version of Conceptual Physics in 1999. To find reviews of the 1997 and 1999 versions, consult our Index List.

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