Copyright © 2010 jsd

Review of Hewitt, Conceptual Physics

1  Overview

Paul G. Hewitt
Conceptual Physics
Prentice Hall [an imprint of Pearson] (2009)

One overall impression of the book is that it is wildly inconsistent. Some passages are obviously very carefully thought out, while other passages are quite thoughtless and careless. The book astutely avoids some of the misconceptions that other texts fall into, but then leaps headlong into other misconceptions.

Another overall impression is that the book panders to the lowest levels of thinking skills. It is expected that many students will begin the introductory physics course without much in the way of critical thinking skills ... but a major purpose of the course should be to help them improve these skills. This book doesn’t do that. It barely even tries to do that. This can be seen in the CONCEPT CHECKS sprinkled throughout the book. For example, in section 38.4 it emphasizes that «Light behaves like waves when it travels in empty space, and like particles when it interacts with solid matter.» A few sentences later we find «CONCEPT CHECK: What causes light to behave like a wave? Like a particle?»

I’m sorry, but that is not a check of the concept. That is only a check for rote regurgitation of the words that appear higher up the page. If you wanted to check the concept, you would ask the student to apply the concept in some setting different from the original setting.

It would be bad enough to rely on rote repetition (to the exclusion of other methods) to teach correct ideas, but one of the problems with rote is that it works equally well for wrong ideas as for right ideas. Alas, the concept featured in section 38.4 is deeply flawed, as discussed in item 47 and item 48.

By way of a pathetic silver lining, given that this version of the wave/particle concept is wrong, perhaps it is a good thing that students will not learn it very deeply.

I could understand it if the CONCEPT CHECKS started out at the rote level and became more sophisticated as we progressed through the book, but alas this is not what happens. This is a grave criticism of a book that features the word «Conceptual» in its title.

Along the same lines: Often the «think!» boxes are just as unthinking as the CONCEPT CHECKS (e.g. section 38.7).

Overall, I find this book to be vastly better than the deplorable reference 1 – but that is very faint praise. As far as I can tell, this book is probably inferior to reference 2.

*   Contents

2  Critical Missing

This book is to be criticized more for what it doesn’t say than for what it does (although what it does say has plenty of problems as well, as discussed in section 3).

I am a big proponent of the building-block approach. That is, given a big scary problem, you pick it apart into individual components. You learn each block separately, then start putting the blocks together, two at a time, then three at a time, and so on ... until you have constructed the finished edifice.

Note that the last step – putting things together to form a finished edifice – is an indispensible part of the process.

Naïve students necessarily start by learning concepts one by one, but it is imperative to move beyond this stage as quickly as possible, even in the most introductory course. A pile of bricks1 and a pile of sticks is better than nothing, but you wouldn’t want to live there. With physics as with bricks, virtually all of the creativity, all of the thinking, and all of the utility come from how you put the things together. Any one block, by itself, isn’t very interesting or useful.

The harshest criticism of this book is that it spends far too much time gawking at individual building blocks, and not nearly enough on putting things together in useful, thoughtful, creative, and/or interesting ways. As long as students see physics as a pile of disconnected factoids, it will not be remembered, and indeed it will not be worth remembering, so why bother teaching it at all?

3  List of Bugs

Here is a list of some of the bugs and other strange things in this book. This is not an exhaustive list – just a sampling.

In chapter 2 the Big Idea is: «An object in mechanical equilibrium is stable, without changes in motion.»

That is just wrong. Equilibrium is not the same as stability. A perfectly balanced wheel gives us equilibrium with zero stability. If we add a weight to the bottom, we get equilibrium with positive stability. If we add a weight to the top instead, we have equilibrium with negative stability. If we add a weight to the side, at the 9:00 position, we no longer have equilibrium.

Figure 1: Equilibrium and/or Stability

Also this is bad pedagogy. It violates the principle that learning proceeds from the known to the unknown. A student who doesn’t know what equilibrium means cannot be assumed to know what stability means. Evidently, even the author doesn’t know what stability means.

The headline of section 3.5 is: «Mass – A Measure of Inertia»

This is bad pedagogy. It puts too much emphasis on terminology and not enough on ideas. Are we defining mass in terms of inertia? What then is the meaning of inertia?

If you are going to assume the students know what inertia is, why not just go all the way and assume they know what mass is?

Books that take a more scientific approach stick to using the scientific term, mass, and don’t even mention inertia.

In section 3.6 it says «Four hundred years ago, people had difficulty with ideas like these ...... Slow, bumpy rides in horse-drawn carriages do not lend themselves to experiments that reveal inertia.»

Four hundred years ago and more, a fast-moving ship sailing over calm seas was quite sufficient to demonstrate relativity, i.e. invariance with respect to uniform motion. Not every person had this experience, but more than a few did. The significance of this was explicitly pointed out in 1632 (reference 3 or reference 4).

In section 4.1, the opening paragraph is a quagmire of contradictions. The headline says «Motion Is Relative» which expresses the principle of Galilean relativity. Then the very next sentence says «Everything moves» which violates the principle of Galilean relativity. (In reality, every object is either moving or not ... depending on an arbitrary choice of reference frame.) The paragraph goes on to say some non-wrong things. Then in the middle of the paragraph, in boldface, it says «An object is moving if its position relative to a fixed point is changing.» That is a gross violation of Galilean relativity. There is no such thing as absolute fixity just as there is no such thing as absolute motion. The lesson should be to teach people never to formulate ideas in terms of statements such as “X moves” or “X is moving” ... and instead to prefer invariant statements such as “X is moving relative to Y”.

In section 6.6 it says «This experiment, said to be done by Galileo from the Leaning Tower of Pisa ....»

It is a cheap shot to say that it was “said” to be done ... when in fact it was not done. We are supposed to be scientists, not smart-aleck lawyers.

There is no valid scientific or pedagogical reason to bring up this bogus story.

This is a big deal because it undercuts the credibility of many other statements throughout the book. For example, in section 19.1 we learn that «According to Huygens, you can replace the crest of any wave by series of equally-spaced wave sources....» OK, let’s stipulate that Huygens said that; the question then remains, was what he said true, or was it as bogus as what was said about the leaning tower of Pisa?

In chapter 5 a sidebar says: «Suppose an airplane flying normally at 80 km/h encounters a wind at a right angle to its forward motion – a crosswind. Will the airplane fly faster or slower than 80 km/h?»

Very few airplanes fly «normally» – or at all – at a speed of 80 km/h. That’s 43 knots.

Furthermore, even setting aside that detail, this question is broken at a deeper conceptual level. The question is underspecified several times over. The pilot’s response to the perturbation is unspecified, the type of speed is unspecified (airspeed versus groundspeed), and even the definition of “forward” is unspecified (heading versus velocity). In aircraft – unlike cars – there is not a one-to-one relationship between heading and direction of motion.

An exercise at the end of chapter 6 refers to a «jumbo jet with a mass of 30,000 kg.» That’s wrong by an order of magnitude.

More importantly, it is a bad practice to use fake data and try to pass it off as real-world data. This is considered scientific misconduct.

In chapter 7 it asks the students: «CONCEPT CHECK: Why do forces always occur in pairs?»

OK, I’ll bite. Please tell me, why do forces always occur in pairs? I cannot imagine how a student is supposed to answer this question. I cannot imagine what purpose is served by asking such a question.

Galileo emphasized that the laws of physics must say what happens. The laws may or may not say how it happens ... and they rarely if ever say why it happens. Recognizing this distinction has been called the epoch, i.e. Day One of modern science. This is what sets physics apart from metaphysics and speculative philosophy.

This is a big deal; asking «why do forces always occur in pairs» sets science back 400 years.

Newton, having gone to school on Galileo, answered a similar question by saying “Hypotheses non fingo.” That is Latin for “I’m not going to pretend to guess”. To say it another way: nobody knows the answer, and nobody ever will, because question is not worth asking. There is alas nothing in this text that trains students to give such a response.

It’s not even 100% guaranteed that the third law is exact. If you express it strictly in terms of particles exerting forces on other particles, there are gross violations, as illustrated by the spinning disk in Feynman volume II figure 17-5; see reference 5.

It is better to reformulate the third law in terms of conservation of momentum (rather than force pairs) – but Hewitt does not do so. This allows us to answer a somewhat different question. Evidence suggests that the non-conservation, if any, must be very small: Kreuzer (1968); Bartlett and van Buren (1986). However, the fact that you can publish any data whatsoever on this topic means that intelligent people are willing to entertain the possibility that the discrepancy is nonzero. They want to see the evidence.

See also item 48. For more about what we mean by words such as “why” and “because”, see reference 6.

In summary, here is how I would respond to such a question. It must be emphasized that this is a response, not an answer, because the question in its original form is not answerable.

  1. The first rule is, all ill-posed questions must be recognized as such. The general topic of ill-posed questions is discussed in reference 7.
  2. Try to transmute it into the question that should have been asked.
  3. A question that begins with «why» should raise lots of red flags.
    • Transmute «why X» to whether X.
    • Transmute «why X» to how do we know X and then how sure are we of that.
    • Transmute «why X» to how could we have predicted X and then what is the evidence for X.
    • In general, always ask in what ways is X consistent (or inconsistent!) with other stuff we know?
  4. Replace force-pairs by conservation of momentum.
  5. Rather than assuming strict conservation, assume there is some small violation, and look for evidence that places upper bounds on the magnitude of the violation.

It would be nifty if the book explained how to do all of that, or any of that, but it doesn’t.

In chapter 7, the mouse says «Most of the cannonball’s momentum is in speed; most of the recoiling cannon’s momentum is in mass.»

First of all, momentum is never “in” mass or “in” speed. And even if it were, it would be proverbially crazy to compare apples and oranges in such a way.

Chapter 9 starts by saying «Energy is the most central concept underlying all of science.»

Really? How do you know? If the concept is so central, why was it not developed until roughly 200 years after the so-called laws of motion?

I cannot imagine why anybody would say such a thing, or ask students to learn such a thing.

I don’t think conservation of energy is more central than conservation of momentum. I don’t think energy is more central than space and time. I don’t think energy is more central than arithmetic. I don’t think tunneling is well explained by focusing on energy transfer to the neglect of other considerations. See also item 50.

The whole discussion of what is “most central” is misguided. There is no center as far as I can tell. It’s not like high-school geometry proofs. Feynman said that knowledge is like a grand tapestry. A forgotten fact is like a hole in the tapestry. You should be able to repair the hole in several different ways, by reweaving down from the top, or up from the bottom, or in from the sides. Any important fact can be rederived in numerous ways, because the things we know are interconnected in innumerable ways.

Chapter 9 repeatedly states that work equals “force × distance”, using a cross as the symbol for multiplication.

It would be better to write it as “force • distance”, using a dot rather than a cross. You could argue that a student reading the book for the first time would not appreciate the distinction between cross-product and dot-product ... but what about somebody reading the book a second time?

Perhaps the assumption is that nobody will ever give this book a second look.

Section 9.2 defines power in terms of work done divided by time interval. This definition is one of the more-or-less conventional possible definitions, and it would be unfair to call it wrong, because people are allowed to define terms however they like, within reason. However, the readers would be better served if they were warned that this definition is guaranteed to be problematic.

Section 9.6 tramples on the distinction between energy and kinetic energy.

This sows confusion for no reason. It would have been super-easy to state things more carefully. The rule should be, if you mean energy say “energy” and if you mean kinetic energy say “kinetic energy”. This involves no extra work and no increase in complexity, and results in a significant increase in clarity and correctness.

Section 9.7 states what is generally called the global law of conservation of energy. It says that the energy of the universe as a whole cannot change. Alas, that is not very useful in practical situations. The law of local conservation law says that energy is conserved right here, right now. The local law is more profound and more useful. See reference 8.

Section 9.11 emphasizes that «The sun is the source of practically all our energy on Earth.»

This pulls the rug out from under the whole discussion of the law of conservation of energy.

Given that the physics energy is conserved, it cannot have any sources or sinks.

Apparently 9.11 is using the word energy to mean something other than the physics energy, presumably some kind of “available energy”. This is one of the homespun meanings of the word energy. Available energy is important, but it is not the same as the physics energy.

Because it is conserved, the physics energy cannot have any sources or sinks. When people talk about a «source of energy», it would usually be more accurate to rephrase it in terms of making energy flow in useful ways. For example, consider the rest-energy tied up in the mass of a large stone. That is an enormous amount of energy, but it usually isn’t considered “available energy” because there is no way to make it flow in useful ways.

I mention this because even if we re-interpret the statement as a claim that the sun drives «practically all» of the energy flows on earth, it’s still an overstatement. There is a nontrivial contribution from radioactivity i.e. nuclear energy. Note that geothermal energy flows can be considered an indirect manifestation of nuclear processes within the earth.

Section 10.3 starts with an example where the observer is in a rotating frame, such that centrifugal forces will be observed. It then proceeds to describe the situation in terms of «centripetal» not centrifugal force. It says «The force you feel from the wall while on a rotating carnival centrifuge is a centripetal force.» That may be literally true in the narrowest sense, but it is bad pedagogy because it is inconsistent with the way that other situations are described.

By way of contrast, consider gravitation. We do not normally introduce gravity by reference to the upward «force you feel from the» floor. That is, we do not normally describe a given force in terms of the constraint-force necessary to oppose the given force.

The fact is, the centrifugal field exists in a rotating reference frame and not otherwise ... as correctly explained in section 10.5. That means the discussion of centripetal forces in a rotating frame should be delayed until the relationship between the centripetal and centrifugal forces can be appreciated.

In chapter 11, the Big Idea is «An object remains in rotational equilibrium if its center of mass is above the area of support.»

That’s weird. First of all, it not uncommon for an object to go out of equilibrium while its center of mass is still above the area of support. An example is the top-heavy bus discussed in item 18.

(Conversely, for what it’s worth, it is possible, and indeed common, for an object in equilibrium to have its center of mass below the area of support ... not above.)

More importantly, the idea of «area of support» is not helpful for analyzing the rotational equilibrium of a bird, aircraft, or spacecraft in flight. It does not even apply to the nut+wrench situations that are prominently featured in section 11.1.

The idea of «area of support» is a watered-down version of the idea of “line of action” of a force, which is not covered in this book. Furthermore, its relationship to equilibrium depends on the notion of force of constraint, which was hinted at in the «Personal Essay» in section 2.1 but never fleshed out to a usable degree.

Continuing the silliness that began with item 17, section 11.5 states «The Rule For Toppling. If the center of gravity of an object is above the area of support, the object will remain upright.» That statement appears directly underneath the picture of a tall, narrow, double-decker bus.

The problem is, the statement is just not true. It is certainly not true for a top-heavy bus while cornering.

In section 11.7 it defines «neutral equilibrium».

There is no such thing. It should have said neutral stability or perhaps neutrally-stable equilibrium.

Section 13.9 says there are «two sets of ocean tides per day – two high tides and two low tides» ... as if that were an inescapable consequence of the laws of physics.

However, the fact is that in the Gulf of Mexico, it is normal to have only one high tide and one low tide per day. This fact is symptomatic of a deeper problem, namely failure to distinguish between (a) the tide-producing stress, and (b) the actual tides that are produced when this stress drives a very complicated response function. See reference 9.

In section 15.2 it says «Einstein’s first postulate of special relativity assumes our inability to detect a state of uniform motion.»

This paints a highly misleading picture of the history. This principle is properly known as Galileo’s principle of relativity. The author must know this, since it was discussed in sections 3.6 and 4.1 of the book.

Near the end of chapter 15 the bird says «In summary – Time dilation: moving clocks run slowly. Length contraction: moving objects are shorter (in the direction of motion).» The sad thing is, those two statements do in fact summarize most of the chapter.

The problem is, those two statements do not correctly summarize special relativity. The chapter fails to mention the breakdown of simultaneity at a distance. We are left with a caricature of relativity that is so incomplete as to be conceptually incorrect and utterly unusable.

See also item 23.

To repeat: Near the end of chapter 15 the bird says «In summary – Time dilation: moving clocks run slowly. Length contraction: moving objects are shorter (in the direction of motion).» Those two statements do in fact summarize most of the chapter.

However, the fact is that those two statements are – at best – matters of opinion. For more than 100 years, the consensus of expert opinion is that a boost (changing the velocity) does not change the length of a ruler, just as a spatial rotation (changing the orientation) does not change the length of the ruler. It may change the projection of the length onto this-or-that coordinate axis, but it does not change the length per se.

It is perfectly possible to teach relativity the modern (post-1908) way, using clocks that mean what they say and rulers that mean what they say; that is, emphasizing proper time, proper length, and invariant mass. See reference 10 and references therein.

See also item 22.

In section 16.2 it says «The equation E=mc2 is more than a formula for the conversion of rest mass into other kinds of energy, or vice versa. It states that energy and mass are the same thing

First of all, that is wrong because the rest energy mc2 is not even numerically equal to the total energy; it is just one contribution to the total energy, as we can see from the more-detailed expression E2p2c2 = m2c4. We see that E=mc2 only when the momentum p is zero, i.e. only in the rest frame of the particle. By way of counterexample, for a photon m=0 and E=pc. The photon is never at rest so it never has E=mc2.

Secondly, at the conceptual level, even if/when two things are numerically equal, that does not necessarily mean they are «the same thing». Despite the title, this book does not seem to care much about conceptual issues.

In section 16.5 the bird says «Look at an airplane’s flight path drawn on a flat map and you’ll see that the line is curved. The same line drawn on the surface of a globe would be a geodesic ....»

That’s untrue several times over.

As a matter of principle: If you want to talk about some model system in which your idealized aircraft flies along a geodesic, that would be OK in moral terms (although of dubious relevance and practicality). However, you have to say what you are doing and not pretend you are doing something else. It is highly improper to pass off models and idealizations as if they were observations about the real world.

Elsewhere section 16.5 offers a «simplified analogy» for gravitation and spacetime curvature, in terms of a marble rolling on a curved surface.

This does not count as misconduct, because it is clearly labeled as a simplified analogy ... but still, it’s wrong physics and bad pedagogy. Better analogies are available, analogies that are simultaneously simpler and far more apt. It turns out that gravitation causes spacetime to be strongly curved in the time direction, and this must be taken into account. Also, the marble does not roll along a geodesic. Methods for constructing simple, highly informative geodesics are available; see reference 11.

Contrary to what it says in section 16.6, according to our modern understanding of spacetime, it is not true that «gravity causes clocks to run slow».

See item 23 for more on this.

In section 17.7 the bird says «Atoms weren’t fully validated until the work of Albert Einstein in the early 1900s.»

That’s just not right. There were plenty of people who understood atoms before Einstein. One major turning point was in 1865 when Loschmidt measured the size of the molecules that make up air. On the other side of the coin, for what it’s worth, there were still people who disbelieved the evidence long after the early 1900s.

The periodic table presented in section 17.8 could be improved.

For one thing, the experimental chemistry and the theoretical atomic physics both indicate that there are 15 lanthanoids, from 57La through 71Lu inclusive. See reference 12.

Contrary to what is depicted near the end of chapter 19, it is not common practice – or safe practice – to use compressed air for lifting cars in a service station.

Contrary to what it says in section 20.7, Bernoulli’s principle is not «a consequence of the conservation of energy».

It’s true that the dynamic pressure ½ρv2 is equal to the kinetic energy per unit volume, and it’s true at the level of dimensional analysis that the overall pressure P has dimensions of energy per unit volume. However, there is more to physics than dimensional analysis. The pressure is simply not numerically equal to the energy per unit volume. Actually, for nonmoving air, the pressure is numerically equal to about 40% of the energy per unit volume.

Bernoulli’s equation can be explained in terms of enthalpy. Enthalpy has the same dimensions as energy, but it’s not the same thing. Also there is no law of conservation of enthalpy, so what the book says is just not true. Not quantitatively, not qualitatively, not conceptually, not at all true.

One of the exercises at the end of chapter 20 asks the student to hold a teaspoon in a jet of water. This is a very complicated hydrodynamical situation. There is virtually no chance that any high-school student will be able to analyze this situation correctly. Not even approximately correctly. The book suggests that the result is explained by Bernoulli’s principle, which is nowhere near being correct. See reference 13.

Contrary to what it says in section 22.7, the mechanism of the so-called greenhouse effect in the context of planetary atmosphere is fundamentally different from the mechanism of an ordinary agricultural greenhouse. The latter works primarily by cutting down on convection.

In section 23.8 a sidebar says «When a car is washed on a cold day, hot water will freeze more readily than warm water because of the energy that the rapidly evaporating water takes with it.»

First of all, the claimed effect is not what actually happens. (There are situations where the Mpemba effect can be observed, but this is not one of them.) In any case, the second part of the statement, i.e. the explanation, is complete hogwash.

Section 24.1 says that «the kinetic energy of the atoms approaches zero, and the temperature of the substance approaches a lower limit. This limit is the absolute zero of temperature.»

In fact, though, the way in which kinetic energy “approaches” zero is wildly different from the way in which temperature “absolute” zero, because of the quantum-mechanical “zero-point motion”. The temperature can get arbitrarily close to zero, but the kinetic energy cannot.

In section 24.3 a sidebar says: «Blow warm air onto your hand from your wide-open mouth. Now reduce the opening between your lips so that the air expands as you blow. Adiabatic expansion – the air is cooled.»

No part of that explanation is correct.

The flow of air through an orifice is never adiabatic. Adiabatic expansion would require doing work against some sort of engine. This should be obvious from the proportionality between energy and temperature for air (or any other polytropic ideal gas).

Also, even if the process were adiabatic, the cooling associated with expansion would simply undo the immediately-prior heating associated with compression.

Section 24.7 says by way of definition that «Entropy is the measure of the amount of disorder in a system.»

Alas that’s not what entropy is.

This misconception is plausible and widespread, but it is still a misconception. The fact that it is plausible makes it all the more pernicious.

Still, if you think about it for a moment, you realize that entropy is a property of the macrostate, whereas disorder is a property of the microstate ... so these things could not possibly be equivalent. If we know what microstate the system is in, even if it is a very disorderly-looking microstate, then the system entropy is zero. See reference 14 and references therein.

Section 25.2 states emphatically that «The source of all waves is something that vibrates.»

That’s certainly not true for Čerenkov radiation. It is not true to any significant degree for the pop of a firecracker or the crack of a whip.

Section 25.3 emphasizes that «The energy transferred by a wave from a vibrating source to a receiver is carried by a disturbance in the medium.»

That’s certainly not true for light. There is no medium. The idea of a luminiferous ether has been dead for more than 100 years.

Section 25.4 emphasizes that «You can calculate the speed of a wave by multiplying the wavelength by the frequency. ... This relationship holds for all kinds of waves, whether they are water waves, sound waves, radio waves, or light waves.»

That relationship is not even remotely true for waves on the surface of a body of water. The propagation of such waves is highly dispersive. It’s also not true, if look at all closely, for waves on a piano string. For a wide range of practical purposes, the group velocity serves as “the” speed of the wave, and that goes like dω/dk not ω/k.

Section 25.6 emphasizes that «Sound waves are longitudinal waves.»

That’s not true for sound waves in solids.

Section 34.6 says «Suppose you fall from a bridge and manage to grab onto a high-voltage power line, halting your fall. So long as you touch nothing else, you will receive no shock at all.»

That’s untrue in principle, and if the voltage is high enough it will be untrue to a quite significant degree. This is an AC circuit, and a dangling person has a nonzero self-capacitance. For a 500,000 volt line, if we model the dangling person as a cylinder 0.15 m in radius with a length of 2 m, I calculate a current of about 8 mA, which is well above the threshold of pain.

є0   8.8542E-12   F/m
radius of cylinder   0.15   m
cutoff   2   m
log factor   2.59    
length   2   m
capacitance   4.2955E-11   F
freq   60   Hz
impedance   61752458   ohm
voltage   500000   V
current   0.008097   A

For an even more spectacular example of self-capacitance in connection with high-voltage power lines, see http://www.youtube.com/watch?v=rkYq17gTCq8

Section 35.3 emphasizes that «If one device fails in a series circuit, current in the whole circuit ceases and none of the devices will work. Some cheap party lights are connected in series. When one lamp burns out, it’s “fun and games” (or frustration) trying to find which bulb to replace.»

That evidently assumes that any device that «fails» becomes an open circuit. That is simply not true. It’s not even true for all strings of decorative lights; in many cases the lamps in such strings are engineered to fail short rather than fail open.

This same misconception is repeated elsewhere in the book. It is simply not correct – and not safe – to assume that every failed device is an open circuit.

The phenomenon cited in item 43 is claimed to be «the main disadvantage of a series circuit.» That’s not true either. The situation is symmetrical, in the sense that if any element in a parallel circuit shorts out, the whole circuit fails.

Again: It is simply not correct – and not safe – to assume that every failed device is an open circuit.

Section 36.4 says «Permanent magnets are made by simply placing pieces of iron or certain iron alloys in strong magnetic fields. Alloys of iron differ; soft iron is easier to magnetize than steel.»

That is misleading several times over. For one thing, in the first sentence it would be better to say “can be made” rather than «are made» since that is by no means the only way of making permanent magnets.

In the second sentence, I generously assume «soft iron» refers to highly pure iron, which is both mechanically soft and magnetically soft, so that the distinction between mechanical softness and magnetic softness does not matter too much. It would be completely unreasonable to expect students to recognize the distinction.

Also in the second sentence, context seems to require that the word «magnetize» must refer to permanent magnetization (as opposed to temporary induced magnetization) ... which means the sentence is entirely false. It is absolutely not easier to create permanent magnetization in soft iron, compared to the alloys that are normally used for permanent magnets.

Alternatively, if you assume that «magnetize» refers to temporarily induced magnetization, the book is wrong for the opposite reason: The silicon steel used in real-world transformers has almost an order of magnitude more permeability than pure mechanically-soft iron ... not to mention the fact that the latter would be wildly impractical in economic terms, and is never used in practice, contrary to what the book says.

Contrary to what it says in the sidebar in section 37.3, airport metal detectors are not limited to detecting iron. The principle of operation does not involve detecting a change in an otherwise-steady field.

Section 38.4 emphasizes that «Light behaves like waves when it travels in empty space, and like particles when it interacts with solid matter.»


For starters, not all matter is solid. How are we supposed to understand what light does in the presence of a rarefied gas? Or a dense liquid?

Furthermore, the fact is that there are innumerable examples of wavelike interactions between light and matter. Familiar examples include:

The «CONCEPT CHECK» featured in section 38.4 is deeply flawed. Strictly speaking, so far as I know, there is not anything that causes light to act like a particle or causes it to act like a wave. Any attempt to think about this «CONCEPT CHECK» will reinforce incorrect ideas about how cause-and-effect works.

See also item 8 and reference 6.

Section 39.4 emphasizes that «Isotopes of an element are chemically identical but differ in the number of neutrons.»

That’s not actually true. Chemical reactions involving deuterium proceed at a different rate compared to hydrogen. The same is true to a lesser (but still noticeable) degree for the isotopes of lithium.

At a more fundamental level, if the sentence is going to talk about the number of neutrons, why not define isotope in terms of the number of protons? This makes the sentence simpler and more correct. As emphasized by Strunk & White among others, if you are going to draw a parallel or draw a contrast, use corresponding words for corresponding ideas.

Section 40.1 emphasizes that «Nuclear fission occurs when the repelling electrical forces within a nucleus overpower the attracting nuclear strong forces. The absorption of a neutron by a uranium nucleus supplies enough energy to cause such an elongation.»

Sorry, the energy argument in the second sentence is not correct. For starters, 236U is lower in energy i.e. more tightly bound than 235U. You should know that fact even without looking it up, because fission is promoted by slow neutrons, and if the neutron were not attracted to the 235U it would never have been absorbed. Alternatively, you could just look it up (reference 15) and find that the binding energies are:

nucleon  bound state energy
235U   −1783870±2 keV
236U   −1790415±2 keV

More generally, despite what the book says in chapter 9, not everything can be analyzed by focusing on energy transfer to the neglect of other considerations. For more about the actual mechanism of induced fission, see reference 16. See also item 10.

4  References

John Denker
“Review of Glencoe Physics: Principles and Problems
John Denker
“Review of Serway and Faughn, Holt Physics
Galileo Galilei,
Dialogo sopra i due massimi sistemi del mondo (1632).
Galileo Galilei,
Dialogue Concerning the Two Chief World Systems
An excellent translation of reference~3 by Stillman Drake (1953).

Shut yourself up with some friend in the main cabin below decks on some large ship, and have with you there some flies, butterflies, and other small flying animals. Have a large bowl of water with some fish in it; hang up a bottle that empties drop by drop into a wide vessel beneath it. With the ship standing still, observe carefully how the little animals fly with equal speed to all sides of the cabin. The fish swim indifferently in all directions; the drops fall into the vessel beneath; and, in throwing something to your friend, you need throw it no more strongly in one direction than another, the distances being equal; jumping with your feet together, you pass equal spaces in every direction. When you have observed all these things carefully (though doubtless when the ship is standing still everything must happen in this way), have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still. In jumping, you will pass on the floor the same spaces as before, nor will you make larger jumps toward the stern than toward the prow even though the ship is moving quite rapidly, despite the fact that during the time that you are in the air the floor under you will be going in a direction opposite to your jump. In throwing something to your companion, you will need no more force to get it to him whether he is in the direction of the bow or the stern, with yourself situated opposite. The droplets will fall as before into the vessel beneath without dropping toward the stern, although while the drops are in the air the ship runs many spans. The fish in their water will swim toward the front of their bowl with no more effort than toward the back, and will go with equal ease to bait placed anywhere around the edges of the bowl. Finally the butterflies and flies will continue their flights indifferently toward every side, nor will it ever happen that they are concentrated toward the stern, as if tired out from keeping up with the course of the ship, from which they will have been separated during long intervals by keeping themselves in the air. And if smoke is made by burning some incense, it will be seen going up in the form of a little cloud, remaining still and moving no more toward one side than the other. The cause of all these correspondences of effects is the fact that the ship’s motion is common to all the things contained in it, and to the air also. That is why I said you should be below decks; for if this took place above in the open air, which would not follow the course of the ship, more or less noticeable differences would be seen in some of the effects noted.

John Denker,
“Introduction to Force, Momentum, Torque, and Angular Momentum”
John Denker,
“Cause and Effect”
John Denker,
“How to Deal with Ill-Posed Questions”
John Denker,
“Conservation as related to Continuity and Constancy and the Continuity of World-Lines”
John Denker,
“Twice-A-Day and Once-A-Day Tides”
John Denker,
“Welcome to Spacetime”
John Denker,
“Tabletop Geodesics, General Relativity, and Embedding Diagrams”
John Denker,
“Periodic Table of the Elements – Cylinder with Bulges”
John Denker,
“Teaspoon Demonstration” in chapter 18 of See How It Flies
John Denker,
“Entropy – Increased by Stirring, Decreased by Observation”
Table of Nuclides
Raymond A. Serway and John W. Jewett, Jr.
Physics for Scientists and Engineers with Modern Physics

This applies equally to real bricks and to toy bricks, such as Legos.
Copyright © 2010 jsd