Grandes Errores y Omisiones
Informe FinalThe David and Lucile Packard Foundation Beca #1998-4248 Revisión de Textos de Enseñanza de Física
en las Escuelas de Nivel Medio
John L. Hubisz, Ph.D., Hubisz@unity.ncsu.edu
At this point we feel that it would not be worthwhile adding eight pages of "Lesser Errors and Confusing Statements."Obviously, NOT RECOMMENDED.
Página 11 La Tabla 1.2 da unidades SI de masa como gramo en vez de kilogramo. Está correcto en el texto de la página 13. 40 La Fig. 2.9 muestra un gráfico de velocidad contra tiempo. El texto se refiere a ello como "posición vs. tiempo" y pide al lector compararlo con un gráfico previo de "posición vs. tiempo". El resto del párrafo discute un gráfico de "posición versus tiempo" para aceleración constante, que aparentemente es lo que 2.9 se suponía que era. No se trata simplemente de una cuestión de que las leyendas de los ejes estén equivocadas, eino que lo mostrado en la Fig. 2.9 es un gráfico segmentado y no una curva uniforme. 40 No menciona a la aceleración promedio. Tienen gráficos de velocidad vs. tiempo para los que podrían haber fácilmente conectado la aceleración a la pendiente de la línea en cada sección, pero no se hizo. La Conexión Matemática de la Edición de Maestros habla de la pendiente, pero sólo sugiere que debe aplicarse a un gráfico de velocidad constante. 41 El ejercicio de Constructor de Habilidad es espantoso. Se les pide a los estudiantes "aplicar la fórmula de la aceleración para calcular la aceleración a 4 segundos" de la distancia versus los datos del tiempo. La única información que tienen de la aceleración es su definición como cambio de velocidad dividia por el cambio en el tiempo. También se les pide que usen la distancia versus la información de tiempo para hacer un gráfico mostrando la aceleración. Se espera que ellos encuentren la velocidad promedio para cada segudno, y que lo grafiquen, pero esto un gigantesco salto a partir de la información en el texto. El único cálculo numérico mostrado en el "material para maestros" está equivocado. 43 y 50 "...una aceleración es un cambio en la velocidad que resulta de acelerar, frenar, o cambiar de dirección." La Causa y el Efecto están aquí al revés. Es la fuerza la que produce aceleración lo que es visto como un cambio en la velocidad. También, la aceleración no es cambio de la velocidad, sino que es un cambio de la velocidad por unidad de tiempo. Continuando en el mismo párrafo, se hace la declaración que "la aceleración causada por movimiento en un círculo se llama "aceleración centrípeta". Nuevamente, el movimiento en un círculo es movimiento acelerado. 44 Los "links" a Vida-Ciencia hablan de la fuerza necesaria para reducir la velocidad de una persona desde 50 km/h en un accidente de automóvil, sin referencia a un intervalo de tiempo. La omisión de un intervalo de tiempo es contraria a lo que los estudianes han aprendido acerca de la aceleración. Además, pierde la oportunidad de hablar sobre la manera en que las bolsas de aire operan aumentando la cantidad de tiempo durante el que la velocidad cambia. Luego usa una elemental regla para calcular la fuerza necesaria basada en su peso. Esto tenderá a forzar la idea de que la fuerza está relacionada con el peso más que sobre la masa. 51 Llene los espacios vacíos: "La distancia viajada en un cierto tiempo se llama _ _ _ _". La Edición de Maestros dice que la respuesta correcta es "velocidad", pero si yo viajo 10 m en 2 segundos mi velocidad no es 10 metros. Aún si el tiempo fuese 1 segundo, la velocidad seguiría sin ser 10 metros. Una distancia no puede equivaler a velocidad 56 "Dado que la fuerza de la gravedad es 9.8 m/s, …" y esto después de haber dicho, sólo una momento antes que la fuerza se mide en newtons. 60 El Calentamiento Previo de Habilidad pide a los estudiantes que consideren la situación de una pulseada en la que ninguna de las personas puede vencer a la otra. Se les pide: "¿Qué es lo que pueden inferir de las fuerzas?". La mayoría de los estudiantes dirán que las dos personas están ejerciendo fuerzas iguales uno sobre el otro. Esto es, desde luego, cierto por la Tercera Ley de Newton, pero no tiene nada que ver con el hecho que los brazos no se están moviendo. No es sorprendente que los estudiantes tengan problemas con la Tercera Ley de Newton cuando ejemplos como este les llevan a creer que las fuerzas son sólo iguales porque hasta el momento hay un empate. La respuesta dada en el material del maestro dice que a causa de que estas dos fuerzas son iguales y opuestas no hay movimiento. Esto está errado. La fuerza que la persona A ejerce sobre la persona B no afecta al movimiento de la persona A. El libro ha ignorado totalmente las fuerzas entre la persona y la mesa, que son muy importantes aquí. 60 Newton's first law is stated incorrectly: "… an object in motion will remain in motion unless acted upon by an outside force." It should say, "will remain in motion at constant velocity." This is a very serious error, as students will commit this to memory for life. 61 "Friction works in the opposite direction to the force of motion."There is no such thing as a force of motion. 64 Activity in Skills Warmup does not control variables and ignores it.Says a long pencil will roll down a ramp more slowly than a short one because greater surface area gives more friction, but it ignores the difference in mass between the pencils.This is particular egregious when they are about to introduce Newton's second law. 76 Text states that the acceleration of gravity on the Moon is 1/6 that on the Earth because the mass of the Moon is 1/6 the Earth's mass.First, the mass of the Moon is about 1/80 of the Earth's not 1/6. Second, they have totally ignored the fact that the acceleration due to gravitation is also related to the radius of the body. 79 and 81 Page 73 correctly lists the four fundamental forces.In the chapter review and in the answer to Check and Explain question 1, they are listed incorrectly. 81 Check your vocabulary - fill in the blank question:"When your body is at rest, you have ______." Expected answer is inertia, which is indeed a true statement, but it implies that you don't have inertia if you are moving. 91- 93 Several times it is stated that the buoyant force on a floating object is greater than its weight. Pictures shown with this label show an object in equilibrium. This might be OK as an introductory statement if they clarified what was meant later, but they don't. They never talk about how much of an object is above the water when floating in terms of the buoyant force.They do mention it at the very end of the section in terms of density difference, but this is inadequate – particularly given the incorrect statements about the relation between buoyant force and weight. 91 and 103 "The force gravity exerts on an object is equal to the object's weight."Sounds like these are two different forces that happen to be equal in this case. Sentence should say, "The property of an object that we call weight is the magnitude of the gravitational force that the Earth exerts on the object." Or better yet, be omitted, as students should already know this. 98 The discussion of Bernoulli's principle states that the air moves faster over the top of wing in order to arrive at the back end at the same time as the air that went under the wing. This is nonsense. 107 Text implies that the force you exert on the floor does work which moves you forward. More nonsense. 112 A frequent error in the entire book is to say an object is a force rather than exerts a force. An example is in the discussion of opening a paint can lid with a screwdriver in the section on levers. The statement is made that the lid is the resistance force. 123 Gets the definition of heat correct unlike later in the book (see note re page 209 below). However, the term heat energy is then used on the next page to mean thermal energy. 130 The science literature connection is a story about a boy that can travel in the 4th dimension. While there is much science that can be learned from science fiction, this particular story is totally inappropriate.There is no science accessible to middle school students to be learned here. The text claims that it explores frames of reference but it does not. In the critical thinking section students are asked how frames of reference might differ in 2-D, 3-D, and 4-D space. The answer given is nonsense. As far as we are concerned, having this story in a science book just gives credence to the new age nonsense such as energy auras that students are bombarded with outside of school. 140 Typical incorrect picture illustrating the difference in solids, liquids, and gases. Shows density of liquid as much closer to the gas than the solid.On page 146, students are asked to draw diagrams showing the spacing of particles in a solid, liquid, and gas.Their drawings will certainly be incorrect since the ones in the text are. 145 "At very high temperatures, over 1,000,000 ºC, gas particles break down, forming a plasma phase." There is no mention of charged ions or dissociated electrons. This sentence could just as well mean that the nucleus comes apart into its constituent protons and neutrons. While the original definition of plasma was a gas in which all atoms were ionized, the term has come to be used for any gas in which a sufficient number of atoms are ionized for electricity to be easily conducted through the gas. In this use of the word, the gas inside a fluorescent light is a plasma, which makes problematic the text's statement that no solid substance can contain a plasma. The Teacher's material does in fact talk about fluorescent and neon lights as plasmas. There is no reason to bring up plasmas here.Most college thermodynamics books don't even discuss plasmas. If plasmas arise later in the context of stars or attempts to create fusion in the laboratory, then a simple statement can be made that a gas in which many atoms are ionized behaves so differently from neutral gases that they are given a special name. 160 "Strong force actually gets larger as the distance between the particles increases." While this statement is true over a very limited distance range, no mention is made of the limitation. The statement makes no contribution to the paragraph and should have been omitted. 209 and 227 Paragraph defining heat energy is less than perfect.a) gives the impression that heat and heat energy are different. b) states "Energy that is transferred from one substance to another is called heat energy." While the next sentence does go on to say that heat flows between objects that have different temperatures, students will take the first sentence as a stand-alone definition of heat energy.In fact, in the chapter review this definition is repeated with no reference to a temperature difference.This would make the energy transferred when a person compresses a spring, heat energy. 210 and 227 Absolute zero is defined as the temperature where molecules are so cold they don't move. This is incorrect. At 0 K atoms have their lowest possible kinetic energy but it is not zero. Also, the fact that this temperature is not attainable in the laboratory should be mentioned and isn't.This error is repeated in the end-of-chapter Check Your Vocabulary section. 215 Defines radiation incorrectly as infrared only. Later states that all rays from the Sun are radiant energy that would imply the Sun only emits infrared. 227 and 228 In Concept Summary and answers to Check Your Knowledge questions, that you know "energy" has been transferred if there is a change in temperature of a substance. The phrase "or a phase change has occurred" should be included. 260 and 266 In an activity on the force between charged objects, students are asked to observe the attraction between a charged balloon and an uncharged cotton ball. There is no information whatsoever in the text that would allow them to be able to correctly explain their observations. This is a major omission. Even if this were not a specific part of this activity, students would certainly notice the balloons being attracted to other objects such as their hands. Thus, providing this explanation is extremely important.Later on students are asked to bend a stream of water with a charged comb. Why do it when you aren't going to explain what causes it? 267, 270 and 283 "The difference in the charges at each end of the wire is called potential difference, or voltage." This is a nonsensical statement.Later, voltage is defined as "the amount of energy available to move charges." This is also incorrect. If one were talking about a battery, then the statement that voltage is proportional to the amount of energy available to move charges would be acceptable. However, as a general statement this is also unacceptable. Ch.11 Uses the word electricity with random meanings. In one sentence it means energy, in the next it may mean electric current, and elsewhere, electric charge. 270 Uses the term "flow rate of electric current". Electric current is itself a rate.It does not have a flow rate. 283 "Resistance is the force opposing the flow of electrons."Electrical resistance is not a force. 290 Refers to the magnetic pole located in the Arctic as the Earth's magnetic north pole when it is really the magnetic south pole. 294 Fig. 12.9 correctly shows the direction of a magnetic field produced by a current in a wire. However, the current is shown leaving the positive terminal of the battery. In the earlier chapter on electricity only electron current was discussed not conventional current, so the current was always shown leaving the negative end of the battery.We might be willing to let this go saying that not many students will notice the difference.However, in Fig. 12.10 the current is again shown leaving the positive end of the battery but this time it is actually labeled as flow of electrons! 302 The statement is made that a generator changes magnetic energy into electric energy. If this were true, the permanent magnets would continually get weaker.In the chapter review, it is correctly stated that generators turn mechanical energy into electric energy. 336 The term "heat wave" is used. Judging from terminology elsewhere in the book, they mean infrared radiation. However, heat wave is not a term used by scientists; it does not make any sense in terms of the definition they have given for heat; and the only meaning the term has for students is several days of unusually warm weather. 343 "Even though compressions are not the same as crests, they correspond to one another. Compressions and crests both indicate the amount of energy in a wave.Rarefactions and troughs also correspond to each other. They indicate the lowest energy."The three sentences are fine.The last is nonsense. 441 Statement is made that only high-frequency light, such as violet light, will supply enough energy to release electrons from metals and that red light does not have enough energy. This is clearly untrue. Green light can release electrons from sodium and even red light can release electrons from potassium. No mention is made of the fact that how much energy is needed depends on the type of metal 565 In talking about radioactive elements, the statement is made that scientists can create elements that never existed before. How would we know that they have never existed?They may well have existed, but since decayed into some other element. At best one can say that they no longer seem to exist in nature on Earth. 568 Talks about half-life, but there is no discussion of the randomness of the process. Students will be left asking, "How do the atoms know when it is their turn to decay?" 571 Have a picture showing the reaction n + U235 ® Ba141 + Kr91 +2n which doesn't balance. 573 "Great amounts of heat are given off during fusion."Again, we have an incorrect use of the word heat. Both in terms of its correct definition and the book's use of it to also mean infrared radiation. The energy produced in the fusion process is gamma radiation not infrared. 573 "Unlike fission, fusion doesn't happen spontaneously."Then how are the fusion reactions in the Sun happening? The sentence could be corrected by adding, "at temperatures usually found on Earth."