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Subtleties with Young's double-slit experiment: Investigation of spatial coherence and fringe visibility
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| Pengarang | : | - |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 86 (No. 9) |
| Halaman | : | 683-689 |
| Abstrak | : | We discuss Young's double-slit experiment using a partially coherent light source consisting of a helium-neon laser incident on a rotating piece of white paper. Such an experiment is appropriate for undergraduate students as an independent project or as part of an advanced lab course. As is well known, the resulting interference pattern is observed to disappear and return, depending on the angular size of the source. Interestingly, while the standard theoretical prediction for the light intensity agrees quite well with experimental data when the fringe visibility is high, the prediction is noticeably off when the visibility is low. A first-principles calculation of the light intensity is performed and shown to agree extremely well with the experimental results for all visibilities. |
Lagrangian vs Hamiltonian: The best approach to relativistic orbits
| Pengarang | : | - |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 86 (No. 9) |
| Halaman | : | 678-682 |
| Abstrak | : | In introductory general relativity courses, free particle trajectories, such as astronomical orbits, are generally developed via a Lagrangian and variational calculus, so that physical examples can precede the mathematics of tensor calculus. The use of a Hamiltonian is viewed as more advanced and typically comes later if at all. We suggest here that this might not be the optimal order in a first course in general relativity, especially if orbits are to be solved with numerical methods. We discuss some of the issues that arise in both the Lagrangian and Hamiltonian approaches. |
New strategy for the Puluj-type experiment
| Pengarang | : | - |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 86 (No. 9) |
| Halaman | : | 670-677 |
| Abstrak | : | The frictional mechanical work is calculated as a function of the rotational direction of the calorimeter in the Puluj-type experimental setup (a calorimeter with water inside). Based on this calculus, a new experimental strategy is proposed for demonstrating the equivalence of heat and mechanical work, which reduces the number of physical quantities that need to be measured during the experiment. |
Which part of a chain breaks?
| Pengarang | : | - |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 86 (No. 9) |
| Halaman | : | 663-669 |
| Abstrak | : | This work investigates the dynamics of a one-dimensional homogeneous harmonic chain on a horizontal table. One end is anchored to a wall; the other (free) end is pulled by an external force. A Green's function is derived to calculate the response to a generic pulling force. As an example, I assume that the magnitude of the pulling force increases with time at a uniform rate β. If the number of beads and springs used to model the chain is large, the extension of each spring takes a simple closed form, which is a piecewise-linear function of time. Under an additional assumption that a spring breaks when its extension exceeds a certain threshold, results show that for large β the spring breaks near the pulling end, whereas the breaking point can be located close to the wall by choosing small β. More precisely, the breaking point moves back and forth along the chain as β decreases, which has been called “anomalous” breaking in the context of the pull-or-jerk experiment. Although the experiment has been explained in terms of inertia, its meaning can be fully captured by discussing the competition between intrinsic and extrinsic time scales of forced oscillation. |
Teaching of the harmonic oscillator damped by a constant force: The use of analogy and experiments
| Pengarang | : | - |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 86 (No. 9) |
| Halaman | : | 657-662 |
| Abstrak | : | A description of oscillations damped by a constant force, despite the importance of the phenomenon, is missing from secondary (i.e., high-school) and university physics textbooks. In this paper, a simple explanation of this type of energy loss is given, using the analogy of a vertical spring-mass system. The predicted result, a linear time reduction in the amplitude of oscillation, can be confirmed in the classroom with the demonstration outlined here. In addition, the possibility of extending the experiment to a laboratory exercise at the university level is described. During the exercise, students distinguish different energy loss mechanisms in harmonic oscillations and measure the values of physical quantities associated with energy loss using a simple analytical procedure. |
Resource Letter GECC-2: The Greenhouse Effect and Climate Change: The Intensified Greenhouse Effect
| Pengarang | : | - |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 86 (No. 9) |
| Halaman | : | 645-656 |
| Abstrak | : | Human activities over the past 200 years have increased Earth's greenhouse effect by about 1% relative to the radiative fluxes that drive the climate system. This Resource Letter introduces the physics of this intensified greenhouse effect and of the processes that govern resultant change in climate and provides resources for further study. Context for this examination is provided in a companion Resource Letter (GECC-1) that examined the radiative fluxes that comprise Earth's climate system and the present-day greenhouse effect. The increase in global temperature and other changes in climate resulting from the intensified greenhouse effect are of great societal concern. Developing prognostic capability to determine these responses to the small perturbations that constitute the intensified greenhouse effect to an accuracy that would be useful to inform policymaking is the major challenge facing climate scientists today. |
Dangerous limits
| Pengarang | : | - |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 87 (No. 10) |
| Halaman | : | 775–777 |
| Abstrak | : | In my last column, I mentioned that, at a high Reynolds number, drag is proportional to speed squared and that this statement can be justified by a dangerous combination of two back-of-the-envelope methods.1 The argument's first ingredient, safe on its own, is dimensional analysis. The conclusion from dimensional analysis is that F, the magnitude of the drag force on an object moving through a fluid, is related to the object's speed v, to its characteristic size L, to its cross-sectional area Acs, to the fluid's density ρ, and to the fluid's kinematic viscosity ν by F12ρv2Acs=f(vLν), (1) where f is a dimensionless function. The left side is itself a first dimensionless group: the drag coefficient cd. On the right side, the argument of f is a second,... |
Stepping in or Stepping On: Mentor Teachers' Preferences for Mentoring Inside and Outside of Interactive Teaching
| Pengarang | : | Gotwalt, Elizabeth Schiavone |
| Nama Majalah/Jurnal | : | Journal of Teacher Education |
| Volume / Edisi | : | 74 (No. 3) |
| Halaman | : | 274-287 |
| Abstrak | : | Much research describes how mentor and novice teachers interact when pupils are not present (e.g., co-planning, debriefing). However, little research investigates how mentor/novice pairs interact when K-12 students are present. This gap in the literature is significant because research suggests that without intentional mediation, novices often fail to see much of what happens in classrooms. This article investigates and conceptualizes what 27 mentors think about mentoring practices that occur outside versus inside of instructional time. An analysis of mentor interviews found that mentors prefer to interact with novices outside of instructional time because they fear undercutting novices' authority and autonomy when interacting with them during instruction. However, findings also suggest that hybrid practices (those that straddle the outside/inside boundary) are valued by mentors although they are rarely enacted. This finding has implications for programs interested in supporting mentors to diversify the practices they employ to support novice teachers. |
The transit of Mercury: November 11, 2019
| Pengarang | : | - |
| Nama Majalah/Jurnal | : | American Journal of Physics |
| Volume / Edisi | : | 87 (No. 10) |
| Halaman | : | 773-774 |
| Abstrak | : | On November 11 between 12:35 and 18:04 Universal Time, the tiny black silhouette of Mercury will slowly pass the Sun's disc. Such a passage is called a transit of Mercury,1 and we are organizing a project to observe, photograph, and measure the transit.2 Our project will bring people together from all over the world so that they can compare their own observations with those of astronomers at distant places and determine the distance between the Sun and Earth. Additional participants in our project, especially from the American continent, are most welcome. Since Mercury's orbit around the Sun is inclined with respect to Earth's orbit, Mercury will usually pass above or below the Sun (relative to an observer on Earth) and be invisible during its conjunction with the Sun. Transits of Mercury, therefore, are quite rare (12–13 per century). The 2019 transit will be the last chance until... |
Teachers' Conceptions of Fairness in Classroom Assessment: An Empirical Study
| Pengarang | : | Firoozi, Tahereh,Zandi, Hamed,Rasegh, Abdollah,Rasooli, Amirhossein |
| Nama Majalah/Jurnal | : | Journal of Teacher Education |
| Volume / Edisi | : | 74 (No. 3) |
| Halaman | : | 260-273 |
| Abstrak | : | With heightened equity pursuits in 21st century schools and the key role of assessment in teachers' concerns with educational equity, scholars have recently attempted to empirically investigate teachers' conceptions of fairness in classroom assessment. This study contributes to this growing literature and draws on interview data from 27 experienced high school teachers to further appreciate the factors that propel teachers' fairness conceptions. The results indicate that the teachers' conceptions of fairness in classroom assessment were influenced by three themes: (a) individual mechanisms, (b) social mechanisms, and (c) dialectical relationships between individual and social mechanisms. These themes underscored how teachers' individual philosophies and experiences interacted with their encounters with social conditions of society, schools, and classrooms to influence their conceptions and articulated practices of fairness in classroom assessments. The results contribute to provoke conversations around assessment fairness education during pre- and in-service programs. |