189 research outputs found

### Some Remarks about Variable Mass Systems

We comment about the general argument given to obtain the rocket equation as
it is exposed in standard textbooks. In our opinion, it can induce students to
a wrong answer when solving variable mass problems.Comment: 2 page

### Locating the source of projectile fluid droplets

The ill-posed projectile problem of finding the source height from spattered
droplets of viscous fluid is a longstanding obstacle to accident reconstruction
and crime scene analysis. It is widely known how to infer the impact angle of
droplets on a surface from the elongation of their impact profiles. However,
the lack of velocity information makes finding the height of the origin from
the impact position and angle of individual drops not possible. From aggregate
statistics of the spatter and basic equations of projectile motion, we
introduce a reciprocal correlation plot that is effective when the polar launch
angle is concentrated in a narrow range. The vertical coordinate depends on the
orientation of the spattered surface, and equals the tangent of the impact
angle for a level surface. When the horizontal plot coordinate is twice the
reciprocal of the impact distance, we can infer the source height as the slope
of the data points in the reciprocal correlation plot. If the distribution of
launch angles is not narrow, failure of the method is evident in the lack of
linear correlation. We perform a number of experimental trials, as well as
numerical calculations and show that the height estimate is insensitive to
aerodynamic drag. Besides its possible relevance for crime investigation,
reciprocal-plot analysis of spatter may find application to volcanism and other
topics and is most immediately applicable for undergraduate science and
engineering students in the context of crime-scene analysis.Comment: To appear in the American Journal of Physics (ms 23338). Improved
readability and organization in this versio

### Working with simple machines

A set of examples is provided that illustrate the use of work as applied to
simple machines. The ramp, pulley, lever and hydraulic press are common
experiences in the life of a student and their theoretical analysis therefore
makes the abstract concept of work more real. The mechanical advantage of each
of these systems is also discussed so that students can evaluate their
usefulness as machines.Comment: 9 pages, 4 figure

### Dilatonic interpolation between Reissner-Nordstrom and Bertotti-Robinson spacetimes with physical consequences

We give a general class of static, spherically symmetric, non-asymptotically
flat and asymptotically non-(anti) de Sitter black hole solutions in
Einstein-Maxwell-Dilaton (EMD) theory of gravity in 4-dimensions. In this
general study we couple a magnetic Maxwell field with a general dilaton
potential, while double Liouville-type potentials are coupled with the gravity.
We show that the dilatonic parameters play the key role in switching between
the Bertotti-Robinson and Reissner-Nordstr\"om spacetimes. We study the
stability of such black holes under a linear radial perturbation, and in this
sense we find exceptional cases that the EMD black holes are unstable. In
continuation we give a detailed study of the spin-weighted harmonics in
dilatonic Hawking radiation spectrum and compare our results with the
previously known ones. Finally, we investigate the status of resulting naked
singularities of our general solution when probed with quantum test particles.Comment: 27 pages, 4 figures, to appear in CQG

### A Research-Based Curriculum for Teaching the Photoelectric Effect

Physics faculty consider the photoelectric effect important, but many
erroneously believe it is easy for students to understand. We have developed
curriculum on this topic including an interactive computer simulation,
interactive lectures with peer instruction, and conceptual and mathematical
homework problems. Our curriculum addresses established student difficulties
and is designed to achieve two learning goals, for students to be able to (1)
correctly predict the results of photoelectric effect experiments, and (2)
describe how these results lead to the photon model of light. We designed two
exam questions to test these learning goals. Our instruction leads to better
student mastery of the first goal than either traditional instruction or
previous reformed instruction, with approximately 85% of students correctly
predicting the results of changes to the experimental conditions. On the
question designed to test the second goal, most students are able to correctly
state both the observations made in the photoelectric effect experiment and the
inferences that can be made from these observations, but are less successful in
drawing a clear logical connection between the observations and inferences.
This is likely a symptom of a more general lack of the reasoning skills to
logically draw inferences from observations.Comment: submitted to American Journal of Physic

### Images in Christmas Balls

We describe light-reflection properties of spherically curved mirrors, like
balls in the Christmas tree. In particular, we study the position of the image
which is formed somewhere beyond the surface of a spherical mirror, when an eye
observes the image of a pointlike light source. The considered problem,
originally posed by Abu Ali Hasan Ibn al-Haitham -- alias Alhazen -- more than
a millennium ago, turned out to have the now well known analytic solution of a
biquadratic equation, being still of great relevance, e.g. for the
aberration-free construction of telescopes. We do not attempt to perform an
exhaustive survey of the rich historical and engineering literature on the
subject, but develop a simple pedagogical approach to the issue, which we
believe to be of continuing interest in view of its maltreating in many
high-school textbooks.Comment: 13 pages, 7 figures plain LaTeX; Also see
http://cft.fis.uc.pt/eef/mirrors.htm, revised version has simplified
formulas, more transparent for a wider audience, one reference adde

### Transversality of Electromagnetic Waves in the Calculus-Based Introductory Physics Course

Introductory calculus-based physics textbooks state that electromagnetic
waves are transverse and list many of their properties, but most such textbooks
do not bring forth arguments why this is so. Both physical and theoretical
arguments are at a level appropriate for students of courses based on such
books, and could be readily used by instructors of such courses. Here, we
discuss two physical arguments (based on polarization experiments and on lack
of monopole electromagnetic radiation), and the full argument for the
transversality of (plane) electromagnetic waves based on the integral Maxwell
equations. We also show, at a level appropriate for the introductory course,
why the electric and magnetic fields in a wave are in phase and the relation of
their magnitudes.Comment: 10 pages, 6 figure

### Bridging Physics and Biology Teaching through Modeling

As the frontiers of biology become increasingly interdisciplinary, the
physics education community has engaged in ongoing efforts to make physics
classes more relevant to life sciences majors. These efforts are complicated by
the many apparent differences between these fields, including the types of
systems that each studies, the behavior of those systems, the kinds of
measurements that each makes, and the role of mathematics in each field.
Nonetheless, physics and biology are both sciences that rely on observations
and measurements to construct models of the natural world. In the present
theoretical article, we propose that efforts to bridge the teaching of these
two disciplines must emphasize shared scientific practices, particularly
scientific modeling. We define modeling using language common to both
disciplines and highlight how an understanding of the modeling process can help
reconcile apparent differences between the teaching of physics and biology. We
elaborate how models can be used for explanatory, predictive, and functional
purposes and present common models from each discipline demonstrating key
modeling principles. By framing interdisciplinary teaching in the context of
modeling, we aim to bridge physics and biology teaching and to equip students
with modeling competencies applicable across any scientific discipline.Comment: 10 pages, 2 figures, 3 table

### A Deeper Look at Student Learning of Quantum Mechanics: the Case of Tunneling

We report on a large-scale study of student learning of quantum tunneling in
4 traditional and 4 transformed modern physics courses. In the transformed
courses, which were designed to address student difficulties found in previous
research, students still struggle with many of the same issues found in other
courses. However, the reasons for these difficulties are more subtle, and many
new issues are brought to the surface. By explicitly addressing how to build
models of wave functions and energy and how to relate these models to real
physical systems, we have opened up a floodgate of deep and difficult questions
as students struggle to make sense of these models. We conclude that the
difficulties found in previous research are the tip of the iceberg, and the
real issue at the heart of student difficulties in learning quantum tunneling
is the struggle to build the complex models that are implicit in experts'
understanding but often not explicitly addressed in instruction.Comment: v2, v3 updated with more detailed analysis of data and discussion;
submitted to Phys. Rev. ST: PE

### Conservation Laws and Energy Transformations in a Class of Common Physics Problems

We analyze a category of problems that is of interest in many physical situations, including those encountered in introductory physics classes: systems with two well-delineated parts that exchange energy, eventually reaching a shared equilibrium with a loss of mechanical or electrical energy. Such systems can be constrained by a constant of the system (e.g., mass, charge, momentum, or angular momentum) that uniquely determines the mechanical or electrical energy of the equilibrium state, regardless of the dissipation mechanism. A representative example would be a perfectly inelastic collision between two objects in one dimension, for which momentum conservation requires that some of the initial kinetic energy is dissipated by conversion to thermal or other forms as the two objects reach a common final velocity. We discuss how this feature manifests in a suite of four well-known and disparate problems that all share a common mathematical formalism. These examples, in which the energy dissipated during the process can be difficult to solve directly from dissipation rates, can be approached by students in a first-year physics class by considering conservation laws and can therefore be useful for teaching about energy transformations and conserved quantities. We then illustrate how to extend this method by applying it to a final example

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