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

Adopting locally appropriate WASH solutions: a case study of rock catchment systems in South Sudan

Sustainability of water infrastructures is a well-known challenge especially in post-conflict countries, where communities have been used to quick and emergency-focused aid. This article presents a case study on how sustainability can be positively influenced by opting for locally-appropriate technologies and by involving communities in its selection. Considering the abundant rainfall and the presence of rock outcrops in some parts of South Sudan, rock catchment systems are locally-appropriate solutions and good alternatives to the more common hand-pump boreholes. In Eastern Equatoria State, Caritas Switzerland successfully constructed eight rock catchment systems. The potential of using runoff water from rock outcrops to ease communities’ water situation and the cost-effectiveness and appropriateness of this technology has generated a lot of interests among communities, local government and other stakeholders, leading surrending communities with similar geological conditions to request for a similar system

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

Genes involved in mushroom body development and behavior in Drosophila

Mushroom bodies (MBs) are the site of multi modal sensory integration critical for associative conditioning in Drosophila. They have been central to research on the structure function relationship in the brain for over one hundred years due to their unique shape and readily accessible physiology. This dissertation incorporates three different approaches to further elucidate the genetic and molecular nature of this structure function relationship. First, the suite of genetic and molecular tools available in Drosophila melanogaster, facilitated the molecular mapping of a 25-year old MB structural mutant called mushroom body miniature B (mbmB) to the gene Pendulin [Pen, also known as importin-!2 (imp-!2)]. Anatomical rescue, protein expression in the brain and functional domain analysis in mbmB mutants have shown that Imp-!2 is necessary for MB development, which likely gives rise to its learning, long term memory and amnesia resistant memory defects. Imp-!2 is a central component of nuclear cytoplasmic trafficking, mitotic spindle orientation, and injury response in the nervous system. The work described in this dissertation provides the first evidence that Imp-!2 also has a critical role in MB development and associative conditioning. Second, MB specific Gal4 lines were used to identify novel genes associated with MB development through the identification of their flanking sequence. Ten Gal4 inserts were localized to introns, exons, and some intragenic regions of eight genes, likely to have interesting and testable roles in MB development and/or function. These candidate genes include: FTZ-F1, Odorant receptor 42a, no extended memory, TAK1-associated binding protein, frizzled, Ecdysoneinduced protein 75B, Casein Kinase 1# and eyeless. Overall, the inserts themselves had minimal effects on MB development, likely due to their positions in non-coding regions. Protein levels in three homozygous MB Gal4 inserts, all found upstream of the frizzled gene, appeared reduced, indicating that these inserts can in fact disrupt protein levels independent of any effects they may or may not have on MB gross morphology. New evidence that genetic background influences MB anatomy is also provided through the analysis of two Gal4 lines in different genetic backgrounds. This work brings to light novel signaling pathways, likely associated with MB anatomy and development, that upon further investigation will aid in our understanding of the molecular nature of how the MBs form. Finally, the influence of MBs on walking was investigated using mutant alleles of several genes with severe MB disruptions and a chemical method for MB ablation. Over the course of fifteen minutes (the initial stages of walking), flies with disrupted MBs showed a decrease in the frequency of walking indicating a role for MBs in the up-regulation of motor coordination during its initial stages. Slight differences in orientation to landmark and velocity were also observed, but attributed to pleiotropy rather than the MB disruptions. These findings were in contrast to conclusions made in previous work demonstrating MB’s involvement in the termination of walking bouts over longer time courses (i.e. MBs down-regulate locomotion). Both sets of data taken together implicate MBs in regulation of motor behaviors in a time dependent fashion, up regulating activity during the initial stages of walking, but suppressing activity thereafter. Therefore, MBs deliver appropriate contextual information to motor output centers in the brain by modifying the quantity of walking (activity) rather than the quality (velocity and orientation). (Refer to PDF file for exact formulas.

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

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

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

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

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