Scattering of a Long Cosmic String by a Rotating Black Hole

The scattering of a straight, infinitely long string by a rotating black hole is considered. We assume that a string is moving with velocity v and that initially the string is parallel to the axis of rotation of the black hole. We demonstrate that as a result of scattering, the string is displaced in the direction perpendicular to the velocity by an amount kappa(v,b), where b is the impact parameter. The late-time solution is represented by a kink and anti-kink, propagating in opposite directions at the speed of light, and leaving behind them the string in a new ``phase''. We present the results of the numerical study of the string scattering and their comparison with the weak-field approximation, valid where the impact parameter is large, b/M >> 1, and also with the scattering by a non-rotating black hole which was studied in earlier works.Comment: 27 pages, 14 figures, to be published in Classical and Quantum Gravit

Rethinking the discovery function of proof within the context of proofs and refutations

Proof and proving are important components of school mathematics and have multiple functions in mathematical practice. Among these functions of proof, this paper focuses on the discovery function that refers to invention of a new statement or conjecture by reflecting on or utilizing a constructed proof. Based on two cases in which eighth and ninth graders engaged in proofs and refutations, we demonstrate that facing a counterexample of a primitive statement can become a starting point of students’ activity for discovery, and that a proof of the primitive statement can function as a useful tool for inventing a new conjecture that holds for the counterexample. An implication for developing tasks by which students can experience this discovery function is mentioned.ArticleInternational Journal of Mathematical Education in Science and Technology. 45(7):1053-1067 (2014)journal articl

Sources of job stress, work engagement and career orientations of employees in a South African fi nancial institution

The objective of this study was to examine the relationship between employees’ sources of job stress, work engagement and career orientations and how they diff er with respect to these variables in terms of socio-demographic contextual factors such as gender, race, employment status and age. A sample of 90 employees participated in this study, and a Sources of Job Stress Scale, the Utrecht Work Engagement Scale and the Career Orientations Inventory were administered. The results indicated a signifi cant relationship between the participants’ sources of job stress, levels of work engagement and career orientations. Signifi cant diff erences regarding these variables were also detected between males and females, blacks and whites, temporary and permanently employed participants and the various age groups. The fi ndings add new knowledge that may inform organisational wellness and career development practices

A near-infrared study of the star forming region RCW 34

We report the results of a near-infrared imaging study of a $7.8 \times 7.8$ arcmin$^2$ region centered on the 6.7 GHz methanol maser associated with the RCW 34 star forming region using the 1.4m IRSF telescope at Sutherland. A total of 1283 objects were detected simultaneously in J, H, and K for an exposure time of 10800 seconds. The J-H, H-K two-colour diagram revealed a strong concentration of more than 700 objects with colours similar to what is expected of reddened classical T Tauri stars. The distribution of the objects on the K {\it vs} J-K colour-magnitude diagram is also suggestive that a significant fraction of the 1283 objects is lower mass pre-main sequence stars. We also present the luminosity function for the subset of about 700 pre-main sequence stars and show that it suggests ongoing star formation activity for about $10^7$ years. An examination of the spatial distribution of the pre-main sequence stars shows that the fainter (older) part of the population is more dispersed over the observed region and the brighter (younger) subset is more concentrated around the position of the O8.5V star. This suggests that the physical effects of the O8.5V star and the two early B-type stars on the remainder of the cloud out of which they formed, could have played a role in the onset of the more recent episode of star formation in RCW 34.Comment: Accepted for publication in The Astronomical Journa

The effect of nitrite and nitrate concentrations on low F/M filament bulking in nitrogen removal activated sludge systems

Includes bibliography.Filamentous bulking and its associated poor sludge settleability is a considerable problem in South African activated sludge plants, as indicated by the two surveys undertaken on these plants in 1985 and 1988 (Blackbeard et al., 1986, 1988). Amelioration of this problem would enable a greater daily flow and load of wastewater to be treated by these plants. From the surveys it is apparent that bulking in long sludge age activated sludge plants is mainly due to the proliferation of the group of so-called low F/M filaments (as classified by Jenkins et al., 1984). In the course of a 4-year research program investigating specific bulking control methods, Gabb et al. (1989) concluded that the selector effect, which was proposed as a method for controlling low F/M filament bulking, was ineffective. This research also indicated that in fully aerobic systems low F/M bulking was ameliorated, but in intermittently aerated anoxic-aerobic systems low F/M filament bulking was promoted. The research presented in this thesis forms part of a follow-up bulking research program into low F/M bulking which led to the formulation of a bulking hypothesis by Casey et al. (1992b) and focuses on the effect of different anoxic-aerobic conditions and their effect on the concentration of nitrate and nitrite entering the aerobic zone/reactor in single intermittently aerated and multi-reactor nitrification-denitrification systems

Stroke in the young

CITATION: De Villiers, R. V. P., February, E. & De Villiers, M. L. 2003. Stroke in the young. South African Medical Journal, 93(11):836-837.The original publication is available at http://www.samj.org.za[No abstract available]Publisher’s versio

Students Opinions on Autopsy and Death

Background: Autopsies are commonly seen on television, but are less common in real life. Worldwide, the autopsy rate has declined drastically over the past half century, from approximately 50% to only five to 22% in the 1990s. These percentages are inflated by the number of forensic autopsies, which suggests that hospital autopsy rates for other purposes are very low. Some students graduate without witnessing a formal autopsy, and some students and medical practitioners consider the autopsy to be of little value. The purpose of this study was to assess the thoughts and feelings of students towards autopsies at the end of their medical training. Method: A survey of final-year medical students’ thoughts and feelings on the autopsy, death and grief was performed by means of a self-administered questionnaire. Informed consent was obtained and the study was approved by the Ethics Committee. Results: Of the 164 respondents, 64 were female and 100 male. Their ages varied from 22 to 40, with a mean of 27,7 years. Only 11% had discussed their wishes with regard to an autopsy on themselves with their family, while 33% had discussed funeral arrangements. Most students thought that aspects of the autopsy should be changed, e.g. the deceased should remain anonymous, the number of students observing an autopsy should be reduced, the atmosphere should be more respectful and the organs should be handled with more care. During training, students have insufficient opportunities to discuss their thoughts and feelings about the autopsy, and issues of grief and death are insufficiently dealt with. Conclusions: Students’ emotional reactions are an important, but unfortunately neglected, aspect of medical training. Student teaching needs to deal effectively with expected reactions through the understanding and management of these emotional responses towards cadaver dissection and other medical procedures. At both undergraduate and postgraduate levels, medical education must emphasise the importance of the autopsy. This means that academic institutions need to perform a sufficient number of autopsies for students to have the opportunity to observe and participate in them.SA Fam Pract 2005;47(1): 47-5