4,559 research outputs found
Using âStudent Technologyâ in introductory physics: a comparison of three tools to study falling objects
Being able to facilitate effective handsâon laboratory experiences in introductory physics courses is a challenging task, even when contemporary laboratory facilities, equipment, and new technologies for data collection and analysis are available. At institutions without adequate resources, especially those in developing countries, we have found that the problem of providing effective laboratory experiences is especially daunting for at least two reasons: 1) the lack of equipment and contemporary measuring devices; and 2) even at institutions that have some laboratory equipment, students who have access to cell phones with digital timing and video capabilities or inexpensive digital cameras are bored with trying to use âoldâfashionedâ apparatus for measurements.Fil: Saraiva Da Rocha, FĂĄbio. Universidade Federal do Pampa; BrasilFil: Fajardo, Fabio. Universidad Nacional de Colombia; ColombiaFil: GrisolĂa, Maricarmen. Universidad de Los Andes; VenezuelaFil: Benegas, Julio Ciro. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - San Luis. Instituto de MatemĂĄtica Aplicada de San Luis "Prof. Ezio Marchi". Universidad Nacional de San Luis. Facultad de Ciencias FĂsico, MatemĂĄticas y Naturales. Instituto de MatemĂĄtica Aplicada de San Luis ; ArgentinaFil: Tchitnga, Robert. University of Dschang; CamerĂșnFil: Laws, Priscilla. Dickinson College; Estados Unido
Fragilidade das terras pelo processo erosivo das chuvas em ĂĄreas antrĂłpicas e declivosas na AmazĂŽnia legal.
Spatio-temporal variability of precipitation in the venezuelan Amazon.
A regiĂŁo amazĂŽnica desempenha um papel fundamental no balanço hĂdrico global. Assim, a anĂĄlise espaço-temporal da precipitação Ă© essencial para a avaliação dos impactos das mudanças climĂĄticas. No entanto, ainda existe uma lacuna de informação na AmazĂŽnia venezuelana. O objetivo deste trabalho Ă© analisar o padrĂŁo espaço-temporal de 12 anos da precipitação na AmazĂŽnia venezuelana a partir de dados de superfĂcie da regiĂŁo. Foram utilizados dados diĂĄrios de precipitação pluvial para um perĂodo de 12 anos (2000-2011) de 10 localidades da AmazĂŽnia venezuelana. As estruturas temporais e espaciais da variabilidade interanual da precipitação foram avaliadas. Foi utilizado o teste nĂŁo paramĂ©trico de Mann-Kendall para determinar se as tendĂȘncias anuais de precipitação em cada local podem ser identificadas em uma sĂ©rie temporal que inclua um componente sazonal. Para avaliar a autocorrelação espacial da precipitação entre as observaçÔes com base no Era-Interim, o Ăndice global de Moran foi usado. Foram distinguidos dois grupos homogĂȘneos de precipitação, quando as localidades sĂŁo separadas por mĂ©dias anuais. Com relação Ă precipitação mensal por localidade, as variĂĄveis (meses) que definem os grupos homogĂȘneos de precipitação. Os resultados mostram que nesta regiĂŁo da AmazĂŽnia prevalece um padrĂŁo unimodal de precipitação nas diferentes localidades, com um longo perĂodo de chuva e um perĂodo menos chuvoso entre dezembro e março. Esse padrĂŁo Ă© explicado pela Zona de ConvergĂȘncia Intertropical que nesses meses se encontra deslocado para latitudes mais ao sul, modulando a dinĂąmica das chuvas na AmazĂŽnia. Outra das principais consequĂȘncias no padrĂŁo de precipitação na AmazĂŽnia Ă© a influĂȘncia da Zona de ConvergĂȘncia do AtlĂąntico Sul com a ocorrĂȘncia dos altos Ăndices pluviais. Ao avaliar a associação espacial local pelo ERA-Interim, observa-se que os grupos de precipitação (Ăndice de Moran) expressam a variabilidade existente em função do nĂvel de significĂąncia dos valores de seus Ăndices locais
Planck Intermediate Results II: Comparison of Sunyaev-Zeldovich measurements from Planck and from the Arcminute Microkelvin Imager for 11 galaxy clusters
A comparison is presented of Sunyaev-Zeldovich measurements for 11 galaxy
clusters as obtained by Planck and by the ground-based interferometer, the
Arcminute Microkelvin Imager. Assuming a universal spherically-symmetric
Generalised Navarro, Frenk & White (GNFW) model for the cluster gas pressure
profile, we jointly constrain the integrated Compton-Y parameter (Y_500) and
the scale radius (theta_500) of each cluster. Our resulting constraints in the
Y_500-theta_500 2D parameter space derived from the two instruments overlap
significantly for eight of the clusters, although, overall, there is a tendency
for AMI to find the Sunyaev-Zeldovich signal to be smaller in angular size and
fainter than Planck. Significant discrepancies exist for the three remaining
clusters in the sample, namely A1413, A1914, and the newly-discovered Planck
cluster PLCKESZ G139.59+24.18. The robustness of the analysis of both the
Planck and AMI data is demonstrated through the use of detailed simulations,
which also discount confusion from residual point (radio) sources and from
diffuse astrophysical foregrounds as possible explanations for the
discrepancies found. For a subset of our cluster sample, we have investigated
the dependence of our results on the assumed pressure profile by repeating the
analysis adopting the best-fitting GNFW profile shape which best matches X-ray
observations. Adopting the best-fitting profile shape from the X-ray data does
not, in general, resolve the discrepancies found in this subset of five
clusters. Though based on a small sample, our results suggest that the adopted
GNFW model may not be sufficiently flexible to describe clusters universally.Comment: update to metadata author list onl
Risk factors for hepatitis B virus infection in Rio de Janeiro, Brazil
BACKGROUND: Despite international efforts to prevent hepatitis B virus (HBV) infection through global vaccination programs, new cases are still being reported throughout the world. METHODS: To supply data that might assist in improving preventive measures and national surveillance for HBV infection, a cross-sectional study was conducted among individuals referred to the Brazilian National Reference Center for Viral Hepatitis (Rio de Janeiro) during a two-year period. Reported risk factors among infected subjects ("HBV-positive") were compared to those of subjects never exposed ("HBV-negative") to HBV. Two subgroups were further identified within the HBV-positive group, "acute" infection and "non-acute" infection. RESULTS: A total of 1,539 subjects were tested for HBV, of which 616 were HBV-positive (79 acute infection and 537 non-acute infection). HBV-positive subjects were more likely to be of male gender (63% versus 47%); and to report multiple sexual partners (12% versus 6%) and illicit drug use (IDU and/or intranasal cocaine use) (6% versus 3%). Among the HBV-positive subgroups, age differed significantly, with 48% being under 30 years of age in subjects acutely infected compared to 17% in those with non-acute infection. CONCLUSIONS: The association of multiple sexual partners with past HBV infection and the age distribution of currently infected subjects suggest that sexual transmission played a major role in the transmission of HBV in this study population. Thus, vaccination during adolescence should be considered
Planck Intermediate Results. IV. The XMM-Newton validation programme for new Planck galaxy clusters
We present the final results from the XMM-Newton validation follow-up of new
Planck galaxy cluster candidates. We observed 15 new candidates, detected with
signal-to-noise ratios between 4.0 and 6.1 in the 15.5-month nominal Planck
survey. The candidates were selected using ancillary data flags derived from
the ROSAT All Sky Survey (RASS) and Digitized Sky Survey all-sky maps, with the
aim of pushing into the low SZ flux, high-z regime and testing RASS flags as
indicators of candidate reliability. 14 new clusters were detected by XMM,
including 2 double systems. Redshifts lie in the range 0.2 to 0.9, with 6
clusters at z>0.5. Estimated M500 range from 2.5 10^14 to 8 10^14 Msun. We
discuss our results in the context of the full XMM validation programme, in
which 51 new clusters have been detected. This includes 4 double and 2 triple
systems, some of which are chance projections on the sky of clusters at
different z. We find that association with a RASS-BSC source is a robust
indicator of the reliability of a candidate, whereas association with a FSC
source does not guarantee that the SZ candidate is a bona fide cluster.
Nevertheless, most Planck clusters appear in RASS maps, with a significance
greater than 2 sigma being a good indication that the candidate is a real
cluster. The full sample gives a Planck sensitivity threshold of Y500 ~ 4 10^-4
arcmin^2, with indication for Malmquist bias in the YX-Y500 relation below this
level. The corresponding mass threshold depends on z. Systems with M500 > 5
10^14 Msun at z > 0.5 are easily detectable with Planck. The newly-detected
clusters follow the YX-Y500 relation derived from X-ray selected samples.
Compared to X-ray selected clusters, the new SZ clusters have a lower X-ray
luminosity on average for their mass. There is no indication of departure from
standard self-similar evolution in the X-ray versus SZ scaling properties.
(abridged)Comment: accepted by A&
Planck intermediate results. III. The relation between galaxy cluster mass and Sunyaev-Zeldovich signal
We examine the relation between the galaxy cluster mass M and
Sunyaev-Zeldovich (SZ) effect signal D_A^2 Y for a sample of 19 objects for
which weak lensing (WL) mass measurements obtained from Subaru Telescope data
are available in the literature. Hydrostatic X-ray masses are derived from
XMM-Newton archive data and the SZ effect signal is measured from Planck
all-sky survey data. We find an M_WL-D_A^2 Y relation that is consistent in
slope and normalisation with previous determinations using weak lensing masses;
however, there is a normalisation offset with respect to previous measures
based on hydrostatic X-ray mass-proxy relations. We verify that our SZ effect
measurements are in excellent agreement with previous determinations from
Planck data. For the present sample, the hydrostatic X-ray masses at R_500 are
on average ~ 20 per cent larger than the corresponding weak lensing masses, at
odds with expectations. We show that the mass discrepancy is driven by a
difference in mass concentration as measured by the two methods, and, for the
present sample, the mass discrepancy and difference in mass concentration is
especially large for disturbed systems. The mass discrepancy is also linked to
the offset in centres used by the X-ray and weak lensing analyses, which again
is most important in disturbed systems. We outline several approaches that are
needed to help achieve convergence in cluster mass measurement with X-ray and
weak lensing observations.Comment: 19 pages, 9 figures, matches accepted versio
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