4,407 research outputs found
Mineralogia de argilas em cambissolos do Sudoeste da Amazônia brasileira.
bitstream/item/40418/1/Boletim-Pesquisa-34-CPATU.pd
Covariant Uniform Acceleration
We show that standard Relativistic Dynamics Equation F=dp/d\tau is only
partially covariant. To achieve full Lorentz covariance, we replace the
four-force F by a rank 2 antisymmetric tensor acting on the four-velocity. By
taking this tensor to be constant, we obtain a covariant definition of
uniformly accelerated motion. We compute explicit solutions for uniformly
accelerated motion which are divided into four types: null, linear, rotational,
and general. For null acceleration, the worldline is cubic in the time. Linear
acceleration covariantly extends 1D hyperbolic motion, while rotational
acceleration covariantly extends pure rotational motion.
We use Generalized Fermi-Walker transport to construct a uniformly
accelerated family of inertial frames which are instantaneously comoving to a
uniformly accelerated observer. We explain the connection between our approach
and that of Mashhoon. We show that our solutions of uniformly accelerated
motion have constant acceleration in the comoving frame. Assuming the Weak
Hypothesis of Locality, we obtain local spacetime transformations from a
uniformly accelerated frame K' to an inertial frame K. The spacetime
transformations between two uniformly accelerated frames with the same
acceleration are Lorentz. We compute the metric at an arbitrary point of a
uniformly accelerated frame.
We obtain velocity and acceleration transformations from a uniformly
accelerated system K' to an inertial frame K. We derive the general formula for
the time dilation between accelerated clocks. We obtain a formula for the
angular velocity of a uniformly accelerated object. Every rest point of K' is
uniformly accelerated, and its acceleration is a function of the observer's
acceleration and its position. We obtain an interpretation of the
Lorentz-Abraham-Dirac equation as an acceleration transformation from K' to K.Comment: 36 page
Saturation properties and incompressibility of nuclear matter: A consistent determination from nuclear masses
Starting with a two-body effective nucleon-nucleon interaction, it is shown
that the infinite nuclear matter model of atomic nuclei is more appropriate
than the conventional Bethe-Weizsacker like mass formulae to extract saturation
properties of nuclear matter from nuclear masses. In particular, the saturation
density thus obtained agrees with that of electron scattering data and the
Hartree-Fock calculations. For the first time using nuclear mass formula, the
radius constant =1.138 fm and binding energy per nucleon = -16.11
MeV, corresponding to the infinite nuclear matter, are consistently obtained
from the same source. An important offshoot of this study is the determination
of nuclear matter incompressibility to be 288 28 MeV using
the same source of nuclear masses as input.Comment: 14 latex pages, five figures available on request ( to appear in Phy.
Rev. C
Comment on "Giant absorption cross section of ultracold neutrons in Gadolinium"
Rauch et al (PRL 83, 4955, 1999) have compared their measurements of the Gd
cross section for Ultra-cold neutrons with an exptrapolation of the cross
section for thermal neutrons and interpreted the discrepancy in terms of
coherence properties of the neutron. We show the extrapolation used is based on
a misunderstanding and that coherence properties play no role in absorption.Comment: 2 pages, 1 postscript figure, comment on Rauch et al, PRL 83,4955
(1999
Sulfation degree of glycosaminoglycans triggers distinct cytoskeleton organisation in mesenchymal stem cells
Glycosaminoglycans (GAGs) comprise the closest cellular environment: they are building
elements of the ECM and can be also found on cells surface. Their biological activity depends
on several parameters among which the negative charge is of prime importance[1]. This
charge is generally associated with the presence of sulfate groups (-OSO3H). Sulfation is
a dynamic modification: it can occur at various positions within the glycan and different
sulfation patterns have been identified for the same organs and cells during their
development. However, the mechanisms of coding and transferring information by
these functionalities are not yet complete understood, mainly because of (i)the complex
physiological microenvironment in which GAGs interactions occur and (ii)the inability to access
homogeneous GAGs[2].
In this work, we propose model surfaces bearing GAGs with different sulfation degree as
platform to investigate the pathways by which mesenchymal stem cells (MSCs) sense and
respond to this peculiar functionality: the -OSO3H. We have selected two natural GAGs for this
study: hyaluronic acid (HA) because it is the only non-sulfated glycan and heparin (HEP) as it is
the GAG with the highest degree of sulfation. To obtain a larger range of sulfation degrees, we
have also prepared a synthetic analogue of HA with a sulfation degree of 1.4 (sHA). All these
GAGs were covalently bonded to aminothiols deposited on gold surfaces. MSCs, both from
bone marrow and adipose tissue, adhered well to all surfaces. Formation of focal adhesions
was observed after only 1h of culture for bone marrow derived MSCs regardless the used
substrate. The presence of –OSO3H groups induced different morphology and cytoskeleton
organisation: formation of longer filopodia and well pronounced actin fibers were visible for
the MSCs from both sources. Moreover, cells were more spread after 24h in contact with –
OSO3H containing surfaces. Cells behaved similarly on both sulfated surfaces (sHA and HEP)
and differences in cell morphology were less obvious: higher sulfation degree induced less
lamellipodia formation while filopodia number and length increased.
In summary, the present study provides evidence that sulfation degree of GAGs triggers
distinct cytoskeleton organisation in mesenchymal stem cells that may be related with the
differentiation of those cells. However, further studies at the molecular level about the exact
mechanism of these processes need to be carried out
Three-loop matching coefficients for hot QCD: Reduction and gauge independence
We perform an integral reduction for the 3-loop effective gauge coupling and
screening mass of QCD at high temperatures, defined as matching coefficients
appearing in the dimensionally reduced effective field theory (EQCD).
Expressing both parameters in terms of a set master (sum-) integrals, we show
explicit gauge parameter independence. The lack of suitable methods for solving
the comparatively large number of master integrals forbids the complete
evaluation at the moment. Taking one generic class of masters as an example, we
highlight the calculational techniques involved. The full result would allow to
improve on one of the classic probes for the convergence of the weak-coupling
expansion at high temperatures, namely the comparison of full and effective
theory determinations of the spatial string tension. Furthermore, the full
result would also allow to determine one new contribution of order O(g**7) to
the pressure of hot QCD.Comment: 19 pages, 2 figures. v2: new Section 6 discussing applications, to
match journal versio
Statistics of Magnification Perturbations by Substructure in the Cold Dark Matter Cosmological Model
We study the statistical properties of magnification perturbations by
substructures in strong lensed systems using linear perturbation theory and an
analytical substructure model including tidal truncation and a continuous
substructure mass spectrum. We demonstrate that magnification perturbations are
dominated by perturbers found within a tidal radius of an image, and that
sizable magnification perturbations may arise from small, coherent
contributions from several substructures within the lens halo. We find that the
root-mean-square (rms) fluctuation of the magnification perturbation is 10% to
20% and both the average and rms perturbations are sensitive to the mass
spectrum and density profile of the perturbers. Interestingly, we find that
relative to a smooth model of the same mass, the average magnification in
clumpy models is lower (higher) than that in smooth models for positive
(negative) parity images. This is opposite from what is observed if one assumes
that the image magnification predicted by the best-fit smooth model of a lens
is a good proxy for what the observed magnification would have been if
substructures were absent. While it is possible for this discrepancy to be
resolved via nonlinear perturbers, we argue that a more likely explanation is
that the assumption that the best-fit lens model is a good proxy for the
magnification in the absence of substructure is not correct. We conclude that a
better theoretical understanding of the predicted statistical properties of
magnification perturbations by CDM substructure is needed in order to affirm
that CDM substructures have been unambiguously detected.Comment: ApJ accepted, minor change
Pairing correlations. Part 1: description of odd nuclei in mean-field theories
In order to extract informations on pairing correlations in nuclei from
experimental mass differences, the different contributions to odd-even mass
differences are investigated within the Skyrme HFB method. In this first paper,
the description of odd nuclei within HFB is discussed since it is the key point
for the understanding of the above mentioned contributions. To go from an even
nucleus to an odd one, the advantage of a two steps process is demonstrated and
its physical content is discussed. New results concerning time-reversal
symmetry breaking in odd-nuclei are also reported.
PACS: 21.10Dr; 21.10.Hw; 21.30.-x.
Keywords: Mean-field theories; Pairing correlations; odd nuclei;Comment: 34 pages, 8 figures. Submitted to Phys. Rev.
Correlations between EMG jaw muscle activity and facial morphology in complete denture wearers
In a sample of fifteen partially edentulous subjects assigned for immediate complete denture treatment, a correlation analysis was performed between pre-extraction facial morphology determined from lateral cephalograms, and EMG activity of the anterior temporal and masseter muscles in maximal clench and tapping of teeth. Marked correlations observed between vertical and sagittal jaw relations and mean voltages of the jaw-closing muscles indicated strong biting activity in subjects with a square facial type. No significant associations were observed between biting strength and the age of the subjects. Correlations between facial morphological changes and changes in EMG biting activity after denture insertion and during 1 year of denture wear indicated that the anterior temporal muscles were extremely sensitive to sagittal changes in anterior occlusion. The masseter muscles were less sensitive, but responded in some instances to changes in vertical jaw relationship.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73816/1/j.1365-2842.1983.tb00105.x.pd
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