58,501 research outputs found
Low level accelerometer test methods are investigated
Problems associated with testing accelerometers to an accuracy where the standard error is less than .0000001 g are centered around the elimination of uncertainties in the acceleration input to the accelerometer. By placing a test rig in free fall, the uncertainty in the earths gravity field can be eliminated
Effects of Low Cell pH and Elevated Inorganic Phosphate on the pCa-Force Relationship in Single Muscle Fibers at Near-Physiological Temperatures
Intense muscle contraction induces high rates of ATP hydrolysis with resulting increases in Pi, H+, and ADP, factors thought to induce fatigue by interfering with steps in the cross-bridge cycle. Force inhibition is less at physiological temperatures; thus the role of low pH in fatigue has been questioned. Effects of pH 6.2 and collective effects with 30 mM Pi on the pCa-force relationship were assessed in skinned fast and slow rat skeletal muscle fibers at 15 and 30°C. At 30°C, pH 6.2 + 30 mM Pi significantly depressed peak force in all fiber types, with the greatest effect in type IIx fibers. Across fiber types, Ca2+ sensitivity was depressed by low pH and low pH + high Pi, with the greater effect at 30°C. For type IIx fibers at 30°C, half-maximal activation (pCa50) was 5.36 at pH 6.2 (no added Pi) and 4.98 at pH 6.2 + 30 mM Pi compared with 6.58 in the control condition (pH 7, no added Pi). At 30°C, n2, reflective of thick filament cooperativity, was unchanged by low cell pH but was depressed from 5.02 to 2.46 in type IIx fibers with pH 6.2 + 30 mM Pi. With acidosis, activation thresholds of all fiber types required higher free Ca2+ at 15 and 30°C. With the exception of type IIx fibers, the Ca2+ required to reach activation threshold increased further with added Pi. In conclusion, it is clear that fatigue-inducing effects of low cell pH and elevated Pi at near-physiological temperatures are substantial
Structure and dynamics of topological defects in a glassy liquid on a negatively curved manifold
We study the low-temperature regime of an atomic liquid on the hyperbolic
plane by means of molecular dynamics simulation and we compare the results to a
continuum theory of defects in a negatively curved hexagonal background. In
agreement with the theory and previous results on positively curved (spherical)
surfaces, we find that the atomic configurations consist of isolated defect
structures, dubbed "grain boundary scars", that form around an irreducible
density of curvature-induced disclinations in an otherwise hexagonal
background. We investigate the structure and the dynamics of these grain
boundary scars
How effective is harassment on infalling late-type dwarfs?
A new harassment model is presented that models the complex, and dynamical
tidal field of a Virgo like galaxy cluster. The model is applied to small,
late-type dwarf disc galaxies (of substantially lower mass than in previous
harassment simulations) as they infall into the cluster from the outskirts.
These dwarf galaxies are only mildly affected by high speed tidal encounters
with little or no observable consequences; typical stellar losses are ,
producing very low surface brightness streams ( mag arcsec),
and a factor of two drop in dynamical mass-to-light ratio. Final stellar discs
remain disc-like, and dominated by rotation although often with tidally induced
spiral structure. By means of Monte-Carlo simulations, the statistically likely
influences of harassment on infalling dwarf galaxies are determined. The
effects of harassment are found to be highly dependent on the orbit of the
galaxy within the cluster, such that newly accreted dwarf galaxies typically
suffer only mild harassment. Strong tidal encounters, that can morphologically
transform discs into spheroidals, are rare occurring in of dwarf
galaxy infalls for typical orbits of sub-structure within CDM cluster
mass halos. For orbits with small apocentric distances (250 kpc), harassment
is significantly stronger resulting in complete disruption or heavy mass loss
( dark matter and stellar), however, such orbits are expected
to be highly improbable for newly infalling galaxies due to the deep potential
well of the cluster.Comment: 15 pages, 11 figures, 4 table
Localization transitions in non-Hermitian quantum mechanics
We study the localization transitions which arise in both one and two
dimensions when quantum mechanical particles described by a random
Schr\"odinger equation are subjected to a constant imaginary vector potential.
A path-integral formulation relates the transition to flux lines depinned from
columnar defects by a transverse magnetic field in superconductors. The theory
predicts that the transverse Meissner effect is accompanied by stretched
exponential relaxation of the field into the bulk and a diverging penetration
depth at the transition.Comment: 4 pages (latex) with 3 figures (epsf) embedded in the text using the
style file epsf.st
The changing tide: Federal support of civilian-sector R and D
The involvement of the Federal government in civilian sector research and development is discussed. Relevant policies are put in an historical perspective. The roles played by industrial research and public funding are reveiwed. Government support of basic an generic research, clientele-oriented applied research, and research with commercial ends is studied. Procurement, anti-trust, and patent policies, all of which affect the climate for private research and development, are examined
Electronically Variable Pressure Regulator (EVPR)
A new programmable electronically variable pressure regulator (EVPR) concept accurately controls the local outlet or remote system pressure. It uses an integral pulse width modulated rare earth permanent magnet motor operating in response to redundant pressure transducer feedback signals. The EVPR is a simple single stage device that does not use dynamic seals or pilot valving. Conversion of partial revolution motor torque to poppet lifting force is accomplished by pure flexure action to avoid using bearings. The flexure drive (called the ROTAX) has a variable lead to minimize motor weight and power consumption. Breadboard tests were completed successfully on two critical design elements of the EVPR: the ROTAX and the motor. The ROTAX cable system was tested for 250,000 cycles without failure. The breadboard motor met the basic design requirements including the design torque and power consumption. Prototype parts were fabricated, and testing of the prototype EVPR has started. It is PC computer controlled to facilitate programming, data acquisition and analysis. A lightweight dedicated microprocessor is planned for the flightweight EVPR
Free Energies of Isolated 5- and 7-fold Disclinations in Hexatic Membranes
We examine the shapes and energies of 5- and 7-fold disclinations in
low-temperature hexatic membranes. These defects buckle at different values of
the ratio of the bending rigidity, , to the hexatic stiffness constant,
, suggesting {\em two} distinct Kosterlitz-Thouless defect proliferation
temperatures. Seven-fold disclinations are studied in detail numerically for
arbitrary . We argue that thermal fluctuations always drive
into an ``unbuckled'' regime at long wavelengths, so that
disclinations should, in fact, proliferate at the {\em same} critical
temperature. We show analytically that both types of defects have power law
shapes with continuously variable exponents in the ``unbuckled'' regime.
Thermal fluctuations then lock in specific power laws at long wavelengths,
which we calculate for 5- and 7-fold defects at low temperatures.Comment: LaTeX format. 17 pages. To appear in Phys. Rev.
Patterned Geometries and Hydrodynamics at the Vortex Bose Glass Transition
Patterned irradiation of cuprate superconductors with columnar defects allows
a new generation of experiments which can probe the properties of vortex
liquids by confining them to controlled geometries. Here we show that an
analysis of such experiments that combines an inhomogeneous Bose glass scaling
theory with the hydrodynamic description of viscous flow of vortex liquids can
be used to infer the critical behavior near the Bose glass transition. The
shear viscosity is predicted to diverge as at the Bose glass
transition, with the dynamical critical exponent.Comment: 5 pages, 4 figure
Helix Formation and Folding in an Artificial Peptide
We study the relation between -helix formation and folding for a
simple artificial peptide, Ala-Gly-Ala. Our data rely on
multicanonical Monte Carlo simulations where the interactions among all atoms
are taken into account. The free-energy landscape of the peptide is evaluated
for various temperatures. Our data indicate that folding of this peptide is a
two-step process: in a first step two -helices are formed which
afterwards re-arrange themselves into a U-like structure.Comment: 15 pages, with 9 eps figure
- …