3,226 research outputs found
How Sandcastles Fall
Capillary forces significantly affect the stability of sandpiles. We analyze
the stability of sandpiles with such forces, and find that the critical angle
is unchanged in the limit of an infinitely large system; however, this angle is
increased for finite-sized systems. The failure occurs in the bulk of the
sandpile rather than at the surface. This is related to a standard result in
soil mechanics. The increase in the critical angle is determined by the surface
roughness of the particles, and exhibits three regimes as a function of the
added-fluid volume. Our theory is in qualitative agreement with the recent
experimental results of Hornbaker et al., although not with the interpretation
they make of these results.Comment: 4 pages, 2 figures, revte
A Ball in a Groove
We study the static equilibrium of an elastic sphere held in a rigid groove
by gravity and frictional contacts, as determined by contact mechanics. As a
function of the opening angle of the groove and the tilt of the groove with
respect to the vertical, we identify two regimes of static equilibrium for the
ball. In the first of these, at large opening angle or low tilt, the ball rolls
at both contacts as it is loaded. This is an analog of the "elastic" regime in
the mechanics of granular media. At smaller opening angles or larger tilts, the
ball rolls at one contact and slides at the other as it is loaded, analogously
with the "plastic" regime in the mechanics of granular media. In the elastic
regime, the stress indeterminacy is resolved by the underlying kinetics of the
ball response to loading.Comment: RevTeX 3.0, 4 pages, 2 eps figures included with eps
Crumpling a Thin Sheet
Crumpled sheets have a surprisingly large resistance to further compression.
We have studied the crumpling of thin sheets of Mylar under different loading
conditions. When placed under a fixed compressive force, the size of a crumpled
material decreases logarithmically in time for periods up to three weeks. We
also find hysteretic behavior when measuring the compression as a function of
applied force. By using a pre-treating protocol, we control this hysteresis and
find reproducible scaling behavior for the size of the crumpled material as a
function of the applied force.Comment: revtex 4 pages, 6 eps figures submitted to Phys Rev. let
Differential loss of spinal interneurons in a mouse model of ALS
Amyotrophic lateral sclerosis (ALS) leads to a loss of specific motor neuron populations in the spinal cord and cortex. Emerging evidence suggests that interneurons may also be affected, but a detailed characterization of interneuron loss and its potential impacts on motor neuron loss and disease progression is lacking. To examine this issue, the fate of V1 inhibitory neurons during ALS was assessed in the ventral spinal cord using the SODG93A mouse model. The V1 population makes up ∼30% of all ventral inhibitory neurons, ∼50% of direct inhibitory synaptic contacts onto motor neuron cell bodies, and is thought to play a key role in modulating motor output, in part through recurrent and reciprocal inhibitory circuits. We find that approximately half of V1 inhibitory neurons are lost in SODG93A mice at late disease stages, but that this loss is delayed relative to the loss of motor neurons and V2a excitatory neurons. We further identify V1 subpopulations based on transcription factor expression that are differentially susceptible to degeneration in SODG93A mice. At an early disease stage, we show that V1 synaptic contacts with motor neuron cell bodies increase, suggesting an upregulation of inhibition before V1 neurons are lost in substantial numbers. These data support a model in which progressive changes in V1 synaptic contacts early in disease, and in select V1 subpopulations at later stages, represent a compensatory upregulation and then deleterious breakdown of specific interneuron circuits within the spinal cord
UBVRIz Light Curves of 51 Type II Supernovae
We present a compilation of UBV RIz light curves of 51 type II supernovae
discovered during the course of four different surveys during 1986 to 2003: the
Cerro Tololo Supernova Survey, the Calan/Tololo Supernova Program (C&T), the
Supernova Optical and Infrared Survey (SOIRS), and the Carnegie Type II
Supernova Survey (CATS). The photometry is based on template-subtracted images
to eliminate any potential host galaxy light contamination, and calibrated from
foreground stars. This work presents these photometric data, studies the color
evolution using different bands, and explores the relation between the
magnitude at maximum brightness and the brightness decline parameter (s) from
maximum light through the end of the recombination phase. This parameter is
found to be shallower for redder bands and appears to have the best correlation
in the B band. In addition, it also correlates with the plateau duration, being
thus shorter (longer) for larger (smaller) s values.Comment: 110 pages, 9 Figures, 6 Tables, accepted in A
The Roles of the Olivocerebellar Pathway in Motor Learning and Motor Control. A consensus paper
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Design impacts of safeguards and security requirements for a US MOX fuel fabrication facility
The disposition of plutonium that is no longer required for the nation`s defense is being structured to mitigate risks associated with the material`s availability. In the 1997 Record of Decision, the US Government endorsed a dual-track approach that could employ domestic commercial reactors to effect the disposition of a portion of the plutonium in the form of mixed oxide (MOX) reactor fuels. To support this decision, the Office of Materials Disposition requested preparation of a document that would review US requirements for safeguards and security and describe their impact on the design of a MOX fuel fabrication facility. The intended users are potential bidders for the construction and operation of the facility. The document emphasizes the relevant DOE Orders but also considers the Nuclear Regulatory Commission (NRC) requirements. Where they are significantly different, the authors have highlighted this difference and provided guidance on the impact to the facility design. Finally, the impacts of International Atomic Energy Agency (IAEA) safeguards on facility design are discussed. Security and materials control and accountability issues that influence facility design are emphasized in each area of discussion. This paper will discuss the prepared report and the issues associated with facility design for implementing practical, modern safeguards and security systems into a new MOX fuel fabrication facility
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On the Origin of SN 2016hil—A Type II Supernova in the Remote Outskirts of an Elliptical Host
Type II supernovae (SNe) stem from the core collapse of massive (>8 M ⊙) stars. Due to their short lifespan, we expect a very low rate of such events in elliptical hosts, where the star formation rate is low, and which are mostly comprised of an old stellar population. SN 2016hil (iPTF16hil) is an SN II located in the extreme outskirts of an elliptical galaxy at z = 0.0608 (projected distance 27.2 kpc). It was detected near peak (M_r ~ −17 mag) 9 days after the last non-detection. The event has some potentially peculiar properties: it presented an apparently double-peaked light curve, and its spectra suggest low metallicity content (Z < 0.4 Z ⊙). We place a tentative upper limit on the mass of a potential faint host at log M/M⊙ = 7.27^(+0.43)_(-0.24) using deep optical imaging from Keck/LRIS. In light of this, we discuss the possibility of the progenitor forming locally and other more exotic formation scenarios such as a merger or common-envelope evolution causing a time-delayed explosion. Further observations of the explosion site in the UV are needed in order to distinguish between the cases. Regardless of the origin of the transient, observing a population of such seemingly hostless SNe II could have many uses, including an estimate the amount of faint galaxies in a given volume, and tests of the prediction of a time-delayed population of core-collapse SNe in locations otherwise unfavorable for the detection of such events
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