13,604 research outputs found
'It Has to Go Down A Little, In Order to Go Around'- Following Feynman on the Gyroscope
In this paper we show that with the help of accessible, teaching quality
equipment, some interesting details of the motion of a gyroscope, typically
overlooked in introductory courses, can be measured and compared to theory. We
begin by deriving a simple relation between the asymptotic dip angle of a
gyroscope released from rest and its precession velocity. We then describe an
experiment which measures these parameters. The data gives excellent agreement
with the model. The idea for this project was suggested by the discussion of
gyroscopic motion in The Feynman Lectures on Physics. Feynman's conclusion
(stated in colloquial terms and quoted in the title) is confirmed and, in
addition, conservation of angular momentum, which underlies this effect, is
quantitatively demonstrated.Comment: 6 pages, 4 figure
Ascent control studies of the 049 and ATP parallel burn solid rocket motor shuttle configurations
The control authority approach is discussed as a major problem of the parallel burn soil shuttle configuration due to the many resulting system impacts regardless of the approach. The major trade studies and their results, which led to the recommendation of an SRB TVC control authority approach are presented
Potential, core-level and d band shifts at transition metal surfaces
We have extended the validity of the correlation between the surface
3d-core-level shift (SCLS) and the surface d band shift (SDBS) to the entire 4d
transition metal series and to the neighboring elements Sr and Ag via accurate
first-principles calculations. We find that the correlation is quasilinear and
robust with respect to the differencies both between initial and final-state
calculations of the SCLS's and two distinct measures of the SDBS's. We show
that despite the complex spatial dependence of the surface potential shift
(SPS) and the location of the 3d and 4d orbitals in different regions of space,
the correlation exists because the sampling of the SPS by the 3d and 4d
orbitals remains similar. We show further that the sign change of the SCLS's
across the transition series does indeed arise from the d band-narrowing
mechanism previously proposed. However, while in the heavier transition metals
the predicted increase of d electrons in the surface layer relative to the bulk
arises primarily from transfers from s and p states to d states within the
surface layer, in the lighter transition metals the predicted decrease of
surface d electrons arises primarily from flow out into the vacuum.Comment: RevTex, 22 pages, 5 figures in uufiles form, to appear in Phys.Rev.
The CANADA-FRANCE REDSHIFT SURVEY XIII: The luminosity density and star-formation history of the Universe to z ~ 1
The comoving luminosity density of the Universe is estimated from the CFRS
faint galaxy sample in three wavebands (2800A, 4400A and 1 micron) over the
redshift range 0 < z < 1. In all three wavebands, the comoving luminosity
density increases markedly with redshift. For a (q_0 = 0.5, Omega = 1.0)
cosmological model, the comoving luminosity density increases as at 1 micron, as at 4400A and as at 2800A, these exponents being reduced by 0.43 and 1.12 for (0.05,0.1)
and (-0.85,0.1) cosmological models respectively. The variation of the
luminosity density with epoch can be reasonably well modelled by an actively
evolving stellar population with a Salpeter initial mass function (IMF)
extending to 125 M_sun, a star-formation rate declining with a power 2.5, and a
turn-on of star-formation at early epochs. A Scalo (1986) IMF extending to the
same mass limit produces too many long-lived low mass stars. This rapid
evolution of the star-formation rate and comoving luminosity density of the
Universe is in good agreement with the conclusions of Pei and Fall (1995) from
their analysis of the evolving metallicity of the Universe. One consequence of
this evolution is that the physical luminosity density at short wavelengths has
probably declined by two orders of magnitude since z ~ 1.Comment: uuencoded compressed tar file containing 8 page Tex file, 2
postscript figures and 2 tables. Ap J Letters, in press. Also available at
http://www.astro.utoronto.ca/~lilly/CFRS/papers.htm
The Formation of Large Galactic Disks: Revival or Survival?
Using the deepest and the most complete set of observations of distant
galaxies, we investigate how extended disks could have formed. Observations
include spatially-resolved kinematics, detailed morphologies and photometry
from UV to mid-IR. Six billion years ago, half of the present-day spiral
progenitors had anomalous kinematics and morphologies, as well as relatively
high gas fractions. We argue that gas-rich major mergers, i.e., fusions between
gas-rich disk galaxies of similar mass, can be the likeliest driver for such
strong peculiarities. This suggests a new channel of disk formation, e.g. many
disks could be reformed after gas-rich mergers. This is found to be in perfect
agreement with predictions from the state-of-the-art LCDM semi-empirical
models: due to our sensitivity in detecting mergers at all phases, from pairs
to relaxed post-mergers, we find a more accurate merger rate. The scenario can
be finally confronted to properties of nearby galaxies, including M31 and
galaxies showing ultra-faint, gigantic structures in their haloes.Comment: Proceedings of the annual meeting of the French Astronomical Society,
2011, 6 pages, 1 Figur
Kerncraft: A Tool for Analytic Performance Modeling of Loop Kernels
Achieving optimal program performance requires deep insight into the
interaction between hardware and software. For software developers without an
in-depth background in computer architecture, understanding and fully utilizing
modern architectures is close to impossible. Analytic loop performance modeling
is a useful way to understand the relevant bottlenecks of code execution based
on simple machine models. The Roofline Model and the Execution-Cache-Memory
(ECM) model are proven approaches to performance modeling of loop nests. In
comparison to the Roofline model, the ECM model can also describes the
single-core performance and saturation behavior on a multicore chip. We give an
introduction to the Roofline and ECM models, and to stencil performance
modeling using layer conditions (LC). We then present Kerncraft, a tool that
can automatically construct Roofline and ECM models for loop nests by
performing the required code, data transfer, and LC analysis. The layer
condition analysis allows to predict optimal spatial blocking factors for loop
nests. Together with the models it enables an ab-initio estimate of the
potential benefits of loop blocking optimizations and of useful block sizes. In
cases where LC analysis is not easily possible, Kerncraft supports a cache
simulator as a fallback option. Using a 25-point long-range stencil we
demonstrate the usefulness and predictive power of the Kerncraft tool.Comment: 22 pages, 5 figure
A Precision Angle Sensor using an Optical Lever inside a Sagnac Interferometer
We built an ultra low noise angle sensor by combining a folded optical lever
and a Sagnac interferometer. The instrument has a measured noise floor of 1.3
prad / Hz^(1/2) at 2.4 kHz. We achieve this record angle sensitivity using a
proof-of-concept apparatus with a conservative N=11 bounces in the optical
lever. This technique could be extended to reach sub-picoradian / Hz^(1/2)
sensitivities with an optimized design.Comment: 3 pages, 4 figure
Deep UV Luminosity Functions at the Infall Region of the Coma Cluster
We have used deep GALEX observations at the infall region of the Coma cluster
to measure the faintest UV luminosity functions (LFs) presented for a rich
galaxy cluster thus far. The Coma UV LFs are measured to M_UV = -10.5 in the
GALEX FUV and NUV bands, or 3.5 mag fainter than previous studies, and reach
the dwarf early-type galaxy population in Coma for the first time. The
Schechter faint-end slopes (alpha = -1.39 in both GALEX bands) are shallower
than reported in previous Coma UV LF studies owing to a flatter LF at faint
magnitudes. A Gaussian-plus-Schechter model provides a slightly better
parametrization of the UV LFs resulting in a faint-end slope of ~ -1.15 in both
GALEX bands. The two-component model gives faint-end slopes shallower than -1
(a turnover) for the LFs constructed separately for passive and star forming
galaxies. The UV LFs for star forming galaxies show a turnover at M_UV ~ -14
owing to a deficit of dwarf star forming galaxies in Coma with stellar masses
below M*=10^8 Msun. A similar turnover is identified in recent UV LFs measured
for the Virgo cluster suggesting this may be a common feature of local galaxy
clusters, whereas the field UV LFs continue to rise at faint magnitudes. We did
not identify an excess of passive galaxies as would be expected if the missing
dwarf star forming galaxies were quenched inside the cluster. In fact, the LFs
for both dwarf passive and star forming galaxies show the same turnover at
faint magnitudes. We discuss the possible origin of the missing dwarf star
forming galaxies in Coma and their expected properties based on comparisons to
local field galaxies.Comment: accepted for publication in Ap
- …