22,781 research outputs found
Analysis of standing vertical jumps using a force platform
A force platform analysis of vertical jumping provides an engaging demonstration of the kinematics and dynamics of one-dimensional motion. The height of the jump may be calculated (1) from the flight time of the jump, (2) by applying the impulse–momentum theorem to the force–time curve, and (3) by applying the work–energy theorem to the force-displacement curve
Collaborative research on V/STOL control system/cockpit display tradeoffs under the NASA/MOD joint aeronautical program
Summarized here are activities that have taken place from 1979 to the present in a collaborative program between NASA Ames Research Center and the Royal Aerospace Establishment (now Defence Research Agency), Bedford on flight control system and cockpit display tradeoffs for low-speed and hover operations of future V/STOL aircraft. This program was created as Task 8A of the Joint Aeronautical Program between NASA in the United States and the Ministry of Defence (Procurement Executive) in the United Kingdom. The program was initiated based on a recognition by both parties of the strengths of the efforts of their counterparts and a desire to participate jointly in future simulation and flight experiments. In the ensuing years, teams of NASA and RAE engineers and pilots have participated in each other's simulation experiments to evaluate control and display concepts and define design requirements for research aircraft. Both organizations possess Harrier airframes that have undergone extensive modification to provide in-flight research capabilities in the subject areas. Both NASA and RAE have profited by exchanges of control/display concepts, design criteria, fabrication techniques, software development and validation, installation details, and ground and flight clearance techniques for their respective aircraft. This collaboration has permitted the two organizations to achieve jointly substantially more during the period than if they had worked independently. The two organizations are now entering the phase of flight research for the collaborative program as currently defined
Dynamics of a Monolayer of Microspheres on an Elastic Substrate
We present a model for wave propagation in a monolayer of spheres on an
elastic substrate. The model, which considers sagittally polarized waves,
includes: horizontal, vertical, and rotational degrees of freedom; normal and
shear coupling between the spheres and substrate, as well as between adjacent
spheres; and the effects of wave propagation in the elastic substrate. For a
monolayer of interacting spheres, we find three contact resonances, whose
frequencies are given by simple closed-form expressions. For a monolayer of
isolated spheres, only two resonances are present. The contact resonances
couple to surface acoustic waves in the substrate, leading to mode
hybridization and "avoided crossing" phenomena. We present dispersion curves
for a monolayer of silica microspheres on a silica substrate, assuming
adhesive, Hertzian interactions, and compare calculations using an effective
medium approximation to a discrete model of a monolayer on a rigid substrate.
While the effective medium model does not account for discrete lattice effects
at short wavelengths, we find that it is well suited for describing the
interaction between the monolayer and substrate in the long wavelength limit.
We suggest that a complete picture of the dynamics of a discrete monolayer
adhered to an elastic substrate can be found using a combination of the results
presented for the discrete and effective medium descriptions. This model is
potentially scalable for use with both micro- and macroscale systems, and
offers the prospect of experimentally extracting contact stiffnesses from
measurements of acoustic dispersion
The Evolution of Post-Starburst Galaxies from to the Present
Post-starburst galaxies are in the transitional stage between blue,
star-forming galaxies and red, quiescent galaxies, and therefore hold important
clues for our understanding of galaxy evolution. In this paper, we
systematically searched for and identified a large sample of post-starburst
galaxies from the spectroscopic dataset of the Sloan Digital Sky Survey (SDSS)
Data Release 9. In total, we found more than 6000 objects with redshifts
between and , making this the largest sample of
post-starburst galaxies in the literature. We calculated the luminosity
function of the post-starburst galaxies using two uniformly selected
subsamples: the SDSS Main Galaxy Sample and the Baryon Oscillation
Spectroscopic Survey CMASS Sample. The luminosity functions are reasonably fit
by half-Gaussian functions. The peak magnitudes shift as a function of redshift
from at to at . This is
consistent with the downsizing trend, whereby more massive galaxies form
earlier than low-mass galaxies. We compared the mass of the post-starburst
stellar population found in our sample to the decline of the global
star-formation rate and found that only a small amount () of all
star-formation quenching in the redshift range results in
post-starburst galaxies in the luminosity range our sample is sensitive to.
Therefore, luminous post-starburst galaxies are not the place where most of the
decline in star-formation rate of the universe is happening.Comment: 26 pages, 24 figures, 8 tables. Accepted for publication in The
Astrophysical Journa
Optimum take-off angle in the long jump
In this study, we found that the optimum take-off angle for a long jumper may be predicted by combining the equation for the range of a projectile in free flight with the measured relations between take-off speed, take-off height and take-off angle for the athlete. The prediction method was evaluated using video measurements of three experienced male long jumpers who performed maximum-effort jumps over a wide range of take-off angles. To produce low take-off angles the athletes used a long and fast run-up, whereas higher take-off angles were produced using a progressively shorter and slower run-up. For all three athletes, the take-off speed decreased and the take-off height increased as the athlete jumped with a higher take-off angle. The calculated optimum take-off angles were in good agreement with the athletes' competition take-off angles
The Magnetic Properties of Heating Events on High-Temperature Active Region Loops
Understanding the relationship between the magnetic field and coronal heating
is one of the central problems of solar physics. However, studies of the
magnetic properties of impulsively heated loops have been rare. We present
results from a study of 34 evolving coronal loops observed in the Fe XVIII line
component of AIA/SDO 94 A filter images from three active regions with
different magnetic conditions. We show that the peak intensity per unit
cross-section of the loops depends on their individual magnetic and geometric
properties. The intensity scales proportionally to the average field strength
along the loop () and inversely with the loop length () for a
combined dependence of . These loop properties are
inferred from magnetic extrapolations of the photospheric HMI/SDO line-of-sight
and vector magnetic field in three approximations: potential and two Non Linear
Force-Free (NLFF) methods. Through hydrodynamic modeling (EBTEL model) we show
that this behavior is compatible with impulsively heated loops with a
volumetric heating rate that scales as .Comment: Astrophysical Journal, in pres
Colloids with key-lock interactions: non-exponential relaxation, aging and anomalous diffusion
The dynamics of particles interacting by key-lock binding of attached
biomolecules are studied theoretically. Experimental realizations of such
systems include colloids grafted with complementary single-stranded DNA
(ssDNA), and particles grafted with antibodies to cell-membrane proteins.
Depending on the coverage of the functional groups, we predict two distinct
regimes. In the low coverage localized regime, there is an exponential
distribution of departure times. As the coverage is increased the system enters
a diffusive regime resulting from the interplay of particle desorption and
diffusion. This interplay leads to much longer bound state lifetimes, a
phenomenon qualitatively similar to aging in glassy systems. The diffusion
behavior is analogous to dispersive transport in disordered semiconductors:
depending on the interaction parameters it may range from a finite
renormalization of the diffusion coefficient to anomalous, subdiffusive
behavior. We make connections to recent experiments and discuss the
implications for future studies.Comment: v2: substantially revised version, new treatment of localized regime,
19 pages, 10 figure
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