1,513 research outputs found
Energetically optimal running requires torques about the centre of mass
Bipedal animals experience ground reaction forces (GRFs) that pass close to the centre of mass (CoM) throughout stance, first decelerating the body, then re-accelerating it during the second half of stance. This results in fluctuations in kinetic energy, requiring mechanical work from the muscles. However, here we show analytically that, in extreme cases (with a very large body pitch moment of inertia), continuous alignment of the GRF through the CoM requires greater mechanical work than a maintained vertical force; we show numerically that GRFs passing between CoM and vertical throughout stance are energetically favourable under realistic conditions; and demonstrate that the magnitude, if not the precise form, of actual CoM-torque profiles in running is broadly consistent with simple mechanical work minimization for humans with appropriate pitch moment of inertia. While the potential energetic savings of CoM-torque support strategies are small (a few per cent) over the range of human running, their importance increases dramatically at high speeds and stance angles. Fast, compliant runners or hoppers would benefit considerably from GRFs more vertical than the zero-CoM-torque strategy, especially with bodies of high pitch moment of inertiaâsuggesting a novel advantage to kangaroos of their peculiar long-head/long-tail structure
The supply and use of organic material at the deep-sea floor
Sediment traps and benthic respirometers have been used to measure the supply of particulate materials to, and the use of organic material by the North Atlantic deep-sea benthos. The area of the sea floor less than 2 kilometers deep is relatively small but accounts for 85% of the total oceanic benthic oxygen consumption, which reflects the primary productivity of the surface waters...
Dynamics of Human Walking
The problem of biped locomotion at steady speeds is discussed through a
Lagrangian formulation developed for velocity-dependent, body driving forces.
Human walking on a level surface is analyzed in terms of the data on the
resultant ground-reaction force and the external work. It is shown that the
trajectory of the center of mass is due to a superposition of its rectilinear
motion with a given speed and a backward rotation along a shortened
hypocycloid. A stiff-to-compliant crossover between walking gaits is described
and the maximum speed for human walking, given by an instability of the
trajectory, is predicted.
Key words: locomotion, integrative biology, muscles, bipedalism, human
walking, biomechanics.Comment: 9 pages, 4 figure
A study of temperature-related non-linearity at the metal-silicon interface
In this paper, we investigate the temperature dependencies of metal-semiconductor interfaces in an effort to better reproduce the current-voltage-temperature (IVT) characteristics of any Schottky diode, regardless of homogeneity. Four silicon Schottky diodes were fabricated for this work, each displaying different degrees of inhomogeneity; a relatively homogeneous NiV/Si diode, a Ti/Si and Cr/Si diode with double bumps at only the lowest temperatures, and a Nb/Si diode displaying extensive non-linearity. The 77â300âK IVT responses are modelled using a semi-automated implementation of Tung's electron transport model, and each of the diodes are well reproduced. However, in achieving this, it is revealed that each of the three key fitting parameters within the model display a significant temperature dependency. In analysing these dependencies, we reveal how a rise in thermal energy âactivatesâ exponentially more interfacial patches, the activation rate being dependent on the carrier concentration at the patch saddle point (the patch's maximum barrier height), which in turn is linked to the relative homogeneity of each diode. Finally, in a review of Tung's model, problems in the divergence of the current paths at low temperature are explained to be inherent due to the simplification of an interface that will contain competing defects and inhomogeneities
Key principle of the efficient running, swimming, and flying
Empirical observations indicate striking similarities among locomotion in
terrestrial animals, birds, and fish, but unifying physical grounds are
lacking. When applied to efficient locomotion, the analytical mechanics
principle of minimum action yields two patterns of mechanical similarity via
two explicit spatiotemporal coherent states. In steady locomotory modes, the
slow muscles determining maximal optimum speeds maintain universal intrinsic
muscular pressure. Otherwise, maximal speeds are due to constant mass-dependent
stiffness of fast muscles generating a uniform force field, exceeding
gravitation. Being coherent in displacements, velocities and forces, the body
appendages of animals are tuned to natural propagation frequency through the
state-dependent elastic muscle moduli.
Key words: variational principle of minimum action (04.20.Fy), locomotion
(87.19.ru), biomechanics (87.85.G-).Comment: Submitted to the Europhysical Letter
Sintered alumina with low dielectric loss
Published versio
BKM Lie superalgebras from counting twisted CHL dyons
Following Sen[arXiv:0911.1563], we study the counting of (`twisted') BPS
states that contribute to twisted helicity trace indices in four-dimensional
CHL models with N=4 supersymmetry. The generating functions of half-BPS states,
twisted as well as untwisted, are given in terms of multiplicative eta products
with the Mathieu group, M_{24}, playing an important role. These multiplicative
eta products enable us to construct Siegel modular forms that count twisted
quarter-BPS states. The square-roots of these Siegel modular forms turn out be
precisely a special class of Siegel modular forms, the dd-modular forms, that
have been classified by Clery and Gritsenko[arXiv:0812.3962]. We show that each
one of these dd-modular forms arise as the Weyl-Kac-Borcherds denominator
formula of a rank-three Borcherds-Kac-Moody Lie superalgebra. The walls of the
Weyl chamber are in one-to-one correspondence with the walls of marginal
stability in the corresponding CHL model for twisted dyons as well as untwisted
ones. This leads to a periodic table of BKM Lie superalgebras with properties
that are consistent with physical expectations.Comment: LaTeX, 32 pages; (v2) matches published versio
Perturbative tests of non-perturbative counting
We observe that a class of quarter-BPS dyons in N=4 theories with charge
vector (Q, P) and with nontrivial values of the arithmetic duality invariant I
:= gcd (Q wedge P) are nonperturbative in one frame but perturbative in another
frame. This observation suggests a test of the recently computed
nonperturbative partition functions for dyons with nontrivial values of the
arithmetic invariant. For all values of I, we show that the nonperturbative
counting yields vanishing indexed degeneracy for this class of states
everywhere in the moduli space in precise agreement with the perturbative
result.Comment: 10 pages, 0 figure
Logarithmic Corrections to Extremal Black Hole Entropy from Quantum Entropy Function
We evaluate the one loop determinant of matter multiplet fields of N=4
supergravity in the near horizon geometry of quarter BPS black holes, and use
it to calculate logarithmic corrections to the entropy of these black holes
using the quantum entropy function formalism. We show that even though
individual fields give non-vanishing logarithmic contribution to the entropy,
the net contribution from all the fields in the matter multiplet vanishes. Thus
logarithmic corrections to the entropy of quarter BPS black holes, if present,
must be independent of the number of matter multiplet fields in the theory.
This is consistent with the microscopic results. During our analysis we also
determine the complete spectrum of small fluctuations of matter multiplet
fields in the near horizon geometry.Comment: LaTeX file, 52 pages; v2: minor corrections, references adde
Dielectric properties characterization of La- and Dy-doped BiFeO3 thin films
The dielectric response of La- and Dy- doped BiFeO3 thin films at microwave frequencies (up to 12 GHz) has been monitored as a function of frequency, direct current (dc) electric field, and magnetic field in a temperature range from 25 to 300 °C. Both the real and imaginary parts of the response have been found to be non-monotonic (oscillating) functions of measuring frequency. These oscillations are not particularly sensitive to a dc electric field; however, they are substantially dampened by a magnetic field. The same effect has been observed when the volume of the characterized sample is increased. This phenomenon is attributed to the presence of a limited number of structural features with a resonance type response. The exact origin of these features is unknown at present. Leakage current investigations were performed on the whole set of films. The films were highly resistive with low leakage current, thereby giving us confidence in the microwave measurements. These typically revealed âN'-type I-V characteristic
- âŠ