bluuebunny/crossref_metadata_2025 huggingface dataset:
Boundary Integral Equation Formulation in Generalized Linear Thermo-Viscoelasticity With Rheological Volume
Authors: El-Karamany A. S.
Published: October 2003

A general model of generalized linear thermo-viscoelasticity for isotropic material is established taking into consideration the rheological properties of the volume. The given model is applicable to three generalized theories of thermoelasticity: the generalized theory with one (Lord-Shulman theory) or with two relaxation times (Green-Lindsay theory) and with dual phase-lag (Chandrasekharaiah-Tzou theory) as well as to the dynamic coupled theory. The cases of thermo-viscoelasticity of Kelvin-Voigt model or thermoviscoelasticity ignoring the rheological properties of the volume can be obtained from the given model. The equations of the corresponding thermoelasticity theories result from the given model as special cases. A formulation of the boundary integral equation (BIE) method, fundamental solutions of the corresponding differential equations are obtained and an example illustrating the BIE formulation is given.
Computational Isotropic-Workhardening Rate-Independent Elastoplasticity
Authors: Mukherjee S. , Liu C.-S.
Published: October 2003

A novel formulation for elastoplasticity has been recently proposed by Liu and Hong. These authors have explored the internal symmetry of the constitutive model for perfect plasticity to ensure that the consistency condition is satisfied at each time step. Moreover, for perfect plasticity, they have converted the usual nonlinear elastoplastic constitutive model into a linear system of ordinary differential equations in redefined variables. The present paper is concerned with general isotropic workhardening. With the present formulation, it is still possible to satisfy the elastoplastic consistency condition at every time step, without the need for iterations even for nonlinear workhardening. The resulting system of ordinary differential equations, however, is, in general, nonlinear. Different strategies for obtaining numerical solutions of these equations are proposed in this paper, one of them based on group theory. Numerical solutions from the different schemes, for a simple illustrative example, are presented in the paper.
Polymer Dynamics as a Mechanistic Model for the Flow-Independent Viscoelasticity of Cartilage
Authors: Fyhrie D. P. , Barone J. R.
Published: October 2003

The initial, rapid, flow independent, apparent stress relaxation of articular cartilage disks deformed by unconfined compressive displacement is shown to be consistent with the theory of polymer dynamics. A relaxation function for polymers based upon a mechanistic model of molecular interaction (reptation) appropriately approximated early, flow independent relaxation of stress. It is argued that the theory of polymer dynamics, with its reliance on mechanistic models of molecular interaction, is an appropriate technique for application to and the understanding of rapid, flow independent, stress relaxation in cartilage.
Modeling of Neutral Solute Transport in a Dynamically Loaded Porous Permeable Gel: Implications for Articular Cartilage Biosynthesis and Tissue Engineering
Authors: Mauck Robert L. , Hung Clark T. , Ateshian Gerard A.
Published: October 2003

A primary mechanism of solute transport in articular cartilage is believed to occur through passive diffusion across the articular surface, but cyclical loading has been shown experimentally to enhance the transport of large solutes. The objective of this study is to examine the effect of dynamic loading within a theoretical context, and to investigate the circumstances under which convective transport induced by dynamic loading might supplement diffusive transport. The theory of incompressible mixtures was used to model the tissue (gel) as a mixture of a gel solid matrix (extracellular matrix/scaffold), and two fluid phases (interstitial fluid solvent and neutral solute), to solve the problem of solute transport through the lateral surface of a cylindrical sample loaded dynamically in unconfined compression with frictionless impermeable platens in a bathing solution containing an excess of solute. The resulting equations are governed by nondimensional parameters, the most significant of which are the ratio of the diffusive velocity of the interstitial fluid in the gel to the solute diffusivity in the gel Rg, the ratio of actual to ideal solute diffusive velocities inside the gel Rd, the ratio of loading frequency to the characteristic frequency of the gel f^, and the compressive strain amplitude ε0. Results show that when Rg>1,Rd<1, and f^>1, dynamic loading can significantly enhance solute transport into the gel, and that this effect is enhanced as ε0 increases. Based on representative material properties of cartilage and agarose gels, and diffusivities of various solutes in these gels, it is found that the ranges Rg>1,Rd<1 correspond to large solutes, whereas f^>1 is in the range of physiological loading frequencies. These theoretical predictions are thus in agreement with the limited experimental data available in the literature. The results of this study apply to any porous hydrated tissue or material, and it is therefore plausible to hypothesize that dynamic loading may serve to enhance solute transport in a variety of physiological processes.
Ash Content Modulation of Torsionally Derived Effective Material Properties in Cortical Mouse Bone
Authors: Battaglia Todd C. , Tsou An-Chi , Taylor Emerson A. , Mikic Borjana
Published: October 2003

The purpose of this study was to evaluate the effects of isolated alterations in mineral content on mouse bone torsional properties. The femora and tibiae from 25 eight-week-old male A/J strain mice were divided into five groups and selectively decalcified from 5% to 20%. The right femora were then tested to failure in torsion while the tibiae were ashed to determine final mineral content of the decalcified bones. Contralateral femora were serially cross-sectioned to determine geometric properties, and effective material properties were then calculated from the geometric and structural properties of each femoral pair. We found that the relationship between ash content and effective shear modulus or maximum effective shear stress could best be characterized through a power law, with an exponential factor of 6.79 R2=0.85 and 4.04 R2=0.67, respectively. This indicates that in a murine model, as with other species, small changes in ash content significantly influence effective material properties. Furthermore, it appears that (in adolescent A/J strain mice) effective shear modulus is more heavily affected by changes in mineralization than is maximum effective shear stress when these properties are derived from whole bone torsional tests to failure.
A Model for the Modulation of Microvessel Permeability by Junction Strands
Authors: Fu Bingmei M. , Chen Bin
Published: October 2003

To investigate the effect of junction strands on microvessel permeability, we extend the previous analytical model developed by Fu et al. (1994, J. Biomech. Eng., 116, pp. 502–513), for the interendothelial cleft to include multiple junction strands in the cleft and an interface between the surface glycocalyx layer and the cleft entrance. Based on the electron microscopic observations by Adamson et al. (1998, Am. J. Physiol., 274(43), pp. H1885–H1894), that elevation of intracellular cAMP levels would increase number of tight junction strands, this two-junction-strand and two-pore model can successfully account for the experimental data for the decreased permeability to water, small and intermediate-sized solutes by cAMP.
The Fluid Shear Stress Distribution on the Membrane of Leukocytes in the Microcirculation
Authors: Sugihara-Seki Masako , Schmid-Scho¨nbein Geert W.
Published: October 2003

Recent in-vivo and in-vitro evidence indicates that fluid shear stress on the membrane of leukocytes has a powerful control over several aspects of their cell function. This evidence raises a question about the magnitude of the fluid shear stress on leukocytes in the circulation. The flow of plasma on the surface of a leukocyte at a very low Reynolds number is governed by the Stokes equation for the motion of a Newtonian fluid. We numerically estimated the distribution of fluid shear stress on a leukocyte membrane in a microvessel for the cases when the leukocyte is freely suspended, as well as rolling along or attached to a microvessel wall. The results indicate that the fluid shear stress distribution on the leukocyte membrane is nonuniform with a sharp increase when the leukocyte makes membrane attachment to the microvessel wall. In a microvessel (10 μm diameter), the fluid shear stress on the membrane of a freely suspended leukocyte (8 μm diameter) is estimated to be several times larger than the wall shear stress exerted by the undisturbed Poiseuille flow, and increases on an adherent leukocyte up to ten times. High temporal stress gradients are present in freely suspended leukocytes in shear flow due to cell rotation, which are proportional to the local shear rate. In comparison, the temporal stress gradients are reduced on the membrane of leukocytes that are rolling or firmly adhered to the endothelium. High temporal gradients of shear stress are also present on the endothelial wall. At a plasma viscosity of 1 cPoise, the peak shear stresses for suspended and adherent leukocytes are of the order of 10 dyn/cm2 and 100 dyn/cm2, respectively.
A New Method for Evaluation of Cavitation Near Mechanical Heart Valves
Authors: Johansen Peter , Manning Keefe B. , Tarbell John M. , Fontaine Arnold A. , Deutsch Steven , Nygaard Hans
Published: October 2003

Evaluation of cavitation in vivo is often based on recordings of high-pass filtered random high-frequency pressure fluctuations. We hypothesized that cavitation signal components are more appropriately assessed by a new method for extraction of random signal components of the pressure signals. We investigated three different valve types and found a high correlation between the two methods r2:0.8806−0.9887. The new method showed that the cavitation signal could be extracted without a priori knowledge needed for setting the high-pass filter cut off frequency, nor did it introduce bandwidth limitation of the cavitation signal.
Numerical Simulation of Wall Shear Stress Conditions and Platelet Localization in Realistic End-to-Side Arterial Anastomoses
Authors: Longest P. Worth , Kleinstreuer Clement
Published: October 2003

Research studies over the last three decades have established that hemodynamic interactions with the vascular surface as well as surgical injury are inciting mechanisms capable of eliciting distal anastomotic intimal hyperplasia (IH) and ultimate bypass graft failure. While abnormal wall shear stress (WSS) conditions have been widely shown to affect vascular biology and arterial wall self-regulation, the near-wall localization of critical blood particles by convection and diffusion may also play a significant role in IH development. It is hypothesized that locations of elevated platelet interactions with reactive or activated vascular surfaces, due to injury or endothelial dysfunction, are highly susceptible to IH initialization and progression. In an effort to assess the potential role of platelet-wall interactions, experimentally validated particle-hemodynamic simulations have been conducted for two commonly implemented end-to-side anastomotic configurations, with and without proximal outflow. Specifically, sites of significant particle interactions with the vascular surface have been identified by a novel near-wall residence time (NWRT) model for platelets, which includes shear stress-based factors for platelet activation as well as endothelial cell expression of thrombogenic and anti-thrombogenic compounds. Results indicate that the composite NWRT model for platelet-wall interactions effectively captures a reported shift in significant IH formation from the arterial floor of a relatively high-angle (30 deg) graft with no proximal outflow to the graft hood of a low-angle graft (10 deg) with 20% proximal outflow. In contrast, other WSS-based hemodynamic parameters did not identify the observed system-dependent shift in IH formation. However, large variations in WSS-vector magnitude and direction, as encapsulated by the WSS-gradient and WSS-angle-gradient parameters, were consistently observed along the IH-prone suture-line region. Of the multiple hemodynamic factors capable of eliciting a hyperplastic response at the cellular level, results of this study indicate the potential significance of platelet-wall interactions coinciding with regions of low WSS in the development of IH.
3-D Finite-Element Models of Human and Monkey Fingertips to Investigate the Mechanics of Tactile Sense
Authors: Dandekar Kiran , Raju Balasundar I. , Srinivasan Mandayam A.
Published: October 2003

The biomechanics of skin and underlying tissues plays a fundamental role in the human sense of touch. It governs the mechanics of contact between the skin and an object, the transmission of the mechanical signals through the skin, and their transduction into neural signals by the mechanoreceptors. To better understand the mechanics of touch, it is necessary to establish quantitative relationships between the loads imposed on the skin by an object, the state of stresses/strains at mechanoreceptor locations, and the resulting neural response. Towards this goal, 3-D finite-element models of human and monkey fingertips with realistic external geometries were developed. By computing fingertip model deformations under line loads, it was shown that a multi-layered model was necessary to match previously obtained in vivo data on skin surface displacements. An optimal ratio of elastic moduli of the layers was determined through numerical experiments whose results were matched with empirical data. Numerical values of the elastic moduli of the skin layers were obtained by matching computed results with empirically determined force-displacement relationships for a variety of indentors. Finally, as an example of the relevance of the model to the study of tactile neural response, the multilayered 3-D finite-element model was shown to be able to predict the responses of the slowly adapting type I (SA-I) mechanoreceptors to indentations by complex object shapes.
Analysis of Prolapse in Cardiovascular Stents: A Constitutive Equation for Vascular Tissue and Finite-Element Modelling
Authors: Prendergast P. J. , Lally C. , Daly S. , Reid A. J. , Lee T. C. , Quinn D. , Dolan F.
Published: October 2003

The effectiveness of a cardiovascular stent depends on many factors, such as its ability to sustain the compression applied by the vessel wall, minimal longitudinal contraction when it is expanded, and its ability to flex when navigating tortuous blood vessels. The long-term reaction of the tissue to the stent is also device dependant; in particular some designs provoke in-stent restenosis (i.e., regrowth of the occlusion around the stent). The mechanism of restenosis is thought to involve injury or damage to the vessel wall due to the high stresses generated around the stent when it expands. Because of this, the deflection of the tissue between the struts of the stent (called prolapse or “draping”) has been used as a measure of the potential of a stent to cause restenosis. In this paper, uniaxial and biaxial experiments on human femoral artery and porcine aortic vascular tissue are used to develop a hyperelastic constitutive model of vascular tissue suitable for implementation in finite-element analysis. To analyze prolapse, four stent designs (BeStent 2, Medtronic AVE; NIROYAL, Boston Scientific; VELOCITY, Cordis; TETRA, Guidant) were expanded in vitro to determine their repeating-unit dimensions. This geometric data was used to generate a finite element model of the vascular tissue supported within a repeating-unit of the stent. Under a pressure of 450 mm Hg (representing the radial compression of the vessel wall), maximum radial deflection of 0.253 mm, 0.279 mm, 0.348 mm and 0.48 mm were calculated for each of the four stents. Stresses in the vascular wall were highest for the VELOCITY stent. The method is proposed as a way to compare stents relative to their potential for restenosis and as a basis for a biomechanical design of a stent repeating-unit that would minimize restenosis.
Numerical Simulation of Flow in Mechanical Heart Valves: Grid Resolution and the Assumption of Flow Symmetry
Authors: Ge Liang , Jones S. Casey , Sotiropoulos Fotis , Healy Timothy M. , Yoganathan Ajit P.
Published: October 2003

A numerical method is developed for simulating unsteady, 3-D, laminar flow through a bileaflet mechanical heart valve with the leaflets fixed. The method employs a dual-time-stepping artificial-compressibility approach together with overset (Chimera) grids and is second-order accurate in space and time. Calculations are carried out for the full 3-D valve geometry under steady inflow conditions on meshes with a total number of nodes ranging from 4×105 to 1.6×106. The computed results show that downstream of the leaflets the flow is dominated by two pairs of counter-rotating vortices, which originate on either side of the central orifice in the aortic sinus and rotate such that the common flow of each pair is directed away from the aortic wall. These vortices intensify with Reynolds number, and at a Reynolds number of approximately 1200 their complex interaction leads to the onset of unsteady flow and the break of symmetry with respect to both geometric planes of symmetry. Our results show the highly 3-D structure of the flow; question the validity of computationally expedient assumptions of flow symmetry; and demonstrate the need for highly resolved, fully 3-D simulations if computational fluid dynamics is to accurately predict the flow in prosthetic mechanical heart valves.
One-Dimensional Viscoelastic Behavior of Fibroblast Populated Collagen Matrices
Authors: Wagenseil Jessica E. , Wakatsuki Tetsuro , Okamoto Ruth J. , Zahalak George I. , Elson Elliot L.
Published: October 2003

Bio-artificial tissues are being developed as replacements for damaged biologic tissues. Their mechanical properties are critical for load bearing applications. Current testing protocols for bio-artificial tissues vary widely and often do not consider viscoelasticity. Uniaxial stretch tests were performed on fibroblast populated collagen matrices (FPCMs) to determine the influence of specific test protocols on the mechanical behavior. The peak force, hysteresis and shape of the force-stretch curve are affected by the stretch rate, rest period, stretch amplitude and the number and magnitude of preconditioning cycles.
Effect of Fiber Orientation and Strain Rate on the Nonlinear Uniaxial Tensile Material Properties of Tendon
Authors: Lynch Heather Anne , Johannessen Wade , Wu Jeffrey P. , Jawa Andrew , Elliott Dawn M.
Published: October 2003

Tendons are exposed to complex loading scenarios that can only be quantified by mathematical models, requiring a full knowledge of tendon mechanical properties. This study measured the anisotropic, nonlinear, elastic material properties of tendon. Previous studies have primarily used constant strain-rate tensile tests to determine elastic modulus in the fiber direction. Data for Poisson’s ratio aligned with the fiber direction and all material properties transverse to the fiber direction are sparse. Additionally, it is not known whether quasi-static constant strain-rate tests represent equilibrium elastic tissue behavior. Incremental stress-relaxation and constant strain-rate tensile tests were performed on sheep flexor tendon samples aligned with the tendon fiber direction or transverse to the fiber direction to determine the anisotropic properties of toe-region modulus E0, linear-region modulus (E), and Poisson’s ratio (ν). Among the modulus values calculated, only fiber-aligned linear-region modulus E1 was found to be strain-rate dependent. The E1 calculated from the constant strain-rate tests were significantly greater than the value calculated from incremental stress-relaxation testing. Fiber-aligned toe-region modulus E10=10.5±4.7 MPa and linear-region modulus E1=34.0±15.5 MPa were consistently 2 orders of magnitude greater than transverse moduli (E20=0.055±0.044 MPa,E2=0.157±0.154 MPa). Poisson’s ratio values were not found to be rate-dependent in either the fiber-aligned (ν12=2.98±2.59, n=24) or transverse (ν21=0.488±0.653, n=22) directions, and average Poisson’s ratio values in the fiber-aligned direction were six times greater than in the transverse direction. The lack of strain-rate dependence of transverse properties demonstrates that slow constant strain-rate tests represent elastic properties in the transverse direction. However, the strain-rate dependence demonstrated by the fiber-aligned linear-region modulus suggests that incremental stress-relaxation tests are necessary to determine the equilibrium elastic properties of tendon, and may be more appropriate for determining the properties to be used in elastic mathematical models.
The Relative Position of EDL Muscle Affects the Length of Sarcomeres Within Muscle Fibers: Experimental Results and Finite-Element Modeling
Authors: Maas Huub , Baan Guus C. , Huijing Peter A. , Yucesoy Can A. , Koopman Bart H. F. J. M. , Grootenboer Henk J.
Published: October 2003

Background : Effects of extramuscular connective tissues on muscle force (experimentally measured) and lengths of sarcomeres (modeled) were investigated in rat. It was hypothesized that changes of muscle-relative position affect the distribution of lengths of sarcomeres within muscle fibers. Method of approach: The position of extensor digitorum longus muscle (EDL) relative to intact extramuscular connective tissues of the anterior crural compartment was manipulated without changing its muscle-tendon complex length. Results: Significant effects of EDL muscle relative position on proximal and distal EDL forces were found, indicating changes of extramuscular myofascial force transmission. EDL isometric force exerted at its proximal and distal tendons differed significantly. Finite-element modeling showed that the distribution of lengths of sarcomeres is altered by changes of muscle-relative position. Conclusions: It is concluded that forces exerted on a muscle via extramuscular myofascial pathways augment distributions of lengths of sarcomeres within that muscle.
Must a Hamiltonian be Hermitian?
Authors: Bender Carl M. , Brody Dorje C. , Jones Hugh F.
Published: October 2003

A consistent physical theory of quantum mechanics can be built on a complex Hamiltonian that is not Hermitian but instead satisfies the physical condition of space–time reflection symmetry (𝒫𝒯 symmetry). Thus, there are infinitely many new Hamiltonians that one can construct that might explain experimental data. One would think that a quantum theory based on a non-Hermitian Hamiltonian violates unitarity. However, if 𝒫𝒯 symmetry is not broken, it is possible to use a previously unnoticed physical symmetry of the Hamiltonian to construct an inner product whose associated norm is positive definite. This construction is general and works for any 𝒫𝒯-symmetric Hamiltonian. The dynamics is governed by unitary time evolution. This formulation does not conflict with the requirements of conventional quantum mechanics. There are many possible observable and experimental consequences of extending quantum mechanics into the complex domain, both in particle physics and in solid state physics.
Nonisochronism in the interrupted pendulum
Authors: Gil Salvador , Di Gregorio Daniel E.
Published: October 2003

We experimentally studied the dependence of the period of the interrupted pendulum as a function of the amplitude for small angles of oscillation. We found a new kind of dependence of the period with the amplitude of the pendulum that indicates that if the interruption is not located on the main vertical axis that contains the point of suspension, the period of the interrupted pendulum is highly nonisochronous and does not converge to a definite value as the maximum amplitude approaches zero. We have developed a simple model that satisfactorily explains the experimental data with no adjustable parameters. This property of the interrupted pendulum is a general property of the parabolic potential consisting of two quadratic forms with different curvatures that join at a point different from the apex or the vertex.
Average distance between a star and planet in an eccentric orbit
Authors: Williams Darren M.
Published: October 2003

The average distance between a star and a planet in the classic two-body problem of celestial mechanics is different from the orbital semi-major axis of the planet if the average is performed over time or angle. The time- and angle-averaged distances are functions of both the semi-major axis and the orbital eccentricity and can differ from the orbital semi-major axis by as much as 50%–100%. This difference contradicts the usual statement found in most introductory and advanced textbooks that the average distance and the orbital semi-major axis are equal.
A basic lock-in amplifier experiment for the undergraduate laboratory
Authors: Libbrecht K. G. , Black E. D. , Hirata C. M.
Published: October 2003

We describe a basic experiment for the undergraduate laboratory that demonstrates aspects of both the science and the art of precision electronic measurements. The essence of the experiment is to measure the resistance of a small length of brass wire to high accuracy using a simple voltage divider and a lock-in amplifier. By performing the measurement at different frequencies and different drive currents, one observes various random noise sources and systematic measurement effects.
On perturbation theory in statistical mechanics
Authors: Fernández Francisco M.
Published: October 2003

We discuss the application of perturbation theory to statistical mechanics both in the operator form of Schwinger and in the scalar form as a sum over states. We briefly consider and compare the use of variational parameters in both approaches. By means of a simple model we show that Schwinger perturbation theory might fail when the eigenfunctions satisfy Dirichlet boundary conditions. On the other hand, the sum over states gives accurate results.
On Mach’s critique of Newton and Copernicus
Authors: Hartman Herbert I. , Nissim-Sabat Charles
Published: October 2003

By maintaining the relativity of all motion, especially rotational motion, Mach denied the existence of absolute motion and of absolute space. Accordingly, he maintained the equivalence of the Ptolemaic and the Copernican systems and the equivalence of rotating-system/fixed-universe and universe-rotating/fixed-system situations. An analysis of the Foucault pendulum shows that Mach’s relativity principle implies that there cannot be a fixed bucket in a rotating universe. Also, Mach’s views violate the physics that he espoused: noninertial experiments, for example stellar aberration and electromagnetic effects, distinguish between a rotating bucket in a fixed universe and a fixed bucket in a rotating universe, between a Copernican universe and a Brahean or Ptolemaic universe, and establish that one cannot ascribe all pertinent observations solely to relative motion between a system and the universe.
The emergence of semiconductors: Nineteenth century modern physics
Authors: Stahl Frieda A.
Published: October 2003

Volta’s invention of the battery at the end of the eighteenth century generated an era of research devoted to electrical conduction. Classical theory served well for currents in metals, but was contradicted by behavior observed in other materials. Such behavior was labeled anomalous and not understood. Similarities between such substances were known by about 1885; the term Halbleiter (semiconductors) first appeared in the literature in 1911. The twentieth century began with other discoveries that more dramatically contradicted classical theory and drove the development of quantum mechanics. In contrast, the earlier anomalies remained baffling until quantum mechanics could be applied. This converse relationship leads to questions about the coverage of history in modern physics courses.
How to hit home runs: Optimum baseball bat swing parameters for maximum range trajectories
Authors: Sawicki Gregory S. , Hubbard Mont , Stronge William J.
Published: October 2003

Improved models for the pitch, batting, and post-impact flight phases of a baseball are used in an optimal control context to find bat swing parameters that produce maximum range. The improved batted flight model incorporates experimental lift and drag profiles (including the drag crisis). An improved model for bat–ball impact includes the dependence of the coefficient of restitution on the approach relative velocity and the dependence of the incoming pitched ball angle on speed. The undercut distance and bat swing angle are chosen to maximize the range of the batted ball. The sensitivity of the maximum range is calculated for all model parameters including bat and ball speed, bat and ball spin, and wind speed. Post-impact conditions are found to be independent of the ball–bat coefficient of friction. The lift is enhanced by backspin produced by undercutting the ball during batting. An optimally hit curve ball will travel farther than an optimally hit fastball or knuckleball due to increased lift during flight.
Helping students develop an understanding of Archimedes’ principle. I. Research on student understanding
Authors: Loverude Michael E. , Kautz Christian H. , Heron Paula R. L.
Published: October 2003

This paper is the first of two that describe how research on student understanding of Archimedes’ principle is being used to guide the development of instructional materials on this topic. Our results indicate that standard instruction on hydrostatics leaves many science and engineering majors unable to predict and explain the sinking and floating behavior of simple objects. A number of serious and persistent difficulties with the concepts and principles used to analyze such behavior are identified. Although some of these difficulties are specific to the concept of the buoyant force, many others seem to reflect lingering confusion about concepts that are widely assumed to be understood by students before the study of hydrostatics begins.
The importance of weightlessness and tides in teaching gravitation
Authors: Galili I. , Lehavi Y.
Published: October 2003

We examine the presentation of the weight, weightlessness, and tides in university-level physics textbooks. Introductory textbooks often do not discuss tidal forces even though their understanding would be useful for understanding weightlessness. The explanations of tides often miss the free gravitational motion of both interacting objects, which is essential for the symmetry of tidal deformation. The shortcomings in the explanations of weightlessness and tides as provided by students and teachers are compared to textbook discussions. We suggest that an explicit discussion of the different definitions of weight and a synergetic presentation of weightlessness and tides might lead to a better understanding of gravitation. Our approach is illustrated by examples of tidal effects appropriate for introductory courses.
Monday Night Football: Physics Decides Controversial Call
Authors: DiLisi Gregory A. , Rarick Richard
Published: October 2003

This article describes the vector analysis of the projection of a three-dimensional event onto a two-dimensional surface. Specifically, the analysis examines a controversial call of a videotaped football play and the observational error created by the camera's projection of that play onto the football field.
A Triple Rainbow?
Authors: Hendry Archibald W.
Published: October 2003

It's a good question — is there such a thing as a triple rainbow? No doubt we've all admired spectacular rainbows, including some double rainbows. I must also admit that most times when I'm out watering the yard on a sunny day, I cannot help myself from trying to trace out the primary and secondary circular bows in the spray. But a triple rainbow?
Series and Parallel Resistors and Capacitors
Authors: Brown Ronald
Published: October 2003

The equations used to calculate the equivalent resistance or capacitance for series and parallel connections of resistors or capacitors, respectively, are often found to be confusing by students. But they are as they are for exactly the same reason. A battery transfers an amount of charge from one surface to the other proportionately to the area of the surfaces and inversely with their separation.
A Demonstration Apparatus for the Cartesian Diver
Authors: Güémez J. , Fiolhais C. , Fiolhais M.
Published: October 2003

The Cartesian diver is a nice toy and an intriguing physics instrument.1–6 Recently we reported an experimental study on the statics and dynamics of the Cartesian diver,7 using a specially designed apparatus that is much larger than the usual models. The Cartesian diver is an interesting example of the so-called “fold catastrophe,” the pressure being the control parameter,7 and this behavior is well observed in our apparatus.
Maximizing the Workshop Experience: An Example from the PTRA Rural Initiatives Program
Authors: Burns Teresa
Published: October 2003

Since the summer of 2001, the Rural Initiative of the Physics Teaching Resource Agents have led summer workshops for high school and middle school physics teachers at several universities nationwide. From the site at Coastal Carolina University where teachers learned about radioactivity and nuclear physics, assistant professor Teresa Burns describes how high school teacher Lucas Mullen was able to build a curriculum tied to South Carolina State Science Standards around the content he learned at the workshop. This is one example of how a workshop experience can lead to better teaching practices through exposure to content material and current accepted practices in physics education research.
Seeing More Clearly: Recent Advances in Understanding Retinal Circuitry
Authors: He Shigang , Dong Wei , Deng Qiudong , Weng Shijun , Sun Wenzhi
Published: October 2003

Among 10 breakthroughs that
Science
announced at the end of 2002 was the discovery of a photosensing (melanopsin-containing) retinal ganglion cell (RGC) and its role in entraining the circadian clock. This breakthrough exemplifies the ultimate goal of neuroscience: to understand the nervous system from molecules to behavior. Light-sensing RGCs constitute one of a dozen discrete RGC populations coding various aspects of visual scenes by virtue of their unique morphology, physiology, and coverage of the retina. Interestingly, the function of the melanopsin-containing RGCs in entraining the circadian clock need not involve much retinal processing, making it the simplest form of processing in the retina. This review focuses on recent advances in our understanding of retinal circuitry, visual processing, and retinal development demonstrated by innovative experimental techniques. It also discusses the advantages of using the retina as a model system to address some of the key questions in neuroscience.
Requirement of Mammalian
Timeless
for Circadian Rhythmicity
Authors: Barnes Jessica W. , Tischkau Shelley A. , Barnes Jeffrey A. , Mitchell Jennifer W. , Burgoon Penny W. , Hickok Jason R. , Gillette Martha U.
Published: October 2003

Despite a central circadian role in
Drosophila
for the transcriptional regulator
Timeless
(
dTim
), the relevance of mammalian
Timeless
(
mTim
) remains equivocal. Conditional knockdown of
mTim
protein expression in the rat suprachiasmatic nucleus (SCN) disrupted SCN neuronal activity rhythms, and altered levels of known core clock elements. Full-length
mTim
protein (mTIM-fl) exhibited a 24-hour oscillation, where as a truncated isoform (mTIM-s) was constitutively expressed. mTIM-fl associated with the mammalian clock
Period
proteins (mPERs) in oscillating SCN cells. These data suggest that
mTim
is required for rhythmicity and is a functional homolog of
dTim
on the negative-feedback arm of the mammalian molecular clockwork.
A Bayesian Networks Approach for Predicting Protein-Protein Interactions from Genomic Data
Authors: Jansen Ronald , Yu Haiyuan , Greenbaum Dov , Kluger Yuval , Krogan Nevan J. , Chung Sambath , Emili Andrew , Snyder Michael , Greenblatt Jack F. , Gerstein Mark
Published: October 2003

We have developed an approach using Bayesian networks to predict protein-protein interactions genome-wide in yeast. Our method naturally weights and combines into reliable predictions genomic features only weakly associated with interaction (e.g., messenger RNAcoexpression, coessentiality, and colocalization). In addition to de novo predictions, it can integrate often noisy, experimental interaction data sets. We observe that at given levels of sensitivity, our predictions are more accurate than the existing high-throughput experimental data sets. We validate our predictions with TAP (tandem affinity purification) tagging experiments. Our analysis, which gives a comprehensive view of yeast interactions, is available at
genecensus.org/intint
.
The Chemistry of Sexual Deception in an Orchid-Wasp Pollination System
Authors: Schiestl Florian P. , Peakall Rod , Mant Jim G. , Ibarra Fernando , Schulz Claudia , Franke Stephan , Francke Wittko
Published: October 2003

The “sexually deceptive” orchid
Chiloglottis trapeziformis
attracts males of its pollinator species, the thynnine wasp
Neozeleboria cryptoides
, by emitting a unique volatile compound, 2-ethyl-5-propylcyclohexan-1,3-dione, which is also produced by female wasps as a male-attracting sex pheromone.
Listeria
Intracellular Growth and Virulence Require Host-Derived Lipoic Acid
Authors: O'Riordan Mary , Moors Marlena A. , Portnoy Daniel A.
Published: October 2003

Listeria monocytogenes
is a Gram-positive intracytosolic pathogen that causes severe disease in pregnant and immunocompromised individuals. We found that
L. monocytogenes
lacking the lipoate protein ligase LplA1 was defective for growth specifically in the host cytosol and was less virulent in animals by a factor of 300. A major target for LplA1, the E2 subunit of pyruvate dehydrogenase (PDH), lacked a critical lipoyl modification when the Δ
lplA1
strain was grown intracellularly, which suggests that abortive growth was due to loss of PDH function. Thus, the use of host-derived lipoic acid may be a critical process for in vivo replication of bacterial pathogens.
New Buzz on the Humble-Bee
Authors: Heinrich Bernd
Published: October 2003

Bumblebees
Their Behaviour and Ecology. by Dave Goulson. Oxford University Press, Oxford, 2003. 253 pp. $97.50, £55. ISBN 0-19-852606-7. Paper, $45, £27.50. ISBN 0-19-852607-5.

In an attempt to draw public attention to the importance of conserving bumble-bee populations, Goulson offers an overview of our current knowledge of the bees' behavior and ecology and suggests topics for future research.
A Hybridization Model for the Plasmon Response of Complex Nanostructures
Authors: Prodan E. , Radloff C. , Halas N. J. , Nordlander P.
Published: October 2003

We present a simple and intuitive picture, an electromagnetic analog of molecular orbital theory, that describes the plasmon response of complex nanostructures of arbitrary shape. Our model can be understood as the interaction or “hybridization” of elementary plasmons supported by nanostructures of elementary geometries. As an example, the approach is applied to the important case of a four-layer concentric nanoshell, where the hybridization of the plasmons of the inner and outer nanoshells determines the resonant frequencies of the multilayer nanostructure.
Bonding Changes in Compressed Superhard Graphite
Authors: Mao Wendy L. , Mao Ho-kwang , Eng Peter J. , Trainor Thomas P. , Newville Matthew , Kao Chi-chang , Heinz Dion L. , Shu Jinfu , Meng Yue , Hemley Russell J.
Published: October 2003

Compressed under ambient temperature, graphite undergoes a transition at ∼17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the π-bonds between graphite layers convert to σ-bonds, whereas the other half remain as π-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form σ-bonds, whereas the nonbridging carbon atoms remain unpaired with π-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.
Nota Bene: Where the Medium Is (Literally) the Message
Authors: Jasny Barbara R.
Published: October 2003

RIKEN Mouse Genome Encyclopedia
DNA Book. Jun Kawai and Yoshihide Hayashizaki, Eds. RIKEN Genomic Sciences Center, Wako, Japan, 2003. 208 pp.
http://genome.gsc.riken.go.jp/DNA-Book

To distribute libraries of DNA samples to researchers, RIKEN's Genomic Sciences Center has printed ready-to-use spots of DNA onto the pages of a book.
King Corn
Authors: Crow James F.
Published: October 2003

George Beadle
An Uncommon Farmer The Emergence of Genetics in the 20th Century. by Paul Berg and Maxine Singer. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 2003. 393 pp. $35, £25. ISBN 0-87969-688-5.

Drawing on the family records, university archives at CalTech and Chicago, and interviews with a large number of their subject's colleagues and associates, Berg & Singer explore Beadle's roles in the early development of molecular genetics as well as in public policy and university administration.
Classifying Predicates and Languages
Authors: Smith Carl H. , Wiehagen Rolf , Zeugmann Thomas
Published: October 2003

The present paper studies a particular collection of classification problems, i.e., the classification of recursive predicates and languages, for arriving at a deeper understanding of what classification really is. In particular, the classification of predicates and languages is compared with the classification of arbitrary recursive functions and with their learnability. The investigation undertaken is refined by introducing classification within a resource bound resulting in a new hierarchy. Furthermore, a formalization of multi-classification is presented and completely characterized in terms of standard classification. Additionally, consistent classification is introduced and compared with both resource bounded classification and standard classification. Finally, the classification of families of languages that have attracted attention in learning theory is studied, too.
Hop-Congestion Trade-Offs for High-Speed Networks
Authors: Kranakis Evangelos , Krizanc Danny , Pelc Andrzej
Published: October 2003

Message transmission in ATM networks is via virtual paths. Packets are routed along virtual paths by maintaining a routing field whose subfields determine the intermediate destinations of the packet. In such a network it is important to construct path layouts that minimize the hop number (i.e. the number of virtual paths used to travel between any two nodes) as a function of edge-congestion (i.e. the number of virtual paths passing through a link). In this paper we construct asymptotically optimal virtual path layouts for chains and meshes.
Methods to Overlap Communications in Parallel Numerical Algorithms
Authors: Calvin C. , Colombet L. , Michallon Ph.
Published: October 2003

We present in this paper general techniques for overlapping communications in parallel numerical kernels. We describe first some dependencies schemes which can be found in most of numerical parallel algorithms and we apply on these schemes methods based on the change of the granularity of the computational tasks. The choice of the granularity in order to obtain a good overlap depends on the main parameters of the target machines. So we present results of benchmarks executed on two parallel distributed memory machines: a Cray T3D and an Intel Paragon. Then we apply the precedent techniques of overlapping on classical numerical kernels, namely: the matrix-vector and the matrix-matrix products and the mono and bi-dimensional FFT. We have implemented to the overlapped versions of these algorithms on a T3D and a Paragon and tuned the parameters of overlapping in order to minimize the total execution time. The results of these experiments demonstrate the accuracy of this approach.
The Complexity of Deciding Strictly Non-Blocking Concentration and Generalized-Concentration Properties
Authors: Dai H. K.
Published: October 2003

Concentrators and generalized-concentrators are interconnection networks that provide respectively pairwise vertex-disjoint directed paths and trees to satisfy interconnection requests. An interconnection network is non-blocking in the strict sense if every compatible interconnection request can be satisfied by a path regardless of any existing interconnections. We present polynomial time computational complexity results for deciding the strictly non-blocking concentration and generalized-concentration properties with small depth, by using b-matching techniques.
Index-Shuffle Graphs
Authors: Baumslag Marc , Obrenić Bojana
Published: October 2003

Index-shuffle graphs are introduced as candidate interconnection networks for parallel computers. The comparative advantages of index-shuffle graphs over the standard bounded-degree "approximations" of the hypercube, namely butterfly-like and shuffle-like graphs, are demonstrated in the theoretical framework of graph embedding and network emulations.

An N-node index-shuffle graph emulates:

• an N-node shuffle-exchange graph with no slowdown, which the currently best emulations of shuffle-like graphs by hypercubes and butterflies incur a slowdown of Ω( log N).

• its like-sized butterfly graph with a slowdown O( log log log N), while the currently best emulations of butterfly-like graphs by shuffle-like graphs incur a slowdown of Ω( log log N).

• an N-node hypercube that executes an on-line leveled algorithm with a slowdown O( log log N), while the slowdown of currently best such emulations of the hypercube by its bounded-degree shuffle-like and butterfly-like derivatives remains Ω( log N). Our emulation is based on an embedding of an N-node hypercube into an N-node index-shuffle graph with dilation O( log log N), while the currently best embeddings of the hypercube into its bounded-degree shuffle-like and butterfly-like derivatives incur a dilation of Ω( log N).
The Construction of Large Scale Reconfigurable Parallel Computing Systems (The Architecture of the SC320)
Authors: Monien Burkhard , Diekmann Ralf , Lüling Reinhard
Published: October 2003

Reconfigurable communication networks for massively parallel multiprocessor systems offer the possibility to realize a number of application demands like special communication patterns or real-time requirements. This paper presents the design principle of a reconfigurable network which is able to realize any graph of maximal degree four. The architecture is based on a special multistage Clos network, constructed out of a number of static routing switches of equal size. Upper bounds on the cut size of 4-regular graphs, if split into a number of clusters, allow minimizing the number of switches and connections while still offering the desired reconfiguration capabilities as well as large scalability and flexible multi-user access. Efficient algorithms configuring the architecture are based on an old result by Petersen27 about the decomposition of regular graphs.

The concept presented here is the basis for the Parsytec SC series of reconfigurable MPP-systems. The currently largest realization with 320 processors is presented in greater detail.
A Process Algebraic Approach to Time Granularity Semantics
Authors: Krishnan Padmanabhan
Published: October 2003

In this paper we develop a theory for timed systems which relate to time granularity. We use the well known equivalence of bisimulation to study the effect of changing granularity. We identify situations where measuring time more accurately has no effect on the equivalence. Similarly we also present a few situations where measuring time less accurately has no effect on the equivalence. We also present properties of the situations where the semantics is indeed altered by a change in time granularity.