12,637 research outputs found

    Ayudar a los alumnos a aprender cómo aprender. La opinión de un profesor-investigador

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    Four basic questions are approached in this paper: 1. What we know about how people learn. 2. How we can help students learn to learn. 3. What the obstacles are to helping students learn. 4. What the expectations are for the empowerment of people. The answers, based on the author's experience and on research done at Cornell University refer widely to significant learning, to heuristic learning and to the conceptual maps

    Pressure on charged domain walls and additional imprint mechanism in ferroelectrics

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    The impact of free charges on the local pressure on a charged ferroelectric domain wall produced by an electric field has been analyzed. A general formula for the local pressure on a charged domain wall is derived considering full or partial compensation of bound polarization charges by free charges. It is shown that the compensation can lead to a very strong reduction of the pressure imposed on the wall from the electric field. In some cases this pressure can be governed by small nonlinear effects. It is concluded that the free charge compensation of bound polarization charges can lead to substantial reduction of the domain wall mobility even in the case when the mobility of free charge carriers is high. This mobility reduction gives rise to an additional imprint mechanism which may play essential role in switching properties of ferroelectric materials. The effect of the pressure reduction on the compensated charged domain walls is illustrated for the case of 180-degree ferroelectric domain walls and of 90-degree ferroelectric domain walls with the head-to-head configuration of the spontaneous polarization vectors.Comment: subm. to PRB. This verion is extended by appendi

    Radius Dependent Luminosity Evolution of Blue Galaxies in GOODS-N

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    We examine the radius-luminosity (R-L) relation for blue galaxies in the Team Keck Redshift Survey (TKRS) of GOODS-N. We compare with a volume-limited, Sloan Digital Sky Survey sample and find that the R-L relation has evolved to lower surface brightness since z=1. Based on the detection limits of GOODS this can not be explained by incompleteness in low surface-brightness galaxies. Number density arguments rule out a pure radius evolution. It can be explained by a radius dependent decline in B-band luminosity with time. Assuming a linear shift in M_B with z, we use a maximum likelihood method to quantify the evolution. Under these assumptions, large (R_{1/2} > 5 kpc), and intermediate sized (3 < R_{1/2} < 5 kpc) galaxies, have experienced Delta M_B =1.53 (-0.10,+0.13) and 1.65 (-0.18, +0.08) magnitudes of dimming since z=1. A simple exponential decline in star formation with an e-folding time of 3 Gyr can result in this amount of dimming. Meanwhile, small galaxies, or some subset thereof, have experienced more evolution, 2.55 (+/- 0.38) magnitudes. This factor of ten decline in luminosity can be explained by sub-samples of starbursting dwarf systems that fade rapidly, coupled with a decline in burst strength or frequency. Samples of bursting, luminous, blue, compact galaxies at intermediate redshifts have been identified by various previous studies. If there has been some growth in galaxy size with time, these measurements are upper limits on luminosity fading.Comment: 34 Total pages, 15 Written pages, 19 pages of Data Table, 13 Figures, accepted for publication in Ap

    Aerodynamic test results of bicycle helmets in different configurations: Towards a responsive design

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    © IMechE 2019. Within the sport of cycling, aerodynamic efficiency is a fundamental criterion for equipment such as bicycle frames, wheels, clothing and helmets. Emerging technologies continually challenge the rules governing the sport as designers, engineers, sports scientists and athletes attempt to gain the edge on their competition. This study compares three-dimensional (3D) printed bicycle helmet prototypes with three commercially available helmets via aerodynamic testing in a wind tunnel. One 3D printed helmet featured a mechanical mechanism allowing two states of ventilation closure to be examined for aerodynamic efficiency, while the other featured electronically adjustable ventilation openings tested at five different states of ventilation closure. Data were collected using an anthropometrically accurate mannequin sitting atop a bicycle in a road-cycling position. The results found that the mechanically controlled prototype offered a 4.1% increase in overall drag experienced by the mannequin with ventilation in the open position compared to the closed position. The electronic prototype showed an increase in drag as ventilation openings increased through the five states, with an overall difference in drag of 3.7% between closed and the maximum opening. These experimental findings indicate that the responsive helmet prototypes can significantly affect the drag force on a cyclist between their closed and open positions and, when understood as being adaptable using sensors and automated controls, may provide new opportunities to modify athlete performance throughout varying stages of training and competition

    Efectos de las claras en la productividad forestal y características del sitio en rodales de Pinus sylvestris

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    A clear-cutting system with soil preparation before replanting is usual for Scots pine stands in many European countries. Additionally, thinning regimes are applied during the rotation. Thus, forest floor is recreated in each rotation and can be influenced by thinning regime. The present study aimed to determine possible effects of thinning on production (evaluated by basal area) and forest-floor status (evaluated by dry mass, carbon and nitrogen content) in pine stands. We used data from four experiments established in 1962 in 25- to 45-year-old pine stands. In 2008, we analysed forest-floor characteristics under the observed stands. The results from basal area evaluation showed different development in treatments on all experiments during and at the end of observation. We observed substantial, but statistically non-significant, differences between treatments in quantity of dry mass (and of carbon and nitrogen) accumulated in humus horizons under Scots pine stands more than 40 years after first thinning.En muchos países europeos es habitual un sistema de tala rasa con preparación del suelo antes de la plantación para pino silvestre. Además, se aplican regímenes de claras durante la rotación. Por lo tanto, el suelo forestal se recrea en cada rotación y puede ser influenciado por el régimen de claras. El presente estudio tuvo como objetivo determinar los posibles efectos de las claras en la producción (evaluada por el área basal) y el estado del suelo forestal (evaluado por peso seco, contenido de carbono y nitrógeno) en los rodales de pino. Se utilizaron datos de cuatro experimentos, iniciado en 1962 en rodales de 25 a 45 años de edad. En 2008, se analizaron las características de suelo forestal de los rodales. Los resultados de la evaluación de área basal mostraron un crecimiento diferente entre los tratamientos en todos los experimentos durante y al final del periodo de observación. Se han observado diferencias importantes, pero estadísticamente no significativas, entre tratamientos en la cantidad de masa seca (y de carbono y nitrógeno) en el horizonte de humus acumulado en los rodales de pino silvestre durante más de 40 años después de la primera entresaca

    Metal plasticity and fatigue at high temperature

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    In several industrial fields (such as automotive, steelmaking, aerospace, and fire protection systems) metals need to withstand a combination of cyclic loadings and high temperatures. In this condition, they usually exhibit an amount—more or less pronounced—of plastic deformation, often accompanied by creep or stress-relaxation phenomena. Plastic deformation under the action of cyclic loadings may cause fatigue cracks to appear, eventually leading to failures after a few cycles. In estimating the material strength under such loading conditions, the high-temperature material behavior needs to be considered against cyclic loading and creep, the experimental strength to isothermal/non-isothermal cyclic loadings and, not least of all, the choice and experimental calibration of numerical material models and the selection of the most comprehensive design approach. This book is a series of recent scientific contributions addressing several topics in the field of experimental characterization and physical-based modeling of material behavior and design methods against high-temperature loadings, with emphasis on the correlation between microstructure and strength. Several material types are considered, from stainless steel, aluminum alloys, Ni-based superalloys, spheroidal graphite iron, and copper alloys. The quality of scientific contributions in this book can assist scholars and scientists with their research in the field of metal plasticity, creep, and low-cycle fatigue

    Improved shaping approach to the preliminary design of low-thrust trajectories

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    This paper presents a general framework for the development of shape-based approaches to low-thrust trajectory design. A novel shaping method, based on a three-dimensional description of the trajectory in spherical coordinates, is developed within this general framework. Both the exponential sinusoid and the inverse polynomial shaping are demonstrated to be particular two-dimensional cases of the spherical one. The pseudoequinoctial shaping is revisited within the new framework, and the nonosculating nature of the pseudoequinoctial elements is analyzed. A two step approach is introduced to solve the time of flight constraint, related to the design of low-thrust arcs with boundary constraints for both spherical and pseudoequinoctial shaping. The solution derived from the shaping approach is improved with a feedback linear-quadratic controller and compared against a direct collocation method based on finite elements in time. The new shaping approach and the combination of shaping and linear-quadratic controller are tested on three case studies: a mission to Mars, a mission to asteroid 1989ML, a mission to comet Tempel-1, and a mission to Neptune
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