Permissible control of general constrained mechanical systems     

Aaron D. Schutte 《Journal of The Franklin Institute》2010,347(1):208-227

This paper develops a unified approach for modeling and controlling mechanical systems that are constrained with general holonomic and nonholonomic constraints. The approach conceptually distinguishes and separates constraints that are imposed on the mechanical system for developing its physical structure between constraints that may be used for control purposes. This gives way to a general class of nonlinear control systems for constrained mechanical systems in which the control inputs are viewed as the permissible control forces. In light of this view, a new and simple technique for designing nonlinear state feedback controllers for constrained mechanical systems is presented. The general applicability of the approach is demonstrated by considering the nonlinear control of an underactuated system.  相似文献   

Evaluation of the "under-21 rule": do young adult soccer players benefit?     

Vaeyens R  Coutts A  Philippaerts RM 《Journal of sports sciences》2005,23(10):1003-1012

Young adult soccer players often encounter difficulty progressing from youth competition to being regularly selected for high-level, open-age competition. In an attempt to increase the playing opportunities of semi-professional and amateur under-21 players in first teams, the Royal Belgian Football Association required national division teams to include at least two players younger than 21 in their match selection (under-21 rule). Over four seasons, the following variables were analysed across 2138 semi-professional and amateur soccer players aged 16-39 years: (1) the number of times a player was selected to be in the first team squad; (2) the number of times a player was selected to play in the starting line-up; and (3) the number of minutes played. Chi-square analysis demonstrated that second and third division teams had complied with the new selection regulations. However, two-way analysis of variance of individual player data revealed no increase in the number of playing minutes in the under-21 group. It would appear that many teams had complied with the new regulations by selecting young players as substitutes. The results highlight the difficulties that talented young soccer players experience progressing from youth to senior competition. We conclude that the rule imposed by the Royal Belgian Football Association has failed to improve the playing opportunities of young adult soccer players. Alternative strategies for increasing playing opportunities for young talented players are required.  相似文献   

Comment on Roper’s discussion of the language and generality of schedule-induced behavior     

Cora Lee Wetherington  Aaron J. Brownstein 《Learning & behavior》1982,10(4):537-539

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Friendship and the Public Stage: Revisiting Hannah Arendt's Resistance to “Political Education”          下载免费PDF全文

Aaron Schutz  Marie G. Sandy 《Educational theory》2015,65(1):21-38

Hannah Arendt's essays about the 1957 crisis over efforts of a group of youth, the “Little Rock Nine,” to desegregate a high school in Little Rock, Arkansas, reveal a tension in her vision of the “public.” In this article Aaron Schutz and Marie Sandy look closely at the experiences of the youth desegregating the school, especially those of Elizabeth Eckford, drawing upon them to trace a continuum of forms of public engagement in Arendt's work. This ranges from arenas of “deliberative friendship,” where unique individuals collaborate on common efforts, to a more conflictual “public stage,” where groups act in solidarity to change aspects of the public world. While Arendt famously asserted in her essay “The Crisis in Education” that political capacities should not be taught in schools, it makes more sense to see this argument as focused on what she sometimes called the conflictual “public stage,” reflecting the experience of the Little Rock Nine. In contrast, Schutz and Sandy argue that Arendt's own work implies that “deliberative friendship,” as described in her essay “Philosophy and Politics” and elsewhere, should be part of everyday practices in classrooms and schools.  相似文献   

Droplet confinement and leakage: Causes,underlying effects,and amelioration strategies     

Aaron P. Debon  Robert C. R. Wootton  Katherine S. Elvira 《Biomicrofluidics》2015,9(2)

The applicability of droplet-based microfluidic systems to many research fields stems from the fact that droplets are generally considered individual and self-contained reaction vessels. This study demonstrates that, more often than not, the integrity of droplets is not complete, and depends on a range of factors including surfactant type and concentration, the micro-channel surface, droplet storage conditions, and the flow rates used to form and process droplets. Herein, a model microfluidic device is used for droplet generation and storage to allow the comparative study of forty-four different oil/surfactant conditions. Assessment of droplet stability under these conditions suggests a diversity of different droplet failure modes. These failure modes have been classified into families depending on the underlying effect, with both numerical and qualitative models being used to describe the causative effect and to provide practical solutions for droplet failure amelioration in microfluidic systems.  相似文献   

The effects of browse time on the Internet on students’ essay scores     

Kim Doan  Aaron Bloomfield 《TechTrends》2014,58(6):63-72

This study examined how 30 minutes of search time on the Web affected students’ essay scores in response to a writing prompt. Expository essays were obtained from 49 fourth- and fifthgrade students enrolled in an elementary school in Virginia, in the United States. Students were placed by random assignment into three groups with the same writing prompt for all three groups. Data analyses using ANOVA indicate that there was statistical significance in two components of writing. The group that received instruction on using the Internet to conduct research in the prewriting phase outperformed the control group in two areas: the total essay score (p=.053) and usage/mechanics (p=.028). The study also produced effect sizes ranging from small to large when the Internet users were compared to the control group. Implications for the future of Internet use in the school system are discussed.  相似文献   

Partisan Media and Discussion as Enhancers of the Belief Gap     

Aaron S. Veenstra  Mohammad Delwar Hossain  Benjamin A. Lyons 《Mass Communication and Society》2014,17(6):874-897

This study examines the roles of partisanship, partisan media use, and political discussion in the development of belief gaps. Using national survey data, we construct models of political identity, media use, and discussion factors predicting beliefs on five contested political issues and find that ideology and partisanship are generally stronger predictors of beliefs than is education. Notably, each has independent effects on belief outcomes. Contrary to some concerns that the Internet especially promotes partisan clustering, use of partisan traditional media—television and radio—is by far the strongest information-related predictor of belief outcomes, whereas partisan social media use and partisan discussion are relatively weak and inconsistent. These findings suggest that political elites continue to exert significant influence over the perceptions of rank and file partisans.  相似文献   

Stereoscopy in Static Scientific Imagery in an Informal Education Setting: Does It Matter?     

C. Aaron Price  H.-S. Lee  K. Malatesta 《Journal of Science Education and Technology》2014,23(6):721-734

Stereoscopic technology (3D) is rapidly becoming ubiquitous across research, entertainment and informal educational settings. Children of today may grow up never knowing a time when movies, television and video games were not available stereoscopically. Despite this rapid expansion, the field’s understanding of the impact of stereoscopic visualizations on learning is rather limited. Much of the excitement of stereoscopic technology could be due to a novelty effect, which will wear off over time. This study controlled for the novelty factor using a variety of techniques. On the floor of an urban science center, 261 children were shown 12 photographs and visualizations of highly spatial scientific objects and scenes. The images were randomly shown in either traditional (2D) format or in stereoscopic format. The children were asked two questions of each image—one about a spatial property of the image and one about a real-world application of that property. At the end of the test, the child was asked to draw from memory the last image they saw. Results showed no overall significant difference in response to the questions associated with 2D or 3D images. However, children who saw the final slide only in 3D drew more complex representations of the slide than those who did not. Results are discussed through the lenses of cognitive load theory and the effect of novelty on engagement.  相似文献   

Introduction of Molecular Methods into a Food Microbiology Curriculum     

Aaron M. Pleitner  Susan R. Hammons  Emily McKenzie  Young‐Hee Cho  Haley F. Oliver 《Journal of Food Science Education》2014,13(4):68-76

Maintaining current, relevant curriculum in undergraduate Food Microbiology courses is essential for training future experts in food quality and safety. Having an understanding of the fundamental techniques (for example, polymerase chain reaction [PCR]) that are used in the food industry and regulatory agencies is critical for students entering the workforce. The purpose of this study was to assess the efficacy of integrating molecular methods into an undergraduate Food Microbiology course in both lecture and laboratory settings. Modules on PCR and pulsed‐field gel electrophoresis (PFGE), both of which are currently used by government agencies and the food industry to investigate the presence and persistence of foodborne pathogens, were developed, introduced, and evaluated among 269 students over 4 y. Multiple teaching and learning styles were incorporated through (i) traditional lecture format on the basics of PCR and PFGE; (ii) hands‐on group activities to build upon the lecture instruction; (iii) performing PCR and PFGE in the laboratory; and (iv) group discussions to analyze results from laboratory exercises. Pre‐ and postinstruction evaluations revealed significant increases in understanding and application of both methods in lecture and laboratory settings as demonstrated by 0.60 and 0.51 mean normalized gains for respective PCR and PFGE lectures and 0.50 and 0.56 mean normalized gains in respective labs. Academic year significantly impacted score improvement, potentially due to hidden factors, such as previous exposure to material and student aptitude. This study provides the platform for successful introduction of molecular techniques in an undergraduate Food Microbiology course. The guidelines and materials developed by our group are publicly available for use by other institutions.  相似文献   

A comparison of the activity demands of elite and sub-elite Australian men's basketball competition   总被引：2，自引：2，他引：0  

Scanlan A  Dascombe B  Reaburn P 《Journal of sports sciences》2011,29(11):1153-1160

The purpose of the study was to describe the differences in the activity demands of sub-elite and elite Australian men's basketball competition. Ten elite (age 28.3 ± 4.9 years, mass 97.0 ± 13.9 kg, height 197.4 ± 8.3 cm) and 12 sub-elite (age 26.1 ± 5.3 years, mass 85.9 ± 13.2 kg, height 191.4 ± 7.6 cm) Australian basketball players participated in the study. Player activity was analysed using video-based time-motion analysis across multiple in-season matches. Customized analytical software was used to calculate player activity into frequencies, mean and total durations (s), and mean and total distances (m) for standing/walking, jogging, running, sprinting, low shuffling, high shuffling, and dribbling movements. Only movement frequency was calculated for jumping and upper body activity. Multivariate analysis of variance revealed that elite players performed significantly more total movement changes (P <0.001), and experienced greater activity workloads while jogging (P <0.01) and running (P <0.002). In contrast, sub-elite players performed significantly more standing/walking (P <0.023) and sprinting (P <0.003) activities. These data suggest that elite basketball competition requires a greater intermittent workload and more sustained activity demands, whereas sub-elite competition may involve greater bursts of activity and longer recovery periods. These differences are likely to reflect variations in player skill and fitness, as well as playing structure between playing standards.  相似文献