Comparing the pre- and post-impact ball and racquet kinematics of elite tennis players' first and second serves: a preliminary study

The aim of this study was to compare the pre- and post-impact three-dimensional kinematics of the ball and racquet during first and second serves performed by elite tennis players. Data were collected from four male and four female right-handed professional players during competition using two high-speed cameras (200 Hz). For each player, one first serve and one second serve from the 'deuce' or right service court that landed within the specified target area were analysed. To test for significant differences between the first and second serves, Wilcoxon tests (P < or = 0.05) were performed on selected parameters. The results indicate that the ball travelled forward and to the left during the flight phase of the toss in all but one trial. The average pre-impact ball forward location for the first serve was significantly more in front and had a higher associated forward ball velocity than the corresponding values for the second serve. On average, the decrease in post-impact ball speed from the first to the second serve was 24.1%. No significant differences between the first and second serves were found in the pre-impact racquet head speed and orientation, which was represented as a unit vector perpendicular to the racquet face. The major adjustments made by the players when going from the first to second serve were a decrease in pre-impact ball forward location (P < or = 0.01) and an increase in the pre-impact racquet vertical and lateral velocities (both P < or = 0.05). This implies that the players tossed the ball closer to the body and imparted topspin and sidespin on the ball by changing the racquet vertical and lateral velocities when going from the first to the second serve.     

School students’ confidence when answering diagnostic questions online

Educational Studies in Mathematics - In this paper, we analyse a large, opportunistic dataset of responses (N = 219,826) to online, diagnostic multiple-choice mathematics questions, provided by...     

Equal Treatment and Cultural Difference in Multi‐ethnic Schools: a critique of the teacher ethnocentrism theory

ABSTRACT

This paper considers claims that students from minority ethnic backgrounds are treated less favourably than their white peers in schools because their teachers make negative, ethnocentric judgements of their behaviour. The paper challenges the empirical basis of these claims and the implicit model of good practice on which they are based. It goes on to examine some of the implications for teacher practice of this discussion. The aim is to try to clarify the meaning of equality of treatment in schools, a right we expect all citizens to enjoy, and to examine the way this principle should be operationalised by teachers in multi‐ethnic schools.     

A comparison of the perceptions of Love, Anger, and Guilt in Portugal and the United States and implications for counseling

This study compared the ways that graduate and undergraduate counseling and psychology students from Portugal and the United States evaluated the feelings of Love, Anger and Guilt. Utilizing a semantic differential, a multivariate analysis of variance was employed in which the independent variables were Country and Gender and the dependent variables were the evaluative and potency scales of Love, Anger and Guilt. There was no significant main effect for gender and there was no significant interaction effect for gender by country. There was a significant main effect for country. It was found that students from the United States evaluated Anger more positively than the Portuguese students and that the Portuguese students evaluated Guilt more positively than the United States students. These findings have implications that are relevant to the counseling process.     

复杂系统、知识管理与组织学习过程动态模型研究

知识经济时代的组织是一个具有无尺度网络结构的复杂系统。知识管理是知识经济时代的必然要求。组织学习作为组织获取核心竞争力和可持续发展能力的重要手段，是一个动态的过程。本文在系统回顾相关研究成果的基础上，从复杂系统的视角对知识经济时代复杂环境下的组织学习动态复杂过程进行了系统的模型整合和创新，并对构建学习型组织可能遇到的障碍进行了一系列对策分析。