The dynamic impact characteristics of tennis balls with tennis rackets

Abstract

The dynamic properties of six types of tennis balls were measured using a force platform and high-speed digital video images of ball impacts on rigidly clamped tennis rackets. It was found that the coefficient of restitution reduced with velocity for impacts on a rigid surface or with a rigidly clamped tennis racket. Pressurized balls had the highest coefficient of restitution, which decreased by 20% when punctured. Pressureless balls had a coefficient of restitution approaching that of a punctured ball at high speeds. The dynamic stiffness of the ball or the ball-racket system increased with velocity and pressurized balls had the highest stiffness, which decreased by 35% when punctured. The characteristics of pressureless balls were shown to be similar to those of punctured balls at high velocity and it was found that lowering the string tension produced a smaller range of stiffness or coefficient of restitution. It was hypothesized that players might consider high ball stiffness to imply a high coefficient of restitution. Plots of coefficient of restitution versus stiffness confirmed the relationship and it was found that, generally, pressurized balls had a higher coefficient of restitution and stiffness than pressureless balls. The players might perceive these parameters through a combination of sound, vibration and perception of ball speed off the racket.     

Recognition of facial emotions among maltreated children with high rates of post-traumatic stress disorder

OBJECTIVE: The purpose of this study is to examine processing of facial emotions in a sample of maltreated children showing high rates of post-traumatic stress disorder (PTSD). Maltreatment during childhood has been associated independently with both atypical processing of emotion and the development of PTSD. However, research has provided little evidence indicating how high rates of PTSD might relate to maltreated children's processing of emotions. METHOD: Participants' reaction time and labeling of emotions were measured using a morphed facial emotion identification task. Participants included a diverse sample of maltreated children with and without PTSD and controls ranging in age from 8 to 15 years. Maltreated children had been removed from their homes and placed in state custody following experiences of maltreatment. Diagnoses of PTSD and other disorders were determined through combination of parent, child, and teacher reports. RESULTS: Maltreated children displayed faster reaction times than controls when labeling emotional facial expressions, and this result was most pronounced for fearful faces. Relative to children who were not maltreated, maltreated children both with and without PTSD showed enhanced response times when identifying fearful faces. There was no group difference in labeling of emotions when identifying different facial emotions. CONCLUSIONS: Maltreated children show heightened ability to identify fearful faces, evidenced by faster reaction times relative to controls. This association between maltreatment and atypical processing of emotion is independent of PTSD diagnosis.     

A preliminary report on the role of yoga asanas on oxidative stress in non-insulin dependent diabetes mellitus

Nineteen subjects of non-insulin dependent diabetes mellitus (NIDDM) between the age group of 30–60 yrs were studied to see the effect of specific yoga asanas on fasting and postprandial blood glucose (FBG, PPG), serum malondialdehyde (MDA) and glycosylated hemoglobin (HbA₁) in addition to drug treatment and diet control. The duration of diabetes ranged from 1–10 years. Patients with renal, cardiac and proliferative retinal diseases were excluded from the study. The same patients served as their own control. Subjects were called in the morning to the cardio-respiratory laboratory and were given training by a yoga expert. Yoga asanas included Suryanamskar, Tadasan, TriKonasan, Padmasan, Pranayam, Paschimottanasan, Ardhmatsyendrasan, Pavanmukthasan, Sarpasan and Shavasan. The asanas were done every day for 40 days for 30–40 min. FBG, PPG, serum MDA and HbA₁ were estimated before and after 40 days of yoga asanas regimen. Significant reduction was seen in FBG from 220 mg/dl to 162 mg/dl, PPG from 311 mg/dl to 255 mg/dl, MDA from 6 nmol/l to 3 nmol/l and HbA₁, from 8.8% to 6.4%. Subjects felt better and were relieved of their stresses and had an improvement in their day to day performance. The decrease was statistically significant (p<0.0001 for FBG and PPG, p<0.001 for MDA and for HbA₁).     

What’s mine is mine; what’s yours is mine: private ownership of ICTs as a threat to transparency

In the face of ubiquitous information communication technology, the presence of blogs, personal websites, and public message boards give the illusion of uncensored criticism and discussion of the ethical implications of business activities. However, little attention has been paid to the limitations on free speech posed by the control of access to the Internet by private entities, enabling them to censor content that is deemed critical of corporate or public policy. The premise of this research is that transparency alone will not achieve the desired results if ICT is used in a one way system, controlled by the provider of information. Stakeholders must have an avenue using the same technology to respond to and interact with the information. We propose a model that imposes on corporations a public trust, requiring these gatekeepers of communication technology to preserve individual rights to criticism and review.     

冠军由基因决定吗?

本文要点: ◆遗传型是机体全部遗传基因的总和,代表了个体的遗传潜力.在决定个体的解剖、生化、生理、行为等特性或遗传表型(如褐色的眼睛、75kg的体重、50 ml/kg/min的最大摄氧量等)上起主导作用.     

Problem Solving, Contention, and Struggle: How Siblings Resolve a Conflict of Interests

In a laboratory setting, 48 sibling dyads age 4 and 6 or 6 and 8 years negotiated the division of six toys. Findings revealed that, in general, children reached divisions while using a preponderance of constructive problem-solving strategies, rather than contentious tactics. The degree of conflict of interests and the quality of sibling relationships predicted the children's use of problem-solving and contentious negotiation strategies, and was related to the types of resolutions achieved. Dyads experiencing low conflict of interests resolved their differences quickly. High conflict of interests coupled with positive relationships and constructive negotiation resulted in longer negotiations and creative, agreeable resolutions. High conflict of interests coupled with more negative relationships and destructive negotiations resulted in children's failures to reach agreement. Developmental differences indicated that older siblings within the pairs took the lead in negotiation, and benefited slightly more from the divisions. Furthermore, children in older dyads were more sophisticated and other oriented in their negotiations.     

Economics of cultural tourism: issues and perspectives

The special issue aims at exploring, with an economic perspective, the interconnections between cultural participation, in all its expressions, and tourism organization and patterns with the purpose of understanding economic effects, emerging trends and policy implications. The expanding notion of the cultural consumption of tourists makes the definition of cultural tourism increasingly elusive. Empirical investigations of the relationships between cultural participation and cultural heritage and tourism offer interesting hints in many directions. This introduction briefly overviews the premise of this special issue, the literature and the several perspectives taken by the included articles. Aside from their cultural topics—general, intangible or temporary—these essays all tackle some important economic dimensions of tourism. We encourage cultural economists to invest more in these fascinating areas as more than just intellectual tourists.