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41.
Describing the plastic deformation of aluminium softball bats 总被引:1,自引:1,他引:0
Hollow aluminium bats were introduced over 30 years ago to provide improved durability over wooden bats. Since their introduction,
however, interest in hollow bats has focused almost exclusively around their hitting performance. The aim of this study was
to take advantage of the progress that has been made in predicting bat performance using finite elements and apply it to describe
bat durability. Accordingly, the plastic deformation from a ball impact of a single-wall aluminum bat was numerically modelled.
The bat deformation from the finite-element analysis was then compared with experiment using a high-speed bat test machine.
The ball was modelled as an isotropic, homogeneous, viscoelastic sphere. The viscoelastic parameters of the ball model were
found from instrumented, high-speed, rigid-wall ball impacts. The rigid-wall ball impacts were modelled numerically and showed
good agreement with the experimentally obtained response. The strain response of the combined bat-ball model was verified
with a strain-gauged bat at intermediate ball impact speeds in the elastic range. The strain response of the bat-ball model
exhibited positive correlation with the experimental measurements. High-speed bat-ball impacts were performed experimentally
and simulated numerically at increasing impact speeds which induced correspondingly increased dent sizes in the bat. The plastic
deformation from the numerical model found good agreement with experiment provided the aluminium work hardening and strain
rate effects were appropriately described. The inclusion of strain rate effects was shown to have a significant effect on
the bat deformations produced in the finite-element simulations. They also helped explain the existence of high bat stresses
found in many performance models. 相似文献
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Background: Demonstration is a widely used method in sports teaching and coaching, based on the assumption that it is more beneficial than verbal instructions or trial-and-error methods for skill acquisition. Although in teaching/coaching situations, the demonstration is usually carried out in front of the learners, in a research context, it is most often presented via a video. However, a direct comparison between these two types of model has rarely been undertaken in a motor context.Purpose: In this study, we aimed to compare the effectiveness of the observation of a live and a videomodel for the early acquisition of a complex judo movement.Research Design: Participants observed either a live or a videomodel executing the task. After observation, they practised for three minutes taking five trials and then performed it for analysis. This procedure was repeated three times. The form and technique of each participant's execution were evaluated using a technical score.Main results: The results indicated a significant improvement in the task execution by the end of the practice session. However, this improvement occurred only for the video-model group between the second and third blocks of practice.Conclusions: The video demonstration seems more effective than the live one for the early acquisition of a completely new complex coordination. This may be due to the simplification of the visual information in the former condition because of its two-dimensionality. This simplification may allow the observer to identify the more key elements that would guide him/her for the subsequent performance of the task. 相似文献
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Prenatal Caloric Intake and the Development of Academic Achievement Among U.S. Children From Ages 5 to 14 下载免费PDF全文
Few studies have examined the relation between maternal caloric intake during pregnancy and growth in child academic achievement while controlling for important confounding influences. Using data from the National Longitudinal Survey of Youth, the current study examined the effects of reduced prenatal caloric intake on growth in scores on the Peabody Individual Achievement Test from ages 5 to 14. While models controlling for within‐family covariates showed that prenatal caloric intake was associated with lower reading and mathematical achievement at age 5, models controlling for between‐family covariates (such as maternal IQ) and unobserved familial confounders revealed only a statistically significant association between siblings differentially exposed to prenatal caloric intake and mathematical achievement at age 5. 相似文献
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Jenny Olin Shanahan Elizabeth Ackley-Holbrook Eric Hall Kearsley Stewart Helen Walkington 《Mentoring & Tutoring: Partnership in Learning》2015,23(5):359-376
This paper identifies salient practices of faculty mentors of undergraduate research (UR) as indicated in the extensive literature of the past two decades on UR. The well-established benefits for students involved in UR are dependent, first and foremost, on high-quality mentoring. Mentorship is a defining feature of UR. As more and different types of colleges and universities strive to meet student demand for authentic scholarly experiences, it is imperative to identify what effective UR mentors do in order to ensure student engagement, quality enhancement, retention, and degree-completion. We offer an original analysis of the literature on UR mentoring in which we identify 10 significant “lessons learned,” or evidence-based practices of effective UR mentors that apply broadly across disciplines, students, institutions, and mentoring approaches. 相似文献