老龄化:运动如何缓解老龄化?

The aging of the population is one of the most signifi-cant challenges of the 21stcentury .It has been esti mated thatby the year 2025,the number of people above age 60 in allthe countries in the world will exceed one billion.Life ex-pectancy continues to…     

Bending-torsional flutter of a cantilevered wing containing a tip mass and subjected to a transverse follower force

This paper is a study of the bending-torsional flutter of a cantilevered wing subjected to a follower force, and containing a lumped mass, at the free end. In addition, a distributed aerodynamic loading is introduced along the wing. This results in a set of nonself-adjoint differential equations with variable, complex coefficients whose solutions are obtainable only in series form. Using the Frobenius method, a direct procedure is employed which retains the exact expression of the Theodorsen function and the unknown coefficients are evaluated on a computer which numerically converge to any prescribed accuracy. It is found that, as a result of the interaction of the two sources of non-conservative loadings, the follower force reduces considerably the critical speed of flows in all cases studied. An increase in the tip mass, however, has a stabilizing influence. The effect of structural damping is also examined and it is shown that internal damping forces may have pronounced influences on the flutter speed of the system.     

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Institut national d'Etude du travail et d'Orientation professionnelle

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UNESCO     

The nature of static friction

The factors concerned in friction are discussed, together with a review of the limitations of the various approaches heretofore used. A more satisfactory view of the basic cause of static friction is developed in relation to modern experimental knowledge by considering the pertinent factors in systems having ideal surfaces, then relating this discussion to conditions in the best obtainable practical situation.     

Forward acceleration of the centre of mass during ski skating calculated from force and motion capture data

The purpose of this paper was to present and evaluate a methodology to determine the contribution of bilateral leg and pole thrusts to forward acceleration of the centre of mass (COM) of cross-country skiers from multi-dimensional ground reaction forces and motion capture data. Nine highly skilled cross-country (XC) skiers performed leg skating and V2-alternate skating (V2A) under constant environmental conditions on snow, while ground reaction forces measured from ski bindings and poles and 3D motion with high-speed cameras were captured. COM acceleration determined from 3D motion analyses served as a reference and was compared to the results of the proposed methodology. The obtained values did not differ during the leg skating push-off, and force–time curves showed high similarity, with similarity coefficients (SC) >0.90 in the push-off and gliding phases. In V2A, leg and pole thrusts were shown to contribute 35.1 and 65.9% to the acceleration of the body, respectively. COM acceleration derived from ground reaction forces alone without considering the COM position overestimated the acceleration compared to data from motion analyses, with a mean difference of 17% (P < 0.05) during leg push-off, although the shapes of force–time curves were similar (SC = 0.93). The proposed methodology was shown to be appropriate for determining the acceleration of XC skiers during leg skating push-off from multi-dimensional ground reaction forces and the COM position. It was demonstrated that both the COM position and ground reaction forces are needed to find the source of acceleration.     

Finding a Better Person-Environment Fit in the Academic Deanship

Individuals seek a balance between the demands of their institutions and the dimensions that make up their personalities and determine their needs (Getzels & Guba, 1957). Academic deans are no exception. Indeed, the fit (or lack of it) between the work environment and the unique characteristics of individuals asked to function within a given setting helps determine whether they experience levels of stress that move them forward in their work or suffer debilitating stress that leads to excessive physical and mental strain. This article examines the fit between academic deans and their work environments in the U.S. and Australia as it relates to multiple role stress factors affecting deans.     

The effect of national culture on partner buyouts in cross-border biotechnology alliances

This study examines how national culture influences the likelihood and rate of buyouts among R&D equity alliances and joint ventures in the biotechnology industry. We hypothesize that the interaction of specific national culture attributes and cultural differences between alliance partners have an impact on: (1) the amount of endogenous uncertainty surrounding the potential integration of the target firm and (2) the marginal rate of learning in hierarchical versus collaborative governance. Applying a competing hazard model to a sample of 173 joint ventures and minority equity collaborations in the biotechnology industry, we found that investing firms from high power distance and high uncertainty avoidance countries are more likely to buy out their alliance partners. Furthermore, greater cultural distance between alliance partners increases the likelihood of partner buyout when investing firms are from high power distance countries.     

Dosuperior teachers employ systematic instructional planning procedures? A descriptive study

The purpose of this study was to examine the extent to which a group of superior elementary and secondary school teachers employed systematic instructional planning practices. The research method employed was a multicase design in which nine teachers (five secondary and four elementary) from a single school district were studied. Qualitative data in the form of written surveys and face-to-face interviews were collected and analyzed. Results revealed that most of the teachers' planning activities centered around six types of decisions. When the teachers' planning activities were compared with a systematic approach to instructional planning, the two approaches were found to be different in several important ways: the teachers did not place much emphasis on specifying objectives, creating objectives-based tests, or making other instructional decisions in light of objectives. Implications for teaching preservice teachers about systematic planning are discussed. The authors would like to thank the nine teachers who participated in this study, and Julia O'Brien, who helped to conduct it.