对随机变量的k-阶矩的讨论

文章从随机变量k-阶矩的定义出发,结合已有的定理推导出一些不等式,进而说明一些相关随机变量k-阶矩存在的条件。     

新型客车磁流变缓速器制动力计算与仿真

随着汽车工业的进步,商用汽车逐步向高速及大型化发展,制动安全问题日益突出。安装辅助制动装置是实现大型车辆安全制动的重要途径之一。文章分析传统电涡流缓速器及液力缓速器优缺点,设计一种新型的单盘型客车磁流变缓速器。该缓速器在挤压-剪切混合模式下工作;以磁流变液（MRF）的流变特性为基础,推导该缓速器制动力矩计算公式,并在MATLAB环境下对该制动力矩公式进行仿真和分析,结果显示客车磁流变缓速器制动力矩主要与磁场强度大小有关,与转速和磁流变液间隙厚度关系很小。     

国家优秀女子速滑运动员直道滑行中下肢功率情况分析

为了有效评价运动员下肢各环节在蹬冰过程中的贡献情况,借助爱捷三维录像解析系统和CY-BEXNORM等动测试装置获得国家健将级女子速滑运动员的运动学和动力学数据,并采用牛顿差值计算方法计算出与实际运动学数据相对应的关节力矩,在此基础上计算出下肢各环节在蹬冰时功率输出值。研究结果表明:髋关节在单支撑阶段适当伸展,有利于增加整个蹬冰阶段的大腿功率输出;高速度、大角度伸膝肌群肌力差,引起两人小腿峰值功率过早出现,减少双支撑蹬冰阶段功率输出;踝关节肌力不足,刻意展踝,加重了伸小腿肌力不足现象,减少小腿在整个蹬冰过程中的作用。     

个体风险模型总理赔额的数字特征、分布及其近似

研究了个体风险模型总理赔额的数字特征,利用矩母函数及母函数求其精确分布,给出了总理赔额的多种近似方法,并举例比较分析近似方法的优劣,给出了模拟程序.     

The influence of variable resistance moment arm on knee extensor performance

Abstract

To enhance muscular strength, resistance training machines with a cam, incorporating a variable resistance moment arm, are widely used. However, little information is available about the influence of the variable resistance moment arm on torque, velocity, and power during muscle contraction. To address this, a knee extensor machine was equipped with a cam or with a semi-circular pulley that imposed a variable or a constant resistance moment arm, respectively. Fourteen physically active men performed two full knee extensions against loads of 40–80 kg in both conditions. Participants developed significantly higher torque with the pulley than with the cam (P < 0.001). The relative differences between pulley and cam conditions across all loads ranged from 8.72% to 19.87% (P < 0.001). Average knee extension velocity was significantly higher in the cam condition than in the pulley condition. No differences were observed in average and peak power, except at 50 and 55 kg. Torque–velocity and power–velocity relationships were modified when the resistance moment arm was changed. In conclusion, whatever the link, namely cam or pulley, the participants produced similar power at each load. However, the torque–velocity and power–velocity relationships were different in the cam and pulley conditions. The results further suggest that the influence of the machine's mechanism on muscular performance has to be known when prescribing resistance exercises.     

Segmental dynamics of soccer instep kicking with the preferred and non-preferred leg

Abstract

Detailed time-series of the resultant joint moments and segmental interactions during soccer instep kicking were compared between the preferred and non-preferred kicking leg. The kicking motions of both legs were captured for five highly skilled players using a three-dimensional cinematographic technique at 200 Hz. The resultant joint moment (muscle moment) and moment due to segmental interactions (interaction moment) were computed using a two-link kinetic chain model composed of the thigh and lower leg (including shank and foot). The mechanical functioning of the muscle and interaction moments during kicking were clearly illustrated. Significantly greater ball velocity (32.1 vs. 27.1 m · s^?1), shank angular velocity (39.4 vs. 31.8 rad · s^?1) and final foot velocity (22.7 vs. 19.6 m · s^?1) were observed for the preferred leg. The preferred leg showed a significantly greater knee muscle moment (129.9 N · m) than the non-preferred leg (93.5 N · m), while no substantial differences were found for the interaction moment between the two legs (79.3 vs. 55.7 N · m). These results indicate that the highly skilled soccer players achieved a well-coordinated inter-segmental motion for both the preferred and non-preferred leg. The faster leg swing observed for the preferred leg was most likely the result of the larger muscle moment.     

三线摆测量刚体转动惯量及其误差分析

本文研究了三线摆法测量刚体转动惯量的误差影响因素并对误差的来源和原因进行了分析,在此基础上对实验操作提出了一些改进的方法,以提高实验的精度。     

定轴转动刚体轴承附加压力的计算

机器飞轮转动时,轴承可能受到一定的附加压力。文章通过均质圆盘转动实例,根据质心运动定理和对固定点的角动量定理,给出一种较为简单的计算这种压力的方法,在此基础上指出了改变这种压力的方法。     

楔形体在楔面受一段均布弯矩作用的弹性应力解

用应力函数及其导数在弹性体边界上的力学意义和海维谷函数的方法，对棋形体在楔面承受一段均布湾知作用的问题进行了受力分析，并得出了其弹性应力解．     

“類”与“禷”祭

《诗经》、《尚书》、《周礼》、《礼记》等经籍提到的“禷”祭(又作“類”祭),是上古中国一项有着深厚民俗文化背景和深远历史影响的祭仪。不幸其礼早亡,汉儒已经难究其详。影响及今的《尚书》古文家说和郑玄《礼记》注等,将“禷”祭解为祭天之礼。但根据经籍中的记载来看,“禷”祭指的是当国有大事时,向上帝或社稷宗庙祈福禳灾的一种“祷祈”仪式,与祭天之礼并不是一回事。据《说文解字》等书,“類”字本义是一种犬,可能是白犬。“禷”祭应是指特以“類”为牺牲的祭仪。《山海经》等说“類”具有“自为牝牡”的神奇特性,这与该书所记“白犬有牝牡,是生犬戎”的传说非常相似。在先秦文献中,“類”字常用来指出自同一祖先的子孙后代。这些材料表明“類”与古代中国某些族群的祖先神话有着密切的关系。“禷”祭之形成,正植根于此种观念之上。“禷”祭当属于曾广为流行于上古中国北方地区的犬祭礼俗中某种特定的类型,它的消亡可能与周人颇具理性色彩的的礼制改革有关。