问题 选择题

测定运动员体能的一种装置如右图所示,运动员的质量为M,绳拴在腰间沿水平方向跨过滑轮(不计滑轮摩擦和质量),绳的另一端悬吊的重物质量为m,人用力向后蹬传送带而人的重心不动,传送带以速度v向后匀速运动(速度大小可调).最后可用v的值作为被测运动员的体能参数,则(   )

A.人对传送带不做功

B.人对传送带做功的功率为mgv

C.人对传送带做的功和传送带对人做的功大小相等,但正、负相反

D.被测运动员的v值越大,表示其体能越好

答案

BCD

题目分析:A、以运动员为研究对象,进行受力分析可知人受摩擦力等于mg,方向水平向左,因人的重心随不动,但是摩擦力的作用点(脚)向右移动,所以摩擦力对人做负功;由牛顿第三定律可知人对传送带的摩擦力水平向右,则人对传送带做正功,功率为;被测运动员的值越大说明运动员越有力,体能越好。故BCD正确

故选BCD

点评:中等难度。注意摩擦力可以对物体做正做功,也可以做负功,还可以不做功;相互摩擦的系统,一对静摩擦力所做功的代数和总等于零。

问答题
单项选择题

Psychologists today recognize childhood as a separate stage of life which can only be understood in its own terms, and they wonder why the Western world took so long to see the folly of regarding children simply as small, inadequately socialized adults. Most psychologists, however, persist in regarding people 70 to 90 years old as though they were 35 years old who just happen to have white hair and extra leisure time. But old age is as fundamentally different from young adulthood and middle age as childhood is—a fact attested to by the organization of modern social and economic life. Surely it is time, therefore, to acknowledge that serious research into the unique psychology of advanced age has become indispensable.
Which one of the following principles, if established, would provide the pest backing for the argument

A.Whenever current psychological practice conflicts with traditional attitudes toward people, those traditional attitudes should be changed to bring them in line with current psychological practice.

B.Whenever two groups of people are so related to each other that any member of the second group must previously have been a member of the first, people in the first group should not be regarded simply as deviant members of the second group.

C.Whenever most practitioners of a given discipline approach a particular problem in the same way, that uniformity is good evidence that all similar problems should also be approached in that way.

D.Whenever a society’s economic life is so organized that two distinct times of life are treated as being fundamentally different from one another, each time of life can be understood only in terms of its own distinct psychology.

E.(E) Whenever psychologists agree that a single psychology is inadequate for two distant age groups, they should be prepared to show that there are greater differences between the two age groups than there are between individuals in the same age group.