问题 单项选择题

如图所示,绳与杆均不计重力,承受力的最大值一定.A端用绞链固定,滑轮O在A点正上方(滑轮大小及摩擦均可忽略),B端吊一重物P,现施加拉力FT将B缓慢上拉(均未断),在杆达到竖直前()

A.绳子越来越容易断

B.绳子越来越不容易断

C.杆越来越容易断

D.杆越来越不容易断

答案

参考答案:B

解析:

因为轻杆,力的作用点在杆的一端,故杆中的作用力沿杆的方向。对B点受力分析,如图,当θ角减小时,绳中拉力减小。杆受到的压力沿杆的方向,故杆无所谓易断不易断。故B项正确

单项选择题
单项选择题

Personality is to a large extent inherent. A-type parents usually (1) A-type offspring. But the environment must also have a profound effect (2) if competition is important to the parents, it is (3) to become a major factor in the lives of their children.

One place where children (4) A characteristics is school, which is, (5) its very nature, a highly competitive institution. Too many schools (6) the "win at all costs" moral standard and measure their success by sporting achievements. The current (7) for making children compete against their classmates or against the (8) produces a two-layer system, in which competitive A types seem in some way better than their B type fellows. Being too (9) to win can have dangerous consequences: remember that Pheidippides, the first marathon runner, (10) dead seconds after saying. "Rejoice, we conquer!"

(11) the worst form of competition in schools is the disproportionate emphasis on examinations. There is, for example, a (12) school that allows pupils to concentrate on those things they do well. The (13) of competition by examination are somewhat questionable, but competition in the certain (14) of failure is positively harmful.

(15) , it is neither practical nor desirable that all A youngsters change into B’s. The world needs types, and schools have an important duty to try to (16) a child’s personality to his possible future employment. It is top management.

If the preoccupation of schools with academic work was (17) , more time might be spent teaching children surer values. Perhaps selection (18) the caring professions, especially medicine, could be made less by good grades in chemistry and more by such (19) as sensitivity and sympathy. It is surely a mistake to choose our doctors exclusively from A type stock. B’s are important and should be (20) .

6()

A.adapt

B.abide

C.adopt

D.absorb