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问题 单项选择题

矩形截面挖去个边长为a的正方形，如图所示，该截面对z轴的惯性矩I2为().

A.A

B.B

C.C

D.D

答案

参考答案：C

解析：

令任意形状截面图形的面积为A，则图形对z轴的惯性矩为：，代入即可求得。

[知识拓展]

[5.4.2 惯性矩和惯性积]

1．定义

设任意形状截面图形的面积为A(图5.4-3)，则图形

对y轴的惯性矩

对z轴的惯性矩

对O点的极惯性矩

对y、z轴的惯性积

2．特征

①图形的极惯性矩是对某一极点定义的，轴惯性矩是对某一坐标轴定义的，惯性积是对某一对坐标轴定义的。

②搬惯性矩、轴惯性矩、惯性积的量纲为[L4]，单位为m4。

③极惯性矩、轴惯性矩其数值均为正：惯性积的数值可正可负。也可能为零。若一对坐标轴中有一轴为图形的对称轴，则图形对这一对坐标轴的惯性积必等于零；但图形对某一对坐标轴的惯性积为零，则这对坐标轴中不一定有图形的对称轴。

④极惯性矩的值恒等于以该点为原点的任一对坐标轴的轴惯性矩之和，即

Ip=Iy+Iz (5.4-7)

⑤组合图形对某一点的极惯性矩或对某一轴的轴惯性矩，分别等于各组分图形对同一点的极惯性矩或对同一轴的轴惯性矩之和，即

组合图形对某一对坐标轴的惯性积，等于各组分图形对同一对坐标轴的惯性积之和，即

单项选择题

男性，35岁，阵发性头痛、心悸、出汗3年，此次因情绪激动发作剧烈头痛、心悸、大汗来急诊，检查发现面色苍白，血压200/130mmHg(26.7/17.3kPa)，心率136次/分。应选用以下哪种检查对患者进行初筛诊断

A．血白细胞计数
B．血糖测定
C．24小时尿游离皮质醇测定
D．甲状腺功能
E．24小时尿VMA定性
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问答题

(46) It is known that the brain shrinks as the body ages, but the effects on mental ability are different from person to person. Interestingly, in a study of elderly men and women, those who had more education actually had more brain shrinkage.
"That may seem like bad news," said study author Dr. Edward Coffey, a professor of psychiatry and of neurology at Henry Ford Health System in Detroit. (47) However, he explained, the finding suggests that education allows people to withstand more brain tissue loss before their mental functioning begins to break down.
The study, published in the July issue of Neurology, is the first to provide biological evidence to support a concept called the "reserve" hypothesis, according to the researchers. In recent years, investigators have developed the idea that people who are more educated have greater cognitive reserves to draw upon as the brain ages; in essence, they have more brain tissue to spare.
(48) Examining brain scans of 320 healthy men and women aged 66 to 90, researchers found that for each year of education the subjects had, there was greater shrink age of the outer layer of the brain known as the cortex. Yet on tests of cognition and memory, all participants scored in the range indicating normal.
"Everyone has some degree of brain shrinkage," Coffey said. "People lose (on average) 2.5 percent per decade starting in adulthood."
There is, however, a "remarkable range "of shrinkage among people who show no signs of mental decline, Coffey noted. Overall health, he said, accounts for some differences in brain size. Alcohol or drug use, as well as medical conditions such as diabetes and high blood pressure, contribute to brain tissue loss throughout adulthood.
In the absence of such medical conditions, Coffey said, education level helps explain the range of brain shrinkage exhibited among the mentally-fit elderly. The more-educated can withstand greater loss.
(49) Coffey and colleagues gauged shrinkage of the cortex by measuring the cerebrospinal fluid surrounding the brain. The greater the amount of fluid, the greater the cortical shrinkage.
Controlling for the health factors that contribute to brain injury, the researchers found that education was related to the severity of brain shrinkage. For each year of education from first grade on, subjects had an average of 1.77 milliliters 11 more cerebrospinal fluid around the brain.
Just how education might affect brain cells is unknown. (50) In their report, the researchers speculated that in people with more education, certain brain structures deeper than the cortex may stay intact to compensate for cortical shrinkage.
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