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问题 问答题

当火车驶近时，观察者觉得它的汽笛的基音比驶去时高一个音（即频率高到9/8倍），设声速为v="340" m/s，求火车的速率.

答案

设汽笛原来的频率为f，分析可知，当火车驶近时，观察者感觉到：

f′= f,

当火车远离时，f″=f，

由题知，=，

解得：v2="v/17=20" m/s，即火车速率为20 m/s.

此题相当于观察者不动，波源以v2相对于介质运动的情况.

单项选择题

1 Millions of teens are on weight-reducing diets. Although maintaining a healthy body weight is important, obsessive dieting can lead to eating disorders. Eating disorders can disrupt lives and sometimes even result in death.
Starving Oneself
2 Anorexia nervosa is an eating disorder in which people starve themselves. Anorexics have an irrational fear of becoming fat and see themselves as much heavier than they really are. They shun food and may exercise for hours every day in an attempt to lose weight.
3 Female anorexics may never mature sexually or may stop menstruating because they lose too much body fat, causing the level of sex hormones to fall. Their body temperature and pulse rate become abnormally low; their hair becomes thin and dry; and their resistance to infection drops.
Bingeing and Purging
4 Patients with bulimia engage in frequent episodes of bingeing (eating a large amount of food in a short time). After a binge, they almost always purge by self-induced vomiting or by using laxatives. Therefore, their weight stays fairly constant. Unlike anorexic patients, bulimics often recognize that their behavior is abnormal and often feel guilty, depressed, and helpless.
5 The frequent purging removes salts from the body, which can eventually lead to muscle weakness, heart failure, and kidney disease. The vomiting gradually destroys tooth enamel and results in unhealthy gums.
Help for People with Eating Disorders
6 Little is known about what causes anorexia nervosa and bulimia. Many people believe that western society’s emphasis on thinness in women is largely to blame. However, recent research suggests that genetic factors might also be involved. Both eating disorders can be managed with a combination of medical treatment, counseling, and family support.

What may be the influence of anorexia on female

A．muscle weakness and heart failure

B．kidney disease and low body temperature and pulse rate

C．dropping resistance to infection

D．destroyed tooth enamel and unhealthy gums
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选择题

某研究性学习小组到野外考察，下面为考察区域地形图，虚线所示为考察线路。读图完成下列问题。

小题1:下列描述可能与实地情况相符的是（  ）

A．①地附近的河流从西南流向东北

B．②低坡度最陡

C．③地分布有茶园

D．④地是观赏瀑布的最佳位置小题2:该考察线路的高差可能是（  ）

A．450米

B．500米

C．550米

D．600米
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