一般而言，山羊皮革和绵羊皮革哪个好？

一根弹性绳沿x轴方向放置，左端在原点O处，用手握住绳的左端使其沿y轴方向做周期为1 s的简谐运动，于是在绳上形成一简谐波。绳上质点N的平衡位置为x=5m，振动传播到质点M时的波形如图所示。求：

（1）绳的左端振动后经多长时间传播到质点N；

（2）质点N开始振动时，绳的左端已通过的路程；

（3）如果从N开始振动计时，画出质点N振动的位移-时间图线。

阅读理解。

     As an aid-fashioned as it sounds, doctors say the best way to lose weight is to eat less and

exercise more. For women who are middle-aged 'or older, experts say an hour of daily exercise

is necessary just to keep weight.

     But for most women, fitting in an hour of exercise in an already busy day is difficult, and it is

even impossible. Especially for many women, most hours are already filled with a full-time job

and raising a family. At the end of the tiring day, at least in the U.S., there is little incentive to

prepare a dinner when fast food is so available--and so much more fattening.

     Nearly twenty million women in the U.S. have the problem of obesity. The United Nations

report notes that America is one of the countries that have the most obese people in the world

and Mississippi has more overweight women than any other state in the U.S.. The U. S. Center

for Disease Control says the number of overweight women increased by 60% between 2004

and 2009 in America. The World Health Organization says more than one billion women in other

nations are in the same shape.

     While there is plenty of advice on how to lose kilograms, a new study is carried out on

middie-aged and older women who keep normal body weight, or BMI (Body Mass Index). "We

found that physical activity was effective in controlling weight only among women who started to gain

weight from a normal BMI ,'I-Min Lee and her colleagues found more exercise did not help.

     "Once a woman is obese, there was no relation between physical activity and weight change

among these women, "Lee said. The researchers say the only way that obese women can lose weight

is to simply cut calories.

1. What makes it difficult for most women to have an hour of exercise every day?

A. Losing confidence in exercising.        

B. Paying all attention to their jobs.

C. Having no interest in exercising.        

D. Being too busy to find spare time.

2. The third paragraph shows us that ______.

A. more and more women are caring about their weight

B. the problem of woman obesity worldwide is serious

C. many organizations are built to care about fat women

D. America has more overweight women than any other country

3. What should an obese woman do to lose weight according to Lee?

A. Eat more vegetables and fruit and less fat.

B. Keep busy all day long and do a lot of exercise.

C. Find a high energy costing and tiring job.

D. Sleep less, eating less and exercising more.

4. From the passage, we know an hour of daily exercise can ______.

A. help people to lose weight          

B. make people put on weight

C. make people keep weight      

D. keep people busy

某车间负荷采用低压三相电源线路供电，线路长度L=50m，允许电压降为5%，保护装置0.4s内可切除短路故障，线路发生最大的短路电流I_k=6.8kA。线路采用铜芯交联聚乙烯绝缘电缆穿铜管明敷，环境温度40℃，电缆导体最高温度90℃，电缆经济电流密度2.0A/mm²，电缆热稳定系数c=137，电压损失百分数0.50%/(kW·km)。车间设备功率数值统计见下表，有功功率同时系数取0.8，无功功率同时系数取0.93。请回答下列问题：

若假定已知车间负荷计算电流厶=55A，按电缆经济电流密度选择电缆导体截面应为( )。

A．25mm²

B．35mm²

C．50mm²

D．70mm²

简述社会化及其必要性？