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试确定下列几个摆球在平衡位置附近来回振动的周期.

（1）如图11-4-3甲所示.悬挂在水平横梁上的双线摆球.摆线长为l，摆线与水平横梁夹角θ；

（2）如图11-4-3乙所示.光滑斜面上的摆球.斜面倾角为θ，摆线长为l；

（3）如图11-4-3丙所示.悬挂在升降机中的单摆，摆长为l，升降机以加速度a竖直向上做匀加速运动.

图11-4-3

答案

（1）T=2π （2）T=2π （3）T=2π

（1）双线摆在垂直于纸面的竖直面里做简谐运动，等效摆长为lsinθ，故振动周期为T=2π；

（2）摆球在光滑的斜面上来回振动，回复力由小球重力沿斜面向下的分力mgsinθ决定，等效重力加速度为gsinθ，其振动周期为T=2π；

（3）升降机竖直向上做匀加速运动时，摆球“超重”，回复力由m(g+a)决定，等效重力加速度为g+a，摆球振动周期为T=2π.

单项选择题

At some time in your life you may have a p desire to do something strange or terrible. However, chances are that you don’t act on your impulse, but let it pass instead. You know that to commit the action is wrong in some way and that other people will not accept your behavior.
Perhaps the most interesting thing about the phenomenon of the behavior is how it can change over the years within the same society, how certain behavior and attitudes once considered taboo can become perfectly acceptable and natural at another point in time. Topics such as death, for example, were once considered so upsetting and unpleasant that it was a taboo to even tall about them. Now with the publication of important books such as "On Death and Dying" and "Learning to Say Goodbye", people have become more aware of the importance of expressing feelings about death and, as a result, are more willing to talk about this taboo subject.
One of the newest taboos in American society is the topic of fat. Unlike many other taboos, fat is a topic that Americans tall about constantly. It’s not taboo to tall about fat; it’s taboo to be fat. The "thin" look is thin, not fat.
In the work world, most companies prefer youthful-looking, trim executives to sell their image as well as their products to the public. The thin look is associated with youth, vigor and success. The fat person, on the other hand, is thought of as lazy and lacking in energy, self- respect. After all, people think, how can people who care about themselves, and therefore the way they look, permit themselves to become fat In an image-conscious society like the U.S., thin is "in", fat is "out".

Which of the following statements is TRUE according to the passage

A. Death is a taboo topic today.
B. It is taboo to talk about fat.
C. The topic of fat is different from many other taboo subjects.
D. Fat is a topic that Americans talk about very little.
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单项选择题

在合同结构图中，凡是用双向箭杆联系的两个单位之间存在（）

A.平等关系

B.合同关系

C.协作关系

D.指令关系
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