如图所示的滑轮组，每个滑轮等重，不计绳重和摩擦。物体重G从200_爱下问搜题

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问题计算题

如图所示的滑轮组，每个滑轮等重，不计绳重和摩擦。物体重G从200N开始逐渐增加，直到1600N时绳子被拉断。每次都匀速拉动绳子将物体提升同样的高度。右图记录了在此过程中滑轮组的机械效率随物体重力的增加而变化的图像。求：

(1) 每个滑轮重多少N?

(2) 绳子能承受的最大拉力是多少N?

(3) 当滑轮组的机械效率低于多少时，使用它将不再省力?

答案

（1）200N（2）600N（3）33.3％.

右图记录了在此过程中滑轮组的机械效率随物体重力的增加而变化的图像。由图可知物重为200N时，

滑轮组的机械效率为50％。G动=3F-G物=200N。物体重G从200N开始逐渐增加，直到1600N时绳子被拉断。F=1/3(G动+G物)="600N." 当滑轮组不再省力时，拉力等于物重。G动=3F-G物，即F="G," G动=3F-F=2F,此时滑轮组的机械效率为=33.3％.

（1）由图可知物重为200N时，滑轮组的机械效率为50％。每个滑轮重

G动=3F-G物=200N。

（2）物体重G从200N开始逐渐增加，直到1600N时绳子被拉断。

F=1/3(G动+G物)=600N.

（3）当滑轮组不再省力时，拉力等于物重。

G动=3F-G物，即F="G," G动=3F-F=2F,

此时滑轮组的机械效率为=33.3％.

单项选择题

口腔健康教育的目的是（）

A.传递口腔健康信息

B.使人们认识到并能终生保持口腔健康

C.吸引人们对口腔健康项目的兴趣

D.保证目标人群对口腔健康项目的参与

E.营造有利于口腔健康的环境

完形填空

Somebody might say, “I want to be a big fish, as big as Bill Gates, in a big pool, as large as Microsoft.” However, we all know it is   36   for a green hand(新手) in the field. Then you have to   37  the question carefully. No matter    38   side you choose to take, you have your chance to succeed. Now the problem is which can provide you more    39   .
I choose to be a big fish in a small pool. A big company may provide you a    40   starting point, but a small company offers you opportunity to practice various     41   . During the     42   , you may     43    yourself, recognize your     44     points and find your potentiality(潜力). What’s more,   45  so many limitations and rules in a small company, if you are competent you have     46    chances to climb to a higher point. Finally, being a big fish (although) in a small pool, gives a green hand     47    self-confidence which is quite important for     48   .
     49   , you should not be confined(使局限) to your small pool, and be     50    with being a big fish there. You should always dream of being a big fish in a big pool!

B．uninteresting

C．unnecessary

D．uncorrectable

C．Immediately