问题 计算题

在2008年北京残奥会开幕式上,运动员手拉绳索向上攀登,最终点燃了主火炬,体现了残疾运动员坚韧不拔的意志和自强不息的精神。为了探求上升过程中运动员与绳索和吊椅间的作用,可将过程简化如下:一根不可伸缩的轻绳跨过轻质的定滑轮,一端挂一吊椅,另一端被坐在吊椅上的运动员拉住,如图所示。设运动员的质量为65 kg,吊椅的质量为15 kg,不计定滑轮与绳子间的摩擦,重力加速度取g=10 m/s2。当运动员与吊椅一起以加速度a=1 m/s2上升时,试求:

(1)运动员竖直向下拉绳的力;

(2)运动员对吊椅的压力。

答案

解:(1)设运动员和吊椅的质量分别为M和m,绳拉运动员的力为F

以运动员和吊椅整体为研究对象,受到重力的大小为(M+m)g、向上的拉力为2F

根据牛顿第二定律得:2F- (M+m)g=(M+m)a,解得:F=440 N

根据牛顿第三定律,运动员拉绳的力大小为440 N,方向竖直向下

(2)以运动员为研究对象,运动员受到三个力的作用:重力大小Mg、绳的拉力F、吊椅对运动员的支持力FN

根据牛顿第二定律得:F+FN-Mg=Ma,解得:FN=275 N

根据牛顿第三定律,运动员对吊椅的压力大小为275 N,方向竖直向下

单项选择题 A1/A2型题
填空题

阅读下面短文,按照句子结构的语法性和上下文连贯的要求,在空格处填入一个适当的词或使用括号中词语的正确形式填空。

     There is a popular belief among parents      1    schools are no longer interested in spelling. No

school  in     2   I have ever taught has ever ignored spelling or considered it unimportant as a basic

skill. There are, however, different ideas about how to teach it, or how much priority(优先) it must

be given over general language development and writing ability. The problem is how to encourage a

child to express himself freely and confidently in writing     3        holding him back.

      If spelling becomes the only focal(焦点的) point of his teacher's interest, clearly     4   bright child

will be likely to "play safe". He will tend to write only words within his spelling range,     5     (choose)

to avoid  adventurous language. That's why teachers often encourage the early use of dictionaries and

pay attention to content rather than technical ability.

    Once I    6     (shock) to read on the bottom of a sensitive piece of writing about a personal

experience. "This work is horrible! There are far too many spelling errors and your writing is terrible.

"   7   may have been a sharp criticism of the pupil's technical abilities in writing, but it was also a sad 

    8   (reflect) on the   teacher who had omitted(忽视) to read the essay,    9   there existed some

beautiful expressions of the     child's deep feelings. The teacher was not wrong to draw attention to

the errors,    10   if his priorities had centered on the child's ideas, an expression of his disappointment

with the presentation would have given    the pupil more motivation to seek improvement.