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问题 多选题

如图所示，细线的一端固定于O点，另一端系一小球，在水平拉力F作用下，小球以恒定速率在竖直平面内由A点运动到B点的过程中（　　）

A．小球的机械能保持不变

B．小球受的合力对小球不做功

C．水平拉力F的瞬时功率逐渐减小

D．小球克服重力做功的瞬时功率逐渐增大

答案

A、小球匀速率运动，重力势能增加，动能不变，故机械能增加，故A错误；

B、小球匀速率运动，动能不变，根据动能定理，合力做功为零，故B正确；

D、重力不变，速度方向与重力的夹角不断增加（大于90度），故根据P=Gvcosθ，重力的瞬时功率的绝对值不断最大；故D正确；

C、小球匀速率运动，合力的功率为零，匀速率运动，小球克服重力做功的瞬时功率不断增加，拉力T不做功，故拉力F的功率不断增加，故C错误；

故选BD．

阅读理解
在校园网上校长与学生在线对话引起了强烈的反响。下面是一些学生提出的问题和校长给予的回答，请你将学生的问题与校长的回答一一配对。
Students’  questions
Headmaster’s  answers
小题1:Which do you think is the best middle school in our town?
A. I think they are all very great. They work really hard. They are all good at teaching and they are good with you all.
小题2:In your dream, what are students in our school like?
B. We know it’s very important for everyone to study subjects and learn other things. I think you should know what you like to do first, and then try to do it better. But you should also try to study better. It’s good for you to find a good job.
小题3:What do you think of our teachers?
C. I think our school is the best. Because you are the greatest, and we also have so many great teachers here. And your parents send you here, because our school is the best.
小题4:I want to join some clubs, but there are not many clubs in our school. Why not have more clubs for us?
D. Every student likes our school and likes to study. They are good at schoolwork and other things. They are smart and healthy. They love their parents and teachers, too.
小题5:I’m not good at all subjects. I hate to study. But my parents want me to study. What can I do?
E. It’s a piece of good advice. We have some clubs for Grade 7 students now. And we are going to have more clubs for all students. I hope every student can have a good time at school.
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单项选择题

大展弦比的机翼（与小展弦比的机翼相比而言）将有（）

A.低失速速度

B.在大迎角时阻力会增加

C.在低空速操纵特性差
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