问题 问答题

在共点力的合成实验中,如图所示,使弹簧秤b按图示位置开始顺时针方向缓慢转动90°角,在这个过程中保持O点位置不动,a弹簧秤的拉伸方向不变,则整个过程中关于a、b弹簧秤的读数变化是(  )

A.a增大,b减小

B.a减小,b减小

C.a减小,b先减小后增大

D.a先小后增大

答案

对点o受力分析,受到两个弹簧的拉力和橡皮条的拉力,如图,其中橡皮条长度不变,其拉力大小不变,oa弹簧拉力方向不变,ob弹簧拉力方向和大小都改变

根据平行四边形定则可以看出b的读数先变小后变大,a的读数不断变小,故ABD错误,C正确.

故选C.

阅读理解

Robert was born in a city. His father was a teacher and mother was a doctor. They got a lot of money and liked their son. The boy felt very happy. The boy was clever but was very lazy. He didn't like to use his head and wouldn't do anything. He didn't do well in his lessons and hardly finished middle school, when he was twenty. Then his parents found some work for him. But he refused.He stayed in bed all the time. They were angry with him, but he never listened to them. Seven years later they died in a traffic accident. He had to live on selling their property(财产). Now he has nothing. He's often hungry and sometimes he has to sleep under a chair in the park One night, as soon as he fell to sleep, he saw a beautiful girl came up to him and asked him to marry(娶)her. He was very happy and followed her. They came in a house He found she was very rich and had a lot of nice things. After dinner she asked him to see her garden. There were all kinds of flowers in it. Suddenly she disappeared(消失)in the forest. And he couldn't find her. He began to cry loudly. "What has happened to you, sir?"a policeman woke him up and asked "I've lost my wife.”Answered Robert. "She's very beautiful and rich. And she loves me very much.” "Perhaps she's gone home," said the policeman with a smile "She must wait for you at home?” "But I can’t remember where she lives!”

小题1:The boy had a __________ childhood(童年).

A. terrible              B. happy            C. common        

小题2:The boy didn't do well in his lessons because__________.

A. he wasn't clever       B. he was very young           C. he wouldn't use his head             

小题3:His parents were angry with him because __________.

A .he was very lazy       B. he was weak in his lessons     C. he refused to go on studying              

小题4:When he was __________, his parents died.

A. twenty               B. twenty-two                 C. twenty-seven   .

小题5:One night he __________ in the park.

A. got a beautiful girl's love        B. got a lot of money     C. had a beautiful dream                 

单项选择题