如右图所示，在圆心为O、半径为r的圆形区域内有方向垂直纸面向里_爱下问搜题

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问题实验题

如右图所示，在圆心为O、半径为r的圆形区域内有方向垂直纸面向里的匀强磁场，一电子以速度v沿AO方向射入，后沿OB方向射出匀强磁场.若已知∠AOB=120°，则电子穿越此匀强磁场所经历的时间是_________.

答案

电子在磁场中做圆周运动的圆心O′及半径R如图所示,由平面几何知识知R=rtan60°.

由R==r·tan60°得=

电子运动的时间为:t==.

完形填空

As a young boy my family was very poor. I would often help do something for the neighbors to 36 a little pocket money.
One day I 37 on the door of an elderly woman and asked 38 she needed me to clean the yard. She asked why I was not in 39 and I showed her my worn 40 , which no longer kept me warm. She 41 me to work in the yard. When I finished, she looked at me and said, “I suppose you want to be 42 now.” She then 43 a jar filled with dollar bills and said, “I’m glad you’ve done a 44 job today but that is not something you should be doing again. I want you to 45 new clothes and get back to school. I also want you to come back to 46 me with your report card and I will 47 you when I see you have worked hard and have some good 48 . Now reach out your little hands and take out as much 49 as you can.”
I put my hands in the jar and 50 so much money that it was 51 for me to buy what I badly needed. Later, I returned several times to see the woman and she did just as she 52 . She looked at my report card and gave me a handful of dollars and some delicious food every time I 53 her that I had an “A”.
I was twelve years old when I moved from that neighborhood. I will never forget the huge 54 this wonderful lady made over my life with her 55 . This is something I hope to do myself in this lifetime over and over again.

C．talked about

单项选择题

下列哪一项对直肠息肉与直肠癌的鉴别没有意义：

A．粘液血便

B．肿物质硬

C．活体组织检查

D．肿物表面光滑

E．大便变细变扁