题干

将质量均为M=1kg的编号依次为1,2,…6的梯形劈块靠在一起构成倾角α=37°的三角形劈面,每个梯形劈块上斜面长度均为L=0.2m,如图所示,质量m=1kg的小物块A与斜面间的动摩擦因数μ1=0.5,斜面与地面的动摩擦因数均为μ2=0.3,假定最大静摩擦力与滑动摩擦力相等.现使A从斜面底端以平行于斜面的初速度v0=4.5m/s冲上斜面,下列说法正确的是(   )

A:若所有劈均固定在水平面上,物块最终从6号劈上冲出

B:若所有劈均固定在水平面上,物块最终能冲到6号劈上

C:若所有劈均不固定在水平面上,物块上滑到5号劈时,劈开始相对水平面滑动

D:若所有劈均不固定在水平面上,物块上滑到4号劈时,劈开始相对水平面滑动

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答案(点此获取答案解析)

B,C

同类题2

阅读下面短文,掌握其大意,然后从各题所给出的A、B、C和D项中,故答案选出最佳选项。

    When I was a young boy, my family was very poor. I would often help do something for the neighbors to 1 a little pocket money.

    One day I 2 on the door of an elderly woman and asked 3 she needed me to clean the yard. She asked why I was not in 4 and I showed her my worn 5, which no longer kept me warm. She 6 me to work in the yard. When I finished, she looked at me and said, “I suppose you want to be 7 now.” She then 8 a jar filled with dollar bills and said, “I'm glad you've done a 9 job today but that is not something you should be doing again. I want you to 10 new clothes and get back to school. I also want you to come back to 11 me with your report card and I will 12 you when I see you have worked hard and have some good 13. Now reach out your little hands and take out as much 14 as you can.”

    I put my hands in the jar and 15 so much money that it was 16 for me to buy what I badly needed. Later, I returned several times to see the woman and she did just as she 17. She looked at my report card and gave me a handful of dollars and some delicious food every time I 18 her that I had an “A”.

    I was twelve years old when I moved from that neighborhood. I will never forget the huge 19 this wonderful lady made over my life with her 20. This is something I hope to do myself in this lifetime over and over again.