题干

如图所示,质量为M的木板静置在光滑的水平面上,在M上放置一质量为m的物块,物块与木板的接触面粗糙。物块m获得初速度v0向右滑动,在滑动过程中下面叙述正确的是(    )

A:若M固定不动,则mM的摩擦力的冲量为零,而M
m的摩擦力做负功

B:若M不固定,则m克服摩擦力做的功全部转化为内能

C:若M不固定,则mM的摩擦力做功,等于m克服M的摩擦力做功

D:不论M是否固定,mM相互作用力的冲量大小相等、方向相反

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答案(点此获取答案解析)

D

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完形填空

    Dale Carnegie rose from the unknown of a Missouri farm to international fame because he found a way to fill a universal human need.

    It was a need that he first 1 back in 1906 when young Dale was a junior at State Teachers College in Warrensburg. To get an 2, he was struggling against many difficulties. His family was poor. His Dad couldn't afford the 3 at college, so Dale had to ride horseback 12 miles to attend classes. Study had to be done  4 his farm-work routines. He withdrew from many school activities 5 he didn't have the time or the  6. He had only one good suit. He tried 7 the football team, but the coach turned him down for being too 8. During this period Dale was slowly 9 an inferiority complex (自卑感), which his mother knew could 10 him from achieving his real potential. She 11 that Dale join the debating team, believing that 12in speaking could give him the confidence and recognition that he needed.

    Dale took his mother's advice, tried desperately and after several attempts 13 made it. This proved to be a 14 point in his life. Speaking before groups did help him gain the 15 he needed. By the time Dale was a senior, he had won every top honor in 16. Now other students were coming to him for coaching and they,  17, were winning contests.

    Out of this early struggle to 18 his feelings of inferiority, Dale came to understand that the ability to  19 an idea to an audience builds a person's confidence. And, 20 it, Dale knew he could do anything he wanted to do—and so could others.