如图所示，长
，高
，质量
的长方体木箱，在水平面上向右做直线运动。当木箱的速度
时，对木箱施加一个方向水平向左的恒力
，并同时将一个质量
的小球轻放在距木箱右端
的P点（小球可视为质点，放在P点时相对于地面的速度为零），经过一段时间，小球脱离木箱落到地面。木箱与地面的动摩擦因数为0.2，其他摩擦均不计。取
。求：
（1）小球从离开木箱开始至落到地面所用的时间；
（2）小球放上P点后，木箱向右运动的最大位移；
（3）小球离开木箱时木箱的速度。
解：（1）小球从离开木箱开始至落到地面所用时间为t
由
得
（2）小球放到木箱后相对地面静止，木箱的加速度为
木箱向右运动的最大位移为
（3）x1小于1m，所以小球不会从木箱的左端掉下木箱
向左运动的加速度为
设木箱向左运动的距离为x2时，小球脱离木箱
设木箱向左运动的时间为t2，由
得
小球刚离开木箱瞬间，木箱的速度方向向左，大小为
完形填空（20小题；每小题1.5分，满分30分） 阅读下面短文，从短文后各题所给的四个选项（A、B、C和D）中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。 Everywhere, you will find "the white-haired boy", sometimes called "the fair-haired" boy．He gets special 36 , as if he were above everybody else．You will find him in school, in college, at home, or 37 you work． In school, he is teacher's 38 , her favorite who can do nothing 39 ．Sometimes she lets him do little jobs for her．He comes to class in the morning, all shiny and clean．" He is always raising his hand, 40 with answers to the teacher' s questions．And he knows all the answers． He gets into your hair, especially if you are at the 41 of the class and die teacher thinks you are slow at learning things．How you 42 the guy! At college, he walks across the school ground as if he 43 it．Sometimes he wears a, colorful football or basketball shirt or sweater, with a bright red, or green or yellow college letter 44 worn on the front of it．What 45 shoulders, what muscles he has! He 46 the girls happy by just smiling at them．He has the mark of _47_ on him． At home, "the fair-haired boy" is 48 choice．Sometimes, he is the oldest son— 49 , the youngest．If you are in between, you are out of 50 ． Then, you discover that there are others who 51 your feelings, ready to tell you their private 52 ．One of them asks, "What does he have that I haven't got?" You ask yourself the same question． 53 , there comes a day when you decide to stop hating him. Is it 54 because he has been made boss and you 55 yourself working for him?
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