题目

如图3-1-12所示，在水平桌面上叠放着质量相等的A、B两块木板，在木板A上放着一个物块C，木板和物块均处于静止状态.已知物块C的质量为m，A、B、C之间以及B与地面之间的动摩擦因数均为μ.用水平恒力F向右拉动木板A使之做匀加速运动，物块C始终与木板A保持相对静止.设最大静摩擦力与滑动摩擦力大小相等，重力加速度为g.则以下判断正确的是（    ） 图3-1-12A.A、C之间的摩擦力可能为零B.A、B之间的摩擦力不为零，大小可能等于μmgC.A、B之间的摩擦力大(2015 •枣庄中考)    A teacher had been made very angry by some of his boys making a high sound during school hours. At last he was forced to 1 any one who should repeat the wrong act. The next morning,a loud sound was heard in the quiet room. The students were very much surprised,and the teacher at once looked around to 2 the person who had made a high sound.   When a boy who was often in bad act,was told he was wrong,he 3 said he hadn’t done it,yet his words were not believed,and he was brought up for punishment. Seeing what was about to take place,Charles,a little boy,got up suddenly from his 4 .“Don’t punish John,sir. I made the high sound. Fm very sorry,I didn't mean to whistle. ” He held out his hand to be punished. Taking the little boy's 5 , the teacher said “Charles,you have done right. I can't punish you after you 6 so nobly. ” Even the youngest child in the school felt 7 of Charles,for every child could see how bravely he had acted.   Charles was truly a brave boy. He had done 8 he knew to be right. True courage may also be shown by 9 to do that which we know to be wrong,10 bad people may laugh at us ,1. A. punish   B. praise   C. fix   D. hurt2. A. hide   B. discover   C. pay   D. wake3. A. actively   B. aloud   C. strongly   D. softly4. A. bag   B. board   C. desk   D. seat5. A. hair   B. arm   C. finger   D. hand6. A. have given   B. have run   C. have acted   D. have fallen7. A. afraid   B. proud   C. sure   D. careful8. A. what   B. which   C. who   D. when9. A. refusing   B. continuing   C. dreaming   D. wanting10. A. but   B. though   C. after   D. until