如图在小木板上固定一个弹簧秤（弹簧秤的质量可忽略不计），弹簧秤下吊一光滑小球一起放在斜面上，木板固定时，弹簧秤的示数为F1，放手后木板沿斜面下滑，稳定时弹簧秤的示数是F2，测得斜面的倾角为θ。则（ ）
A．放手后弹簧为拉伸状态，小球加速度为gsinθ－
gcosθ
B．放手后弹簧为压缩状态，小球加速度为gsinθ－gcosθ
C．木板与斜面的动摩擦因数为
D．木板与斜面的动摩擦因数
答案：C
题目分析：设小球的质量为m，木板的质量为M，开始静止时，对小球有
,（1）
当放手后木板沿斜面下滑，稳定时弹簧秤的示数是F2，此时示数肯定减小，减小的部分产生了加速度，所以此时对小球有：
（2），即
，所以AB错误，
将小球和木板看做一个整体，整体的加速度和小球的加速度相等，故有
，（3）
又知道
（4）
联立解得
，C正确，D错误
故选C
点评：做本题的关键是先用隔离法研究小球，然后再用整体法研究木板
I am my mother’s third child. When I was born, the doctor gently explained to my mother that my left arm was 36 , below the elbow (肘). Then he gave her some 37 , “Don’t treat her any 38 from the other girls. Demand more.” And she 39 ! My mother had to work to 40 my family. There were five girls in our family and we all had to 41 . Once when I was about seven, I came out of the 42 , “Mom, I can’t peel (削……皮) potatoes. I only have one hand.” “You get back to peel those potatoes, and don’t ever use that as a(n) 43 for anything again!” Of course I could peel potatoes — with my good hand, while holding them down with my __44 arm. There was always a 45 , and Mom knew it. “If you try hard 46 ,” she said, “you can do anything.” Once in the second grade, our teacher had each of us race across the monkey bars (攀爬架). When it was my turn, I 47 my head. Some kids 48 . I went home crying. After work the next afternoon, Mom 49 me back to the school playground. “Now, 50 up with your right arm,” she advised. She stood by 51 I practiced, and she 52 me when I made progress. I’ll never forget the 53 time I was crossing the bars. The kids were standing there with their mouths open. It was the way with everything. When I 54 I can’t handle (处理) things, I see Mom’s smile again. She had the heart to 55 anything. And she taught me I could, too.
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