（10分）如图，人重600牛，木块A重400牛，人与A、A与地面间的动摩擦因数均为0.2，现人用水平力拉绳，使他与木块一起向右匀速直线运动，滑轮摩擦不计，求(1)人对绳的拉力.(2)人脚给A的摩擦力方向和大小。
（1）100N；（2）静摩擦力方向向右；大小100牛
本题考查整体隔离法的应用，先以整体为研究对象，整体受到水平向右的两个拉力和水平向左的摩擦力，所以绳子的拉力等于摩擦力的一半，再以人为研究对象，静摩擦力等于绳子的拉力
设绳子的拉力为FT,物体与地面间的摩擦力为FfA,
（1）取人和木块为整体,并对其进行受力分析,如图甲所示,由题意可知FfA=μ(mA+m人)g=200 N.
由于系统处于平衡状态,故2FT=FfA,所以FT=100 N.
（2）取人为研究对象,对其进行受力分析,如图乙所示.
由于人处于平衡状态,故FT=Ff人=100 N
由于人与木块A处于相对静止状态,故人与木块A之间的摩擦力为静摩擦力.
由牛顿第三定律可知人脚对木块的摩擦力方向向右,大小为100 N
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| On a cold winter day, I waited in line to see my hero, Jack Canfield, the author of The Success Principles, who was going to deliver a speech on his new book. During his talk, Jack 1 his wallet, pulled out a hundred-dollar bill, and said, ''Who wants this?" 2 shot up in the audience; people leaned forward to see whom Jack would 3 . But I jumped up, ran up the 4 to the stage, and grabbed the bill. As I was launching myself in the air, 5 raced through my mind- was I about to be humiliated (羞辱) in front of 800 people? Would they 6 security guards and take me from the stage? 7 my desire for bold (大胆的)action was louder than any 8 voice. As I got the bill from his hand, he said, "That's it! We can't wait for the 9 to come to us. We must take 10 to create what we want!'' After his talk, I 11 to formally meet Jack and boldly asked for his personal e-mail address. Over the next months, I sent him emails 12 my points of view and dreams with him. He kindly e-mailed back lots of encouragement. But my life got busy 13 other things. I stopped e-mailing Jack. A year later, my dreams had worn away. I thought Jack would inspire me into action, like a giant arrow that would show me the way. I e-mailed him, and then again - but got no 14 . As I sat down at my computer to 15 my e-mails, I 16 woke up. What was I doing? I was waiting! Now I remembered the crowd, most likely 17 that hundred-dollar bill, while they sat glued to their chairs. Usually we all have a "Jack" for whom we wait - whether it's a person, a place or a thing. We 18 believe the gifts of life are just around the corner, and that everything will come in a(n) 19 way. So we don't try. We 20 . But as Wayne Gretzky said,"You'll always miss 100% of the opportunities you don't take!" So I stopped waiting and started my writing career. Now what are you waiting for? | ||||
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