2022英语周报七年级下册52期答案

17.【解析】(1)设小球到达C点时速度为vc,小球从A点运动至C点的过程,由动能定理有mg(5Rsin37°+1.8R)- anglos3?°·5R=mt(2分)解得v(2分)(2)小球沿BC管道做圆周运动,设在C点时管道对小球的作用力为FN,由牛顿第二定律有FN-mgv(1分)其中r满足r+reos37"=1.8R(1分)解得FN=6.6mg(1分)由牛顿第三定律可得小球对管道的作用力为6,6mg,方向竖直向下(1分)(3)要使小球不脱离轨道,有两种情况:情况一:小球能滑过圆周轨道最高点,进入EF轨道,则小球在最高点应满足mR≥mg小球从C点到圆周轨道的最高点过程,由动能定理mgR-mg·2R=方mv-方m晚(1分)可得R≤R=0.92R(1分)情况二:小球上滑至四分之一圆周轨道的最高点时,速度减为零,然后滑回D.则由动能定理有-pmgR一mg·K=-2m(1分)解得R≥2.3R(1分)所以要使小球不脱离轨道,竖直圆周轨道的半径R应该满足R’≤0.92R或R′≥2.3R(1分)

第三节One possible wersionParagraph 1Some time later, he woke up under a tree in the samevalley. He was alone, worried about his dog and he knewhe was going to get into more trouble with his wife. Then henoticed a curious thing his hair was over his shouldersand his beard was over a foot long! So strange was he thathe decided immediately to get back to the village. As soonas he got there, he realized that everything was differentand there were lots of new houses. Surprisingly, he didn'trecognize the people in the villageParagraph 2:When he got to his house, he saw that it was old andbandoned. To his sorrow, his wife had died two years before and the two children had left away. Despite the arment with his wife, he was regretful for the past. Facingthis sharp change, Rip was determined to change his way ofliving and never be a lazy man. He became a diligentfarmer and kept pace with the new society. In his sparetime, he would still go to the old inn that had been rebuiltto tell some stories about his adventure in the valley, whichappealedto the children a lo

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