2022七下英语周报答案39

18.解:(1)根据题意可知,带电粒子的运动轨迹如图所示在第三象限,根据动能定理有gU=m2(2分)C解得;=(1分)M(2)设粒子在第二象限内的运动时间为t在y轴方向有L=t(2分)1.E在x轴方向有号L=2··2(2分)联立解得公3U(1分)(3)在第一象限内,设粒子离开第二象限时沿x轴正方向的分速度大小为v,合速度大小为以,速度v与y轴正方向的夹角为0则L=:(2分)tan==3(1分)粒子在磁场中运动的速度v=(1分COs由洛伦兹力提供向心力有B=m三(1分)矩形磁场的最小面积是图中以QA为长,AC为宽的矩形面积QA=2rsin=L(1分)AC=r-rcos 8=L(1分)6解得S=Q4XAC=L2。(1分)

第二节One possible versionWhen Norah heard of her mothers plan, she was terhappy. She couldnt wait to see Dan at once. Hardly hadthey met Dan in his yard when Norah shouted excitedlyShe ran right up to him as if she had known him all of herlife and she was climbing all over him like a monkeyAnd of course every time I turned around. she d comeback over and hug me again, " Dan said. When it was timeto leave, Norah and her mom passed the rose bush by thedoor. Dan picked a big red rose and gave it to Norah, saying, "This is a symbol of our friendship.The mother and her daughter hane been visiting himceely, erer since. Every time they went there, the old andthe young would play games and tell each other secretsorah considers Dan as a family member and Dan has neverbeen alone again. "Norah helped me recover from my sad-ness and gave me something to live for, Dan said, with afresh shave and haircut

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