英语周报北师大20答案

10.AC【解析】对小球b:FB=mg;对小球a、b整体由平衡条件得 FACOS53°=(2m+m)g,解得FA=5mg,故FA:FB=5:1,A项正确,B项错误;剪断绳子瞬间,弹簧B的弹力不变,则小球b的合力仍为0,故此时小球b的加速度为0,弹簧A的弹力不变,则小球a的合力与原来绳子的拉力T大小相等,T(2m+m)gtan53°=4mg,故此时小球a的加速度a=4mg=2g,C项正确,D项错误。

第三节One possible version:Work plays an extremely important role in Americanculture. sope ople find retirement difficultas they may feeluseless without jobs. Retirement can also bring financialproblems as Social Security cheeks dont provide enoughmoney. However, some senior citizens feel happy aboutretirement as they finally get time to do what they reallywant. American government is solving retired citizensproblems and considering their special needs, which is wiseand necessary, because everyone will get oid.

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