2022年英语周报七年级广州答案

书面表达One possible versionDear Mr. SmithI'm Li Hua. chan writingto inform you that an English speech contest will be heldthe lecture hall next Friday and we would like to invite yoube one of the judges of the contest. The speech contestwith the theme"Stop wasting Food"is intended to arousestudents'awareness of reducing food waste and a sense ofresponsibilityWe will greatly appreciatc it if you could accept ournvitation. Looking forward to your reply.ua

18.解:(1)设B下滑的加速度为a,2min37=2ma(1分)由匀变速运动L=2(1分)解得:=√D32(2分)(2)碰前B的连度为vo由动能定理2mg1sin7=2x2m(1分)碰撞过程动量守恒:2m0=(2m+m)u(2分)碰撞点处弹簧压缩量为x1,kx= mosin37°(1分碰后恰好不分离,此时弹簧压缩量为x,对AB有(m+2m)ssin37°-kx=(m+2m)a(1分)对滑块B有:2mgin7°=2ma(1分)解得:x=0在原长处恰好不分离碰后到运动到原长处的过程中由动能定理:W,-(m+2m) aisin37=0-5(m+2m)(1分)弹簧弹力做功W=2kx1x1(1分)9m解得:k(1分)3)经分析知碰后AB简诣运动,平衡位置时弹簧压缩量为kx2=3mgsi37°(1分根据简诸运动可知弹簧振子的振幅为x2。x2=30gin37在最低点时系统弹性势能最大,由机械能守恒:E=(m+2m)Bsim3722(1分)解得:Em=5.76mg(1分)

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