八下44期英语周报 2020_2021 答案

第一节Dear Mr GeorgeI am Li Hua, a volunteer of the activity"Englishi Communication to the Mountains". Now, I'm writing toexpress my sincere apology to you for not being present atthe activity to be held next Monday.The reason for my absence is that I suddenly fell illMy parents sent me to hospital, and I have to stay therefor a week. As for my volunteer work, I have asked myclassmate Tom to do it.Sincerely hoping that you can accept my apologiesYours.Li Hua

18.解:(1)当A球运动到最低点时,由几何关系可知,B球上升的高度h=√(3L)2 (4Lx-L=4L(1分)设小球B的质量为m,根据机械能守恒定律有Mg3L=mgh(2分)解得:M(1分)⊙DA(2)设刚释放A球时的加速度a,对A研究:Mg-T=Ma(1分)对B研究:T一mg=ma(1分)解得:a=÷g(1分)(3)设改变后B球的质量为m,根据题意有:(M-m)g=(M m')a(2分解得:m=-M(1分)设A球运动到最低点时细线与水平方向的夹角为0,根据几何关系c0=5(1分)设A球运动到最低点时的速度大小为v,B球的速度大小为v2,则vcOs0=v(1分)根据机械能守恒:3MgL一mgh=Mt mn(2分)解得:=5√28L(1分)

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