英语周报 2018-2022 九年级 AHW 19答案

4.B【解析】对B球进行受力分析,如图所示。当R>r时,R越大、r越小4FN越向下倾斜,FN、F№2都越大,即斜面对B球的支持力越大,根据牛顿第三定律,B球对斜面的压力也越大A错误;FN、FN2的合力与重力mg等大反向,在右侧力的三角形中,竖直边大小等于mg,当斜面倾角0一定时,F2的方向保持不变,R=r时,FN1恰好垂直于F,此时F最小,B正确;将两个球作为一个整体,挡板对A球的支持力等于两球重力沿斜面向下的分力,斜面倾角0一定时,重力沿斜面向下的分力一定,与R及r无关,C错误;小球半径确定时,当斜面倾角θ逐渐增大,两球沿斜面向下的分力增大,因此A对挡板的压力一直增大,D错误。故选B。Bmg

书面表达One possible version:Dear Jack,I am happy to know that you are to spend some time in China. I recommend you stay in Beijing, the capital andcultural center of China.First of all, there are a lot of museums where precious relics of historical interest are waiting for you toexplore. Besides, the exceptional Peking Opera performances there are also what you can t miss. And the city isalso best known for its abundance of ancient buildings, which are still well preserved.I hope that you will have a good time in China.Yours sincerely,Li Hua

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