英语周报初三20期答案

7.B【解析】根据物块通过AB段和通过CD段的时间相等,在BC段做匀加速直线运动,所以物块在AB和CD段不可能做匀速运动,由于速度发生变化,物块所受的洛伦兹力就会发生改变,从而导致摩擦力发生改变所以在AB段和CD段不可能做匀减速运动,A错误;由时间相等和位移相等,推出物块在AB段的运功和CD段的运动应该是完全一样的过程,所以v4=vc,B正确;由于物块在AB段的运动和CD段的运动完全一致,可得位移也相同,但是CD段物块做加速度减小的减速运动,而BC段物块做匀加速直线运动,所以CD段所用的时间更多,C错误;物块在AC段由动能定理得2 mgl sinθ-W:=0,所以AB段克服摩擦力做功W=2 mg/ sinθ,又AB段和CD)段运动完全一样,所以在CD段克服摩擦力做功也为W4,即物块通过AD段的过程中机槭能减少了4 ng l sinθ,D错误。故选B。

第二节书面表达Dear Henry,Im Li Hua from your English speaking class lastterm. I m writing to ask for your helpNow I am preparing to attend MUN next weckHowever,have some difficulty with concepts andexpressions of the current affairs and speech skills. Iwonder if I can borrow your Dictionary of englishMedia. which you presented in class. Furthermore.I amnot quite sure about how to make a convincing andpersuasive speech at MUN. Could you be so kind as to giveme some advice?I know you have a very busy schedule, but I'd be verygrateful if you could spare some time to give me face-to-acc instructions. Thank you for your kindnessI'm looking forward to your replyYoursLi Hua

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