7.【解析】(1)对A、C分析,有mg=2ma1,v=2a1h(1分)解得vA=√gh(1分)(2)B放在A上后,设A、C仍一起加速,则mgAung=maz解得a2=0(1分):即B放在A上后,A、C以速度vA做匀速运动,此时B做匀加速运动加速度am=x四=(1分)设经过时间t,B的速度达到υA,且B刚好运动至A的左端则有v=aBt1(1分A的长度L=5A-=z1-、1解得L=2h(1分)(3)加上力F后,B的速度达到vA后,A和C仍匀速,B加速,此时B的加速度a=F+4pmg=2g4(1分)加速时间a2==×kh(1分)B相对A的位移△s=A-sB=wt2-2wt2=4(1分)A、B共速后都向右加速,设经时间t3,B滑出A对B有a=4m(1分)对A有a=mm=g(1分)B相对A的位移△=sB-A=(t+2amG)UALacti)(1分)解得6=√五=(1分)B滑出A时的速度为vB=v+aB·=5(1分)

书面表达One posssible versionDear Mr. Smith,I'm Li Hua, a senior 3 student. Learning that your City Culture department is looking for an English editorthis summer, I'm eager to apply for the position.As far as I'm concerned, city culture refers to a citys history, geography, customs, future, and so on. Asa native, I am quite familiar with our citys traditional culture. Besides, I have a good command of ErI' m especially good at English writing. Additionally, Im cooperative and creative, which will give me anadvantage for the position.I'll be greatly honored if I' m chosen for the job. If I'm admitted, please email me at lihua @16com.Yours sincerely,Li Hu

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