列车员站在站台上的一个位置迎接列车进站，列车进站停下前运动视为匀减速直线运动，第一节车厢通过用了3s，第二节车厢通过用了4s，设各节车厢长度相同，车厢间隙不计，车厢节数足够多，求火车停下时列车员面对的是哪节车厢．
设车厢的长度为L，则3s内和4s内的平均速度分别为
v1=
，v2=L 3 L 4
某段时间内的平均速度等于中间时刻的瞬时速度，两个中间时刻相差3.5s．
则a=
=v1-v2 △t
=L 12 3.5
．L 42
7s内的平均速度等于3.5s末的速度，等于
，从该时刻到停止所需的时间t′=2L 7
=12s．2L 7 a
所以列车从减速到停止所需的时间t=15.5s．
列车的位移x=
at2=2.86L．知火车停下时列车员面对的是第3节车厢．1 2
答：火车停下时列车员面对的是第3节车厢．
 NOTICE CLEAN AND GREEN WEEK The Social Studies Society and Library Club are working together to organize Clean and Green Week activities to encourage recycling. (A) BEST INVENTION COMPETITION The competition is divided into two parts: Individual (个人) and Team. Each team should be made up of three members. The inventions must be made out of small or used materials. PRIZES
You can get the application forms (申请表) from the library during opening hours. Deadline for the submission (提交) of inventions: 31 January 2005. Please call us at 792-4868 for further information. (B) ‘RECYCLE TO SAVE OUR EARTH’ EXHIBITION CUM VIDEO SCREENING An exhibition on recycling is now on in the library. Videos on the subject will also be screened. Please refer to the library notice board for the timetable of the screening dates and time. DO YOUR PART TO SAVE THE EARTH START RECYCLING TODAY Organizers: Library Club & Social Studies Society 1. If you want to know the screening dates, you can _______. A. fill in the application form B. make a telephone call at 792-4868 C. go to the exhibition hall D. refer to the library notice board 2. What can we get from the notice? A. The exhibition is organized by Library Club. B. The exhibition on recycling will be on in the library. C. The students have to send their inventions before 31 January 2005. D. The students can make their inventions from all kinds of materials. 3. What is the purpose of the activities? A. To get more book vouchers. B. To invent more useful things. C. To encourage students to recycle things. D. To make students’ life more colorful. 4.If you want more information, you can_________. A. surf the internet. B.telephone the teacher C. call at 792-4868 D. read the guide book. |