一列长200米的火车，匀速通过一座6700米的隧道，用了6.9分钟，问_爱下问搜题

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问题问答题

一列长200米的火车，匀速通过一座6700米的隧道，用了6.9分钟，问此火车经过隧道时的速度为多大？爆破需要引火线，某引火线燃烧的速度为0.8厘米/秒，若人在点燃引火线后立即以5米/秒的速度迅速跑到600米以外的安全区，问选用的引火线至少应多长？一辆汽车以36千米/小时的速度行驶到距一悬崖绝壁875米处时按了一声喇叭，声音以340米/秒向前传播，遇到绝壁后又反射回来，问：在行驶的汽车里，驾驶员要经过几秒钟才听到喇叭响的回声？

答案

（1）火车的路程s=L_火车+L_隧道=200m+6700m=6900m，火车的运动时间t=6.9min=414s，

火车的速度V=

s

t

=

6900m

414s

≈16.67m/s．

（2）人跑到安全区需要的时间t=

s

v

=

600m

5m/s

=120s，引火线的长度L=V_导火线t=0.8cm/s×120s=96cm．

（3）汽车的速度V_车=36km/h=10m/s，声音传到绝壁的时间t₁=

s

v

=

875m

340m/s

≈2.57s；

在此时间内汽车前进的距离s₁=V_车t₁=10m/s×2.57s=25.7m，此时车到绝壁的距离s₂=s-s₁=875m-25.7m=849.3m；

以车为参照物，声音相对于车的速度V'=V+V_声=340m/s+10m/s=350m/s，声音传到车的时间t₂=

s2

V′

=

849.3m

350m/s

=≈2.43s，

驾驶员听到回声的时间t=t₁+t₂=2.57s+2.43s=5s．

答：（1）火车经过隧道时的速度是16.67m/s；（2）引火线长度至少为96cm；（3）驾驶员要经过5s才听到喇叭响的回声．

选择题

Photography can be ______ excellent hobby for kids. [ ]

A. /

B. a

C. an

D. the

完形填空

阅读下面短文，掌握其大意，然后从36—55各题所给的四个选项（A、B、C和D）中，选出最佳选项。
The night started out ordinarily enough. Jack and Tara, our   36  , were in their small beds surrounded by toy animals. My husband, Pat, and I went to sleep   37 .
38  after midnight, Jack quietly touched me. “Mommy, I want to stay with you. I promise I won’t take much room.” I shifted   39  to make room. My son soon fell asleep.
An hour later, Tara cried and I carefully went downstairs. “Mommy, my   40  is all stuffed up（堵住）. I can’t really breathe very well. Will you please   41  with me a little while?” Tara doesn’t often get up at night,   42  I decided to be there for her. She soon fell asleep. I returned to my own bed. I was determined to sleep. But to my   43  , I found Jack sleeping on my side of the bed. He looked so   44  that I couldn’t remove him. Quietly and carefully, I lay down between 45  and Jack.
I did manage to get some sleep   46  I heard Tara’s voice again. I returned to   47 Tara. “Mom, please lie down with me a little bit, I had a bad dream, and I’m so   48 .” “Of course, honey. It was only a dream. Mommy’s here and   49  is okay.” We fell asleep   50  each other’s arms.
Even in my sleepy state, I thought of   51  these nights of merry-go-round（一连串的繁忙活动）beds will be over. In their place, my twins will be grown.   52  before I’m ready, our children’ rooms will be much too   53  and empty. So it is especially on days when I’m worn out after a night on this mom’s merry-go-round that I remind   54  that these very days and nights are, in fact. “the good old days”, ever so   55 .

D．impatiently

A．even though

C．no matter when

C．disappointment

A．my children