一只小船运载木料逆水而行，经过某桥下时，一块木料不慎落入水中_爱下问搜题

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问题填空题

一只小船运载木料逆水而行，经过某桥下时，一块木料不慎落入水中，经30分钟后才发觉，立即回程追赶，在桥下游5千米处赶上木料．设小船顺流和逆流时划行速度相同，则小船回程追赶所需时间是______小时，水流速度是______．

答案

以河岸为参照物，设船的划行速度为V_船，水流的速度为V_水，则木料落水后即以速度V_水顺流而下，设船向上游运动的时间（即题中的30分钟）为t₁，船向下游运动的时间（即船回程追赶的时间）为t₂，则在t₁时间内小船向上游前进的路程S₁=（V_船-V_水）t₁

在时间t₁+t₂内木料向下游漂流的路程S₂=V_水（t₁+t₂）；

在时间t₂内小船向下游行驶的路程S₃=（V_船+V_水）t₂；

依题意得：S₃=S₂+S₁，即

（V_船+V_水）t₂=V_水（t₁+t₂）+（V_船-V_水）t₁=V_水t₂+V_船t₁；

于是：V_船（t₁-t₂）=0

因为V_船≠0，所以t₁=t₂=30分钟=0.5小时；

故水流速度V_水=

S2

t1+t2

=

5km

0.5h+0.5h

=5km/h；

故答案为：0.5；5km/h．

完形填空

阅读短文，根据短文内容及首字母提示，在空白处填入一个适当的单词。

Do you like animals? Do you want to keep animals? Yes, many people like animals and want to keep a lovely animal as a （71） p_____. It's a pity that they don't have a suitable place to keep them. However, keeping fish may be the best choice.

Why keep them?

People keep dogs and cats because they enjoy their friendship. Fish can not be friendly, but they have beautiful shapes and colors. It's a pleasure to watch as they swim （72）i_____ their tank（水槽）.

What do they need?

First of all, the fish need a suitable （合适的） tank, and you must fill the tank （73） w_____ suitable water. It is also good for （74） t_____ to have a few water plants in the tank to keep the water pure （纯净）. Putting a few snails （蜗牛） in the tank is a good （75） i_____. They will keep it （76） c_____

What about feeding them?

There are many kinds of fish food （77） t_____ you can get in the shops. Most are suitable for all kinds of fish. You should feed your fish only （78） o_____ a day. It is important not to give them （79） t_____ much food. Just give them as much as they can （80） e_____ up in about 15 minutes. This will keep them healthy.

判断题

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