搜题请搜索小程序“爱下问搜题”
问题 多选题

一条河宽度为d，河水流速为v1，小船在静水中的速度为v2，要使小船在渡河过程中所行路程最短，则（　　）

A．当v1＜v2时，s=d

B．当v1＜v2时，s=

v
 21
+v
 22
v2
d

C．当v1＞v2时，s=

v1
v2
d

D．当v1＞v2时，s=

v2
v1d

答案

A、当静水速大于水流速，即v1＜v2时，合速度方向可以垂直于河岸，渡河位移最短，最小位移就是河的宽度即s=d．故A正确，B错误．

C、当静水速小于水流速，即v1＞v2时，合速度方向不可能垂直于河岸，即不可能垂直渡河，当合速度的方向与静水速的方向垂直时，渡河位移最短．

设此时合速度的方向与河岸的夹角为θ，sinθ=

v2
v1

则渡河的最小位移x=

d
sinθ
=s=
v1
v2
d，故C正确，D错误．

故选AC．

填空题

短文填词

A little child was playing one day with a very valuable vase.

He put his h________into it and could not draw

it back out.His father tried________vain to get

his hand out.They were thinking of breaking the vase

________(这时) his father said, "Now, my son,

try one more time.Hold your fingers out straight as you see me

d________and then pull."4.________

To________astonishment, the little fellow said.

"Oh no, father, I couldn't pull my fingers out like that because if

I did I________drop my penny." You may think it

is stupid________the boy to stick to penny at the

cost of his hand.Aren't we l________that little boy

sometimes?We are busy holding on to the world's

________(无价值的) penny that prevents us from

enjoying________(自由) belonging to us.
查看答案
问答题 案例分析题

结合图文材料，回答问题。

广西于2009年8月至2010年4月发生了旱灾，旱情分布见图乙。据图乙描述广西旱情的空间分布特点；请列举两项影响广西旱情分布的自然原因。
查看答案