某小船在静水中的速度大小保持不变，该小船要渡过一条河，渡河时小船船头垂直指向河岸．若船行至河中间时，水流速度突然增大，则( )
A．小船渡河时间不变
B．小船航行方向不变
C．小船航行速度不变
D．小船到达对岸地点不变
答案：A
题目分析：因为分运动具有等时性，所以分析过河时间时，只分析垂直河岸方向的速度即可，渡河时小船船头垂直指向河岸，即静水中的速度方向指向河岸，而其大小不变，因此，小船渡河时间不变，故A正确；
当水流速度突然增大时，由矢量合成的平行四边形法则知船的合速度变化，航行方向变化，因而小船到达对岸地点变化，故B、C、D错误．
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| I live in Albuquerque, New Mexico. I used to give a lot of money to the homeless,feeling 1 for them. But as time passed, I became a mom with no home, a huge 2 , and hardly any income. As a result, I 3 giving money to the people on the side of the road and became very 4 . Things started to change for me 5 . I had a home, a backyard for my daughter,and plenty of food, and I started to 6 myself out of debt. One day we saw a homeless person with the 7 , "Will work for food." I 8 . My daughter commented. "Mommy, you used to always give money to those people in 9 ." I replied, "Honey, they just use that money for 10 , or other bad things." She didn't respond. 11 when I said that, I didn't feel right. Three days later, I was driving to 12 my daughter from school. A man was standing on the corner, and something deep 13 me said,"Just help the guy." So I 14 down my window, and he ran over excitedly. He said,"God bless you, I only need 77 cents." I 15 into my ashtray (烟灰缸) and strangely enough, there 16 three quarters and two pennies. I picked them up and gave them to him. He 17 with joy and tears in his eyes."Wow, you just made it 18 for me to see my mom for Christmas! Thank you so much; the 19 that had this great sale is leaving in 20 minutes!" It was a moment I'll 20 forever. I think that man won't forget it either, but I was the one who got the best gift in life-GIVING. | ||||
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