问题 多选题

如图所示,两只完全相同的均匀光滑小球置于半径为R的圆柱形容器中,且小球的半径r满足2R>2r>R,则以下关于A、B、C、D四点的弹力大小说法中正确的是(  )

A.D点的弹力大小一定大于一个小球的重力大小

B.D点的弹力大小等于A点的弹力大小

C.B点的弹力大小恒等于一个小球重力大小的2倍

D.C点的弹力大小一定大于一个小球的重力大小

答案

A、D以上面的小球为研究对象,分析受力情况:重力G、D点的弹力F1和C点的弹力N1,如图1所示.

设两球连线与竖直方向的夹角为θ.由平衡条件得:

F1=Gtanθ,N1=

G
cosθ

由题,2r>R,则由数学知识得知,F1不一定大于G,cosθ<1,则N1 一定大于G.即D点的弹力不一定大于小球的重力,C点的弹力一定大于小球的重力.故A错误,D正确.

B、C以两球整体为研究对象,分析受力如图2所示:总重力2G、D点的弹力F1、A点的弹力F2、B点的弹力N.由平衡条件得到:

F1=F2,即D点的弹力大小等于A点的弹力大小.

N=2G,即B点的弹力恒等于一个小球重力的2倍.故BC均正确.

故选BCD.

阅读理解

阅读理解。

  A man was sitting in the park watching the ducks on the pond, when a

woman went to him. She had a white dog. She sat down on a bench, took out a

magazine and began to read. The dog was sitting between her and the man. After

it had been sitting quietly for about ten minutes, the dog turned and looked up at

the man.

  "Excuse me,"the dog said, "but you look like a kind man. Please buy me

from this terrible woman."

  The man could hardly believe his ears. A dog spoke to him in English!

  "She is so mean to me, " the dog continued, "She never pets me or plays

with me. She doesn't feed me properly and she often beats me when she's in a

bad mood, which is most of the time.”

  The man was amazed to hear the dog say all this. He was even more

surprised when the dog added, "And I'm really a very good dog. I'm quite

famous, too. I won a medal in the last war and was twice wounded in action. I've

saved three children- two from drowning and one from a fire."

  The man decided that he would try to buy the dog from the woman. He

would pay a lot of money if necessary. The dog would soon earn tens of

thousands of dollars on television. It could be the world’s first talking dog.

  Excuse me,"he said to the woman, "but I love your little dog. Will you sell

him to me? "

  I'll be glad to sell him," the woman said. "You can have him for ten

dollars."

  "Ten dollars!” the man said. "Only ten dollars? Why so little?" "Because, "

the woman said, "he always tells lies."

1. The dog explained that it had _____.

A. stopped a fireman from saving three children  

B. never been in the war

C. acted bravely in dangerous situations        

D. refused a medal in the last war

2.The man would like to pay _____ to buy the dog.

A. ten dollars            

B. only a little        

C. a lot of money    

D. nothing

3.The woman was _____.

A. happy to sell her dog                      

B. happy to feed her dog

C. hoping her dog would earn much money      

D. hoping her dog would help the man

单项选择题