问题 选择题

某人用手竖直向上提着一只行李箱并处于静止状态,如图所示.下列说法中正确的是

A.行李箱的重力与行李箱对人手的拉力是一对平衡力

B.行李箱的重力与人手对行李箱的拉力是一对平衡力

C.行李箱的重力与地球对行李箱的吸引力是一对平衡力

D.行李箱的重力与人手对行李箱的拉力是一对作用力与反作用力

答案

答案:B

分析:平衡力是指作用在同一个物体上,大小相等、方向相反的一对力;而作用力和反作用力是指相互作用的物体之间产生的两个力.

解答:解:A、行李箱的重力作用在行李箱上,而行李箱对人手的拉力作用在人手上,故不可能为平衡力;故A错误;

B、行李箱的重力人手对行李箱的拉力都作用在行李箱上,且使行李箱处于平衡状态,故是一对平衡力,故B正确;

C、行李箱的重力与地球对行李箱的吸引力都是地球作用在行李箱上的力,故不可能为平衡力;故C错误;

D、行李箱的重力的施力物体为地球,而人手对行李箱的拉力是人施加给行李箱的力,施力物体是人,不是两个物体间的相互作用力,故D错误;

故选B.

点评:判断平衡力的关键在于看是否是同一个物体受到的相互平衡的力;而作用力与反作用力是两个相互作用的物体之间的力.

填空题
完形填空
完形填空。
     I agree with the saying that some people “see old memories as a chance to deal with the past and
unite past and present.” Many people are so    1   by things that happened in their past that they
are not able to focus on the   2   . For example, in the book Ceremony, the main character, Tayo,
cannot concentrate on the present because he can’t forget his troubled childhood and    3   continues
to relive things that happened during that time.
     However, past memories can help people to    4    in the present. A    5   example of people
learning from the past would be the Marshall Plan. After the   6   of World War II there were many
war-torn countries around the world in need of    7   assistance to help rebuild their countries, and
the United States would have to be the one to    8    that assistance. Many American politicians
thought it was    9   for the US government to spend money abroad on countries that would not be
able to repay it for a long time. However, George Marshall, a former US general, remembered how the
exact same    10   of "why should we spend money on war-torn nations that really owe us reparations
(赔款)?" had been used after World War I towards Germany. The   11    of assistance towards
Germany after World War I had caused a gigantic economic depression in Germany that had made the
Mark (German money) virtually    12   . The German people became so desperate that they started
   13   an extreme German nationalist named Adolf Hitler, who eventually started World War II.
Marshall knew that if the US did not   14   war-torn Germany and, especially, Japan, we could    15  
have a World War III on our hands.
( ) 1. A. shocked
( ) 2. A. memory
( ) 3. A. constantly
( ) 4. A. forget
( ) 5. A. personal
( ) 6. A. conclusion
( ) 7. A. medical
( ) 8. A. need   
( ) 9. A. reasonable
( )10. A. statement
( )11. A. intension
( )12. A. hopeless
( )13. A. refusing
( )14. A. stop   
( )15. A. similarly
B. troubled
B. situation
B. innocently
B. recover
B. scientific
B. decision
B. political
B. provide
B. necessary
B. argument
B. request
B. worthless
B. employing
B. warn
B. simply
C. punished
C. present   
C. ridiculously
C. enjoy   
C. historical
C. invasion
C. economic
C. find   
C. difficult
C. suggestion
C. influence
C. worldwide
C. supporting
C. resist
C. regularly
D. annoyed      
D. future        
D. rarely        
D. compare      
D. commercial    
D. extension    
D. lawful        
D. prevent      
D. foolish      
D. introduction  
D. lack          
D. valuable      
D. charging      
D. help          
D. unlikely