问题 选择题

一个静止的质量为M的不稳定原子核,当它放射出质量为m、速度为v的粒子后,原子核剩余部分的速度为(  )

A.-v

B.

-mv
(M-m)

C.

-mv
(m-M)

D.

-mv
M

答案

根据动量守恒定律研究整个原子核:

0=mv+(M-m)v′

v′=

-mv
(M-m)
  

故选B.

完形填空
完形填空。
     I know my brother better than anyone else in the world. Since the moment of his birth, I've been part of
his life. I had spent seven  1  years in the world with the attention of those around me. But when Rohan was
born, life  2  Suddenly I had to  3  my toys and there were no more bedtime stories or my own choice of food.
     Finally, I felt my  4  at my brother. The poor baby had no idea what had made me so unhappy. Maybe he
found me strange, the only person in this house  5  did not like him. Whatever the reason was, he loved  6  me
around. However, I seldom talked to him and  7  asked him to leave me alone.
     Then all that changed. I hardly remember everything of that day six years ago,  8  I do remember that I
was feeling very angry. Somehow my feet led me to my brother's bed. My hand, completely  9  from my mind,
reached through the bars (护栏). At once, he reached his small hand and softly touched  10  And that was all
I needed. Through all the unhappiness of the day, that one moment changed  11  How could I hate someone
who made me feel so important? For the   12  time, I saw my brother, not through the eyes of a child who
was no longer favored (宠爱), but through the eyes of a  13  
     To this day, I can't imagine life  14  my brother. It is unbelievable how much I have  15  about life from a
seven-year-old boy. Above all, Rohan has taught me that one should not care only for himself.
( )1. A. happy   
( )2. A. began   
( )3. A. collect 
( )4. A. fun     
( )5. A. which   
( )6. A. showing 
( )7. A. always  
( )8. A. but     
( )9. A. far     
( )10. A. one    
( )11. A. nothing
( )12. A. first  
( )13. A. student
( )14. A. except 
( )15. A. cared  
B. hard      
B. continued 
B. choose    
B. anger     
B. where     
B. carrying  
B. already   
B. so        
B. sick      
B. both      
B. everything
B. second    
B. parent    
B. besides   
B. learned   
C. lonely   
C. changed  
C. hide     
C. fear     
C. who      
C. meeting  
C. hardly   
C. or       
C. different
C. mine     
C. something
C. last     
C. teacher  
C. without  
C. talked   
D. busy      
D. saved     
D. share     
D. worry     
D. what      
D. following  
D. almost    
D. for        
D. free     
D. them       
D. anything  
D. next                      
D. sister    
D. through   
D. heard     
单项选择题 A1型题