问题 选择题

一匀强电场的电场强度E随时间t变化的图象如图所示,在该匀强电场中,有一个带电粒子在t=0时刻由静止释放,若带电粒子只受电场力作用,则下列说法中正确的是(  )

A.带电粒子只向一个方向运动

B.0~2s内,电场力做正功

C.2s末带电粒子回到原出发点

D.4s末带电粒子回到原出发点

答案

带电粒子在t=0时刻由静止释放,在0-1s内带电粒子沿着电场力方向做匀加速直线运动,

1-2s内沿原方向做匀减速直线运动,2s末速度减为零;

2-3s内沿反方向做初速度为零的匀加速直线运动,

3-4s内继续做匀减速直线运动,4s末速度减为零,回到出发点.

A、由上分析可知,带电粒子做往复运动.故A错误.

B、0~1s内电场力做正功,1~2s内电场力做负功.故B错误.

C、D,2s末带电粒子离原出发点最远,4s末带电粒子回到原出发点.故C错误,D正确.

故选D

完形填空
完形填空

     There is a zoo in our city. My parents often take me there on _ 1 _. I like animals. I have a lot of _ 2   
in my room. In the    3   , I can see tigers, elephants, monkeys, pandas, bears, snakes and many other 
   4   .Some animals are friendly but some are not. Tigers, bears, lions are    5 _. That is why they have to
stay in    6   . But I do not think it is good for animals to stay in cages. They should be free. The animals in
cages can't be    7   . I think the most interesting animals in the zoo are the dolphins. I like    8    them swim  and jump. They swim so fast and jump so high. They can play with a ball. They can stand up and walk 
   9 _ water! They are very    10    to people. If you fall into water and can't swim, they may come to help
you.

( )1. A. summer  
( )2. A. real    
(     )3. A. zoo      
( )4. A. tigers    
( )5. A. dangerous
( )6. A. houses    
( )7. A. nice    
( )8. A. looking   
( )9. A. in        
( )10. A. friendly

B. month
B. ugly      
B. park      
B. people    
B. cute  
B. zoos      
B. happy      
B. seeing
B. at        
B. shy        
C . weekdays    
C. toy          
C. bank        
C. friends      
C. interesting    
C. cages        
C. sad          
C. watching      
C. with          
C. warm        
D. weekends      
D. your          
D. library        
D. animals        
D. ugly          
D. pools          
D. sorry          
D. listening      
D. on            
D. cold          
单项选择题