问题 选择题

如图6-3-12所示,A、B为平行金属板,两板相距为d,分别与电源两极相连,两板的中央各有一小孔M和N.今有一带电质点,自A板上方相距为d的P点由静止自由下落(P、M、N在同一竖直线上),空气阻力忽略不计,到达N孔时速度恰好为零,然后沿原路返回.若保持两极板间的电压不变,则错误的说法是(  )

图6-3-12

A.把A板向上平移一小段距离,质点自P点自由下落后仍能返回

B.把A板向下平移一小段距离,质点自P点自由下落后将穿过N孔继续下落

C.把B板向上平移一小段距离,质点自P点自由下落后仍能返回

D.把B板向下平移一小段距离,质点自P点自由下落后将穿过N孔继续下落

答案

答案:B

选B.电容器两极板间电压U保持不变.若质点P运动到N孔,速度为零,重力做功与电场力做功相等,则不论A板上移还是下移,重力做功都与电场力做功相等,质点均能沿原路返回.若把B板向上平移一小段距离,重力做功变小,电场力做功不变,因此质点可以原路返回.若把B板向下平移一小段距离,质点到达N孔后有一竖直向下的速度,即将穿过N孔继续下落.

完形填空
完形填空。
     Mrs. Brown lived in a small town near a big farm. One Friday afternoon after she finished     1     
housework, she went to her small shop. She opened the window of the shop and     2     out side. She
liked to see the green farm. There were many cows, horses and small animals     3     the farm. Suddenly
she saw a kangaroo under her window. It was interesting to see that it was    4      an old jacket. It stood
there and looked hungry.  Mrs. Brown gave it some bread     5    water. The kangaroo ate and drank
them     6    . Then it became happy and jumped away. Suddenly something dropped on the     7     from    
its jacket pocket. It was a wallet!She picked it up, opened it and found there was $ 300 and a photo in it.
That was a man's photo with his name "John" on it. John? She looked more _  8     and remembered she
had a brother many years ago. His name was also John. "Is this man my lost brother?" she thought. She
took the wallet with her and began to     9     the man. She asked many people in the town, and then she
came to the farm and asked the farmer. To her great surprise, it was John, her lost brother. They were
very excited and    10_  to see each other.
( )l. A. his      
( )2. A. looked  
( )3. A. for      
( )4. A. wears    
( )5. A. or      
( )6. A. all      
( )7. A. back    
( )8. A. careful  
( )9. A. find    
( )10. A. sad    
B. our      
B. saw      
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B. wearing  
B. and      
B. none    
B. water    
B. carefully
B. call    
B. angry    
C. their    
C. look      
C. on        
C. wear      
C. but      
C. any      
C. river    
C. care      
C. look for  
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D. her          
D. see          
D. from        
D. has          
D. nor          
D. many        
D. ground      
D. careless    
D. look after  
D. happy        
填空题