问题 选择题

如图所示,一质量为m的小球套在光滑竖直杆上,轻质弹簧一端固定于O点,另一端与该小球相连,小球在A处时弹簧处于原长。现将小球从A点由静止释放,沿竖直杆运动到B点,已知OA长度小于OB长度。在小球由A到B的过程中

A.加速度大小等于重力加速度g的位置有两个

B.加速度大小等于重力加速度g的位置有三个

C.小球运动到与O点等高的位置时,弹簧弹力的功率不为零

D.弹簧弹力对小球先做正功再做负功

答案

答案:B

题目分析:在A处由于弹簧处于原长状态,弹力为零,在竖直方向上只受重力,所以加速度大小等于重力加速度,当小球运动到与O点等高的位置时,弹力沿水平方向,在竖直方向上没有分力,故小球在竖直方向仍只受重力作用,加速度等于重力加速度,弹簧从A到B过程中先是被压缩,后伸长,所以还在存一个位置,弹簧的弹力为零,在此位置加速度等于重力加速度,故加速度大小等于重力加速度g的位置有三个,A错误;B正确;小球运动到与O点等高的位置时,弹力沿水平方向,速度为竖直方向,两者垂直,所以弹力的功率为零,C错误;在刚开始运动过程中弹簧被压缩,弹力方向与运动方向的夹角大于九十度,弹力做负功,过了与O点等高位置,弹簧仍处于压缩状态,但是弹力方向与速度方向夹角小于九十度,此过程做正功,等小球到了与A关于关于O点对称点后,弹簧恢复原长,此后弹簧处于拉伸状态,拉力方向与速度方向夹角大于九十度,故做负功;故D错误;

完形填空
完形填空。
     Wang Lin is thirteen years old. He lives in the countryside in Shandong Province. Today is his   1  .
His parents cook a nice dinner for him, but the boy says, "I won't have the dinner   2   after the dinner
today is over. And my parents are   3   Guangzhou tomorrow. I don't want them to go."
     It is the   4   birthday Wang Lin spends with his parents these three years. In the past three years,
his parents   5   in Guangzhou and worked to make monkey. Tomorrow they will   6   leave home to
go to Guangzhou.
     Wang Lin is not the only stay-at-home child. In the countryside of China, there are   7   20,000,000
children staying at home without their parents. Stay-at-home children face a lot of problems. Many of 
  8   stay with their grandparents. They get less care from parents. They feel   9  . Some of them don't
study well because their parents can't help them  10  their lessons. 
    The Chinese government (政府) now cares about the children very much and is trying to solve (解决)
their problems.
( )1. A. time         
(     )2. A. so            
(     )3. A. looking for 
(     )4. A. one          
(     )5. A. stayed       
(     )6. A. get to       
(     )7. A. a number of  
(     )8. A. them         
(     )9. A. surprised    
( )10. A. at           
B. birthday     
B. because      
B. waiting for  
B. first        
B. kept         
B. forget to    
B. more than      
B. they         
B. unhappy      
B. with         
C. festival    
C .although    
C. leaving for 
C. two         
C. played      
C. have to         
C. too many    
C. we          
C. interested  
C. for         
D. class    
D. but      
D. coming to
D. a second                       
D. had      
D. ask to   
D. much too 
D. us       
D. excited  
D. on       
单项选择题