问题 选择题

一物体质量为,放在斜面上静止,受一沿斜面向上的力作用,产生沿斜面向上的加速度,若将此沿斜面上的力的F的大小变为,其他条件不变,则物体加速度将

A.大于

B.在之间

C.等于

D.等于

答案

答案:A

题目分析:当水平拉力变为2F时,物体所受的滑动摩擦力大小没有变化,根据牛顿第二定律对前后两种进行研究,分析两次加速度之比,进行选择.解答:解:设物体所受的滑动摩擦力大小为f,当拉力变为2F时物体的加速度为a′.根据牛顿第二定律得

F-f=ma

2F-f=ma′

,即a′>3a

故选A。

点评:本题考查对牛顿第二定律的理解能力,公式F=ma中F是合力,不是拉力,不能简单认为拉力变为3倍,加速度就变3倍.

完形填空
完形填空。
     My dearest Aunt Mei lives in the __1__. She grows __2__ there and sells them in town. Sometimes
 she can't sell out all the vegetables, __3__ she brings (带来) some to us. Aunt Mei is old and short, but
 she's very __4__. She can walk __5__.
    Aunt Mei has no__6__ and she lives with her only white cat. Every summer, I go to stay with her for a
 week or two. She often __7__ me to a farm nearby (在附近). Life in the country is interesting. I can go
 fishing and I can __8__ birds singing. But Aunt Mei doesn't __9__ me swim. She says it's dangerous.
    Now the farmers (农民) don't do farm work __10__ animals. They use tractors. All of them look
 happy.
( ) 1. A. farm      
( ) 2. A. vegetables
( ) 3. A. but      
( ) 4. A. fine      
( ) 5. A. slowly    
( ) 6. A. money    
( ) 7. A. brings    
( ) 8. A. look      
( ) 9. A. agree    
( ) 10.A. for      
B. school  
B. rice    
B. so      
B. good    
B. fast    
B. food    
B. takes    
B. read    
B. like    
B. with    
C. country    
C. wheat      
C. or        
C. happy      
C. near      
C. home      
C. lets      
C. listen    
C. let        
C. from      
D. town              
D. animal            
D. and                
D. interesting        
D. far                
D. children          
D. wants              
D. hear              
D. ask                
D. between            
多项选择题