问题 计算题

如图所示,一辆载重卡车沿平直公路行驶.车上载有质量均为m的A、B两

块长方体水泥预制件。已知预制件左端与车厢前挡板的距离为L,A、B间以及B与车厢间的动摩擦因数分别为,各接触面间的最大静摩擦力等于滑动摩擦力。卡车以速度匀速行驶时,因前方出现障碍物而制动并做匀减速直线运动。问:

(1)卡车制动的加速度满足什么关系时,预制件A相对B滑动,而B相对车厢底板静止?

(2)卡车制动后为保证司机安全,在B相对车厢底板静止的情况下,预制件A不与车厢前挡板碰撞,则卡车从开始制动到停止所经历的时间应满足什么条件?

答案

(1)(2) 

题目分析:解:(1)若A相对B滑动,则有:   (2分)

即:  (1分)

若B相对车厢底板滑动,则有:   (2分)

即:   (1分)

要使A相对B滑动,需满足   (1分)

要使B相对于车厢底板静止,需满足  (1分)

联立以上各式得: (1分)

(2)卡车制动后,设A的位移为s1,有 (1分)

卡车的位移为s,有:  (2分)

要使A不与车厢的前挡板相碰,应满足   (1分)

 (1分)

故:   (1分)

设卡车制动时间为t,则有  (2分)

 (1分)

单项选择题
完形填空
完形填空。
     Last January I was given a chance to work in another country. At first, I lived a   1   life, but soon I
met a group of new friends and they were really nice.
     Then something   2   happened. One of our friends suffered from kidney stones (肾结石), which had
to be   3   surgically as soon as possible. But it was not as   4   as that! Our friend could not   5   the
operation, which put him into deep   6  . I didn't know how to help him because I didn't have any money
either.   7  , I never lost hope. Instead I tried to think of other   8   and told him not to worry because
everything would be all right.
     Before I left the hospital, I talked to the surgeon and asked him to do the surgery, saying I would pay
the expenses the next day   9   I had that kind of money. Well, I must have been   10   because the
surgeon agreed to do it! But how was I going to get that much money?
     All I knew was that I needed to help my friend! Although it could be   11   to deal with this kind of
situation, I decided to make some efforts. I tried to borrow money but failed. I tried   12   my things but
no one wanted them. Then I had the idea of talking to my   13  . I asked him if I could have my next two
months' salary in advance-without even thinking on how I would   14   for two months with no money.
He was surprised and asked me the reason. I told him that I had a friend who was really in need.   15  
I convinced him too and then he gave my two months' salary without any   16  !
     Upon getting the money, I ran to the hospital and paid all the bills. My friend didn't know how to
thank me. I just said, "Come on! Give me a   17  !"
     Friendship may not only be found at the happy moment but sometimes you can find it on the   18  
time in your life! I've done a good thing by being a (an)   19   friend and I think it is well worth the   20  .
( )1. A. private      
( )2. A. strange      
( )3. A. removed      
( )4. A. terrible      
( )5. A. stand        
( )6. A. anxiety      
( )7. A. Thus          
( )8. A. reasons      
( )9. A. even if      
( )10. A. encouraging  
( )11. A. challenging  
( )12. A. saving      
( )13. A. colleague    
( )14. A. adapt        
( )15. A. Luckily      
( )16. A. analysis    
( )17. A. favo        
( )18. A. best        
( )19. A. supportive  
( )20. A. tolerance    
B. lonely    
B. wrong      
B. reduced    
B. painful    
B. consider  
B. confusion  
B. Otherwise  
B. options    
B. as if      
B. annoying  
B. disturbing
B. handling  
B. relative  
B. survive    
B. Slightly  
B. dignity    
B. hug        
B. longest    
B. sensitive  
B. sympathy  
C. colorful    
C. unexpected  
C. examined    
C. simple      
C. perform      
C. embarrassment
C. However      
C. symptoms    
C. so that      
C. astonishing  
C. amazing      
C. donating    
C. boss        
C. continue    
C. Constantly  
C. hesitation  
C. promise      
C. strangest    
C. aggressive  
C. pressure    
D. normal          
D. unfair          
D. lifted          
D. special        
D. afford          
D. dissatisfaction
D. Therefore      
D. expectations    
D. in case        
D. convincing      
D. scaring        
D. selling        
D. surgeon        
D. wander          
D. Occasionally    
D. remark          
D. reply          
D. toughest        
D. attentive      
D. effort