问题 选择题

将一小球以初速度为υ从地面竖直上抛后,经过4s小球离地面高度为6m,若要使小球竖直上抛后经2s到达相同高度,g取10m/s2,不计阻力,则初速度υ0应(  )

A.大于υ

B.小于υ

C.等于υ

D.无法确定

答案

由题,对于第一情况:h=6m,t1=4s,则得h=vt1-

1
2
g
t21

代入解得v=21.5m/s

    同理h=v0t-

1
2
g
t22
,代入解得v0=13m/s

所以v0<v.故选B

完形填空
完形填空。
     What must you do when you receive a present 1        your birthday? You have to sit down and write a thankyou note. The words "Thank you" are   2    important. We have to use them very often. We say them when someone gives us a drink,   3      us to pick up things, lends us a book or gives us a gift.
     Another important word is "please".   Many.people        4      to use it. It is rude to ask someone to do something without saying "please".We have to use it when we   5   something, too. It may be a book 6      a pencil, mor rice or help.  We have to use "please" to make the request pleasant.
     We have     7       "Sorry", too.  When we have   8  someone, we'll have to go up and say we're sorry.When we have forgotten something, we will also say "sorry".  We can make people     9 wrongs by saying it.
     These three words are  10      . Our children must learn to use them.  They are pleasing words to use in any languages.
( )1. A. in                    
( )2. A. so                    
( )3. A. helps                  
( )4. A. want                  
( )5. A. ask for                
( )6. A. but                    
( )7. A. learn to say          
( )8. A. fell                  
( )9. A. remember              
( )10.A. difficult but important
B. at                    
B. very                  
B. has                    
B. forget                
B. say                    
B. and                    
B.  to learn and say      
B. touched                
B. forget                
B.  simple but important  
C. for                    
C. rather                
C. lets                  
C. like                  
C. need                  
C. or                    
C.  learn and say        
C. felt                  
C. need                  
C.  useful but difficult  
D. to                     
D. such                      
D. makes                      
D. remember                  
D. want                      
D. as                        
D.  to learn to say       
D. hurt                      
D. think                      
D.  helpful but difficult    
单项选择题