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_爱下问搜题

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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

t

21

代入解得v=21.5m/s

同理h=v₀t-

1

2

g

t

22

，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

在建筑工程保险中，第三者责任保险实质上是一种以工程施工为经营活动，以工地为场所的（）。

A.公众责任保险

B.雇主责任保险

C.职业责任保险

D.产品责任保险