问题 问答题

备受世界关注的中国首艘航空母舰“辽宁舰”已经开始进行直升机和战斗机的舰载机训练,海空陆战队队员也投入了低空跳伞训练,当直升飞机悬停在离航母甲板224m高时,队员离开飞机做自由落体运动,一段时间后打开降落伞,以12.5m/s2加速度做匀减速下降.为了队员的安全,要求队员落地时的速度不能超过5m/s.(g取10m/s2)求:

(1)队员打开降落伞时离甲板的高度至少是多少?

(2)队员在空中的最短时间是多少?

答案

(1)设打开降落伞时,运动员离地面的高度至少为h,此时速度为v1,落地时速度为v,由匀变速直线运动规律:

v21
=2g(H-h)

v2

-v21
=-2ah

联立以上两式,解得:h=99m

(2)设运动员在空中加速时间为t1,位移为h1,加速阶段末速度为v2

则v2=gt1

h1=

1
2
gt21
                        

设运动员减速运动时间为t2,减速下降的高度为h2

h2=v2t2-

1
2
a
t22

v=v2-at2

又h1+h2=H,

联立以上各式解得,运动员在空中飞行的最短时间t=t1+t2=8.6s

答:(1)离甲板的高度至少是99m

(2)最短时间是8.6s

单项选择题 A1/A2型题
阅读理解

阅读理解

     Singles Day falls on every November 11th, and as the name indicates, this relatively new holiday is

one exclusively for people who are still living the single life. I was a bit surprised when I googled "Singles

Day" on the Internet, to find that China is the only country in the world that has set aside a special day for

singles to celebrate their lives. To find out more about this celebration, please read on.

      An old story goes that once there were four single men, leading very boring lives. None of them were married, or had lovers, or did anything exciting. They just sat around all day and played Mahjong.

     One day they played Mahjong from 11 in the morning until 11 at night. During the game, no matter

who won, the winning card was always the "four columns" card (the card shows four independent,

parallel columns in two lines). Even more of a coincidence, it was Nov 11, or 11/11. In order to

commemorate the day, they nicknamed it Singles Day.

     Singles Day was first celebrated at various universities in Nanjing, capital city of Jiangsu Province

during the 1990s. It got the name Singles Day because the date consists of four "ones". These college

students have since graduated, and carried their university tradition into society. Singles Day is now a

special day for all fashionable youths.

     The main way to celebrate Singles Day is to have dinner with your single friends, but it's important

that each person pay their own fee to show their independence. People also hold "blind date" parties in

an attempt to say goodbye to their single lives.

     Many singles also choose to say goodbye to their single lives on this day. Many attend "blind date"

parties and many people choose to marry on this day. In addition to meaning "single" the four "ones" of

the date can also mean "only one" as in "the only one for me." Some people will use this date and this

meaning to tell their special someone that they are the only "one" in their heart.

1.Singles Day got its name because _______.

A. the persons who play Mahjong are four singles

B. the date consists of four "ones"

C. only single persons celebrate the day

D. it was first celebrated at universities in Nanjing

2.Which of the following is wrong?

A. Singles Day was first celebrated in Nanjing's universities.

B. Many singles choose Singles Day to say goodbye to their single lives.

C. The only way to celebrate the Singles Day is to have dinner with your single friends.

D. It was these college graduated students that carried their university's tradition into society.

3.We can infer from the passage that ______.

A. the four singles did not get married

B. Singles Day receives great popularity among young people

C. when having dinner, one pays the fee for all his friends

D. on Singles Day many attend parties where blind people were present

4.What is the best title for the passage?

A. The Origin of Singles Day.    

B. How to Celebrate Singles Day.

C. Singles Day.                

D. Say Goodbye to Singles Day.