问题 问答题

据报道,一儿童玩耍时不慎从45m高的阳台上无初速掉下,在他刚掉下时恰被楼下一保安发现,该保安迅速由静止冲向儿童下落处的正下方楼底,准备接住儿童.已知保安到楼底的距离为15m,为确保安全能稳妥接住儿童,保安将尽力节约时间,但又必须保证接儿童时没有水平方向的冲击,不计空气阻力,将儿童和管理人员都看做质点,设保安奔跑过程中是匀变速运动且在加速或减速的加速度大小相等,g取10m/s2,求:

(1)保安跑到楼底的平均速度

(2)保安奔跑时的最大速度

(3)保安加速时的加速度.

答案

(1)儿童自由落体的时间为t,则有:h=

1
2
gt2,解得:t=
2h
g
=
2×45
10
s
=3s,则保安的平均速度为:
.
v
=
s
t
=
15
3
m/s=5m/s

(2)保安先做加速再匀减速运动,由平均速度等于初末速度之和的一半可得

.
v
=
0+vm
2

解得:vm=2

.
v
=2×5m/s=10m/s

(3)保安的加速度为:a=

vm
t
2
=
10
3
2
m/s2=
20
3
m/s2

答:

(1)保安跑到楼底的平均速度为5m/s

(2)保安奔跑时的最大速度为10m/s

(3)保安加速时的加速度为

20
3
m/s2

单项选择题
阅读理解

The two largest islands of the British Isles are Great Britain and Ireland. England, Wales and Scotland together form Great Britain. Scotland, lying in the northern part of Great Britain and bordering(接壤)England on the south, is half the size of England and Wales, having an area of about 76 000 square kilometers. Most of Northern Scotland is mountainous area known as the Scottish Highlands. In the center of Scotland there are the Central Low-lands, and the south is waving, hilly area known as the Southern Uplands which rise to 800m. There are many rivers in Great Britain, but they are not very long. The rivers which run to the east are longer than the ones which run to the west.

In Scotland the main rainy winds come from the Atlantic Ocean. The west part is wet with an average(平均) rainfall of up to 200cm while the east is dry with about 75cm. The winters are cold with an average January temperature of 4℃. The summers are cold and warm but rarely hot.

1. This passage mainly tells us____ .

A. how many parts form Great Britain

B. the positions of England, Wales and Scotland

C. the areas of Great Britain

D. Something about Scotland

2. England lies____ .

A. in the south of Great Britain          B. to the south of Scotland

C. to the north of Wales                D. in the north of Scotland

3. This passage also tells us____ .

A. the population of Great Britain        B. the history of England and Wales

C. the size of Scotland                 D. the position of Northern Ireland

4. The Central lowlands_____ .

A. lie between the Scottish Highlands and the Southern Uplands

B. lie to the north of Scottish Highlands

C. lie to the south of the Southern Uplands

D. border England on the south

5. The writer says that in Scotland there is more rain____ .

A. in the Highlands than in the Southern Uplands     B. in the west part than in the east

C. in the Central Lowlands than in the Highland      D. in the east than in the west