问题 问答题

A、B两车在同一直线上运动,质量为103Kg的A车在前,以恒功率80KW从静止开始行驶,行驶过程所受阻力为104N,经3s后速度达到稳定.之后在后面的B车以速度vB=16m/s与A车同向行驶,当A、B两车相距x=20m时,B车开始刹车,做匀减速运动.计算过程g=10m/s2

(1)A车从开始行驶到速度稳定时经过的路程x是多少?

(2)为避免两车相撞,刹车后B车的阻力至少应为车总重的多少倍?

答案

(1)对A车,根据动能定理,有:

Pt-fs=

1
2
mv2-0 ①

A车达到最大速度时,根据平衡条件,有:

F-f=0 ②

功率为:

P=Fv ③

联立解得:

v=8m/s

s=20.8m

(2)以以A车为参考系,B车做匀减速直线运动,初速度:16-8=8m/s,末速度为零,位移为20m,根据速度位移关系公式,有:

-

v20
=2ax

解得:

a=

-
v20
2x
=
-82
2×20
=-1.6m/s2

根据牛顿第二定律,有:

-f=ma

故:

f
mg
=
-a
g
=
1.6
10
=0.16

答:(1)A车从开始行驶到速度稳定时经过的路程s是20.8m;

(2)为避免两车相撞,刹车后B车的阻力至少应为车总重的0.16倍.

阅读理解

阅读理解

     Do you know why different animals or pests(昆虫) have their special colours? Colours in them

seem to be used mainly to protect themselves.

     Some birds like eating locusts(蝗虫), but birds cannot easily catch them. Why? It is because

locusts change their colours together with the change of the colours of crops(庄稼). When crops

are green, locusts look green. But as the harvest(收获) time comes, locusts change to the same

brown colour as crops have. Some other pests with different colours from plants are easily found

and eaten by others. So they have to hide themselves for lives and appear only at night.

     If you study the animal life, you'll find the main use of colouring is to protect themselves. Bears,

lions and other animals move quietly through forests. They cannot be easily seen by hunters. This is

because they have the colours much like the trees.

     Have you ever found an even more strange act? A kind of fish in the sea can send out a kind of

very black liquid(液体) when it faces danger. While the liquid spreads over(散开), its enemies(敌人)

cannot find it. And it immediately swims away. So it has lived up to now though it is not strong at all.

1. From the passage we learn that locusts __________.

A. are small animals

B. are easily found by birds

C. are dangerous to their enemies

D. change their colours to protect themselves

2. How can pests with different colours from plants keep out of danger?

A. They run away quickly.

B. They have the colours much like their enemies.

C. They hide themselves by day and appear at night.

D. They have to move quietly.

3. Bears and lions can keep safe because __________.

A. they have the colours much like the trees

B. they move quietly

C. they like brown and grey colours

D. they live in forests

4. Why can the kind of fish live up to now?

A. Because it is very big and strong.

B. Because the liquid it sends out can help it escape from its enemies.

C. Because the liquid it sends out can kill its enemies.

D. Because it swims faster than any other fish.

5. Which is the best title for this passage?

A. The Change of Colours for Animals and Pests.

B. Colours of Different Animals and pests.

C. The Main Use of Colours for Animals and Pests.

D. Some Animals and Pests.

单项选择题