问题 问答题

如图所示装置由AB、BC、CE三段轨道组成,轨道交接处均由很小的圆弧平滑连接,其中轨道AB段是光滑的,BC、CE段是粗糙的.水平轨道BC的长度s=5m,轨道CE足够长且倾角θ=37°,A、D两点离轨道BC的高度分别为h1=4.30m、h2=0.60m.现让质量为m的小滑块自A点由静止释放.已知小滑块与轨道BC、CE间的动摩擦因数均为μ=0.5,重力加速度g取10m/s2,sin37°=0.6、cos37°=0.8.求:

(1)小滑块最终静止的位置距B点的距离.

(2)小滑块经过D点时的速度大小.

答案

(1)小物块从A→B→C过程中,由动能定理得

 mgh1-μmgs=

1
2
mvc2                                          

将h1、s、μ、g代入得:vc=6m/s

小物块沿CE段上滑的加速度大小mgsinθ+μmgcosθ=ma1

a1=10m/s2

小物块沿CE段上滑的最大距离x=

vc2
2a1
=1.8m                   

小物块沿CE段下滑的加速度大小mgsinθ-μmgcosθ=ma2      

a2=2m/s2

小物块第2次经过C点时的速度vc′=

2a2x

vc′=

6
5
5
m/s

小物块沿CD段向左运动时加速度大小μmg=ma3        

a3=5m/s2

小物块沿CD段向左运动的距离x1=

vc2
2a3
=0.72m

故小物块最终停止的位置距B点的距离为s-x1=4.28m           

(2)第1次通过D点的速度v1=

vc2-2a1
h2
sinθ
=4m/s

第2次通过D点的速度v2=

2a2(x-
h2
sinθ
)
=1.79m/s.

故两次经过D点的速度分别为4m/s、1.79m/s.

阅读理解

阅读理解。

     Animals, including insects, don't have a "language" like ours. They do not "talk" to each other in words

or sentences. But if we watch them, we can see that they do have their ways of communicating with each

other.

     Can you see the rabbit's tail? When rabbits see this white tail moving up and down, theyrun away. They

know that they are in danger. The rabbit has told them something without making a sound. It has given

them a signal.

     Many other animals use this kind of "language". When a cobra(眼镜蛇) is angry, it raises its head and

makes itself look fierce. This worns other animals. When a bee has found food, it. goes back to its home.

It cannot " tell" the other bees where the food is by speaking tothem, but it does a little dance. This tells the bees where the food is.

    Some animals "say" things by making sounds. A dog barks, for example, when a stranger comes near, A

cat purrs(发出呼噜声) when it is pleased. Some birds make several different sounds, each with its own

meaning. Sometimes we human beings speak in the same way. We make a sound like "Oh" or "Ah" when

we are frightened or pleased or when we drop something on our toes.

1. Which one is TRUE according to the passage?

A. Animals do have a language like that of human beings.            

B. Bees 'communicate with each other by dancing.            

C. Animals can use words.            

D. Animals are brave and friendly.

2. A rabbit uses its tail to _________ .

A. worn other rabbits when they are in danger            

B. tell other rabbits where the food is            

C. make itself look fierce            

D. help it to run fast

3. When a cobra is angry, it ________ .

A. makes a loud noise            

B. raises its head            

C. moves up and down          

D. does a little dance

4. Several different sounds can be made by a _________

A. rabbit    

B. bee          

C. bird          

D. fish

5. From the passage, we know that _________use words and sentences to express ideas and thoughts.

A. all living things            

B. all animals            

C. some birds                

D. human beings

单项选择题