问题 选择题

如图所示,轮子的半径均为R=0.20m,且均由电动机驱动以角速度w=8.0rad/s逆时针匀速转动,轮子的转动轴在同一水平面上,轴心相距d=1.6m.现将一块均匀木板轻轻地平放在轮子上,开始时木板的重心恰好在O2轮的正上方,已知木板的长度L>2d,木板与轮子间的动摩擦因数均为=0.16(滑动摩擦力等于最大静摩擦力).则木板的重心恰好运动到O1轮正上方所需的时间是(  )

A.1s

B.0.5s

C.1.5s

D.条件不足,无法判断

答案

轮子的线速度v=Rω=0.2×8m/s=1.6m/s,木板的加速度a=μg=1.6m/s2,当木板达到轮子线速度所经过的位移x=

v2
2a
=
1.62
2×1.6
m=0.8m<1.6m,知木板先做匀加速直线运动再做匀速直线运动,匀加速直线运动的时间t1=
v
a
=
1.6
1.6
s=1s
,位移为0.8m,则匀速直线运动的位移x2=1.6-0.8m=0.8m,匀速直线运动的时间t2=
0.8
1.6
s=0.5s
,所以运动的总时间t=1.5s.故C正确,A、B、D错误.

故选C.

阅读理解

Did you visit the Shanghai World Expo last summer? What impressed (给……留下印象)  you? 21st Century Kids invites four kids to talk about their experiences. They are back from the Expo. What did they bring us? Let’s see what they have to say.

I went to the SAIC-GM Pavilion (上汽集团通用汽车馆). I found my dream car there. It is a green car. Its name is “Leaf”. Chinese engineers made the car. Its roof is in the shape of a leaf. The roof takes in CO2 and changes it intoelectricityfor the car.

The Japan Pavilion is full of high technology. The “Wonder Camera” is cool. It can find a smiling face in a crowd. When you’re smiling, it takes a photo of you. I want to invent something like that in the future.

My favorite pavilion is the Germany Pavilion. I watched the Energy Source show. Visitors made a ball swing back and forth (来回摇摆) by using only their voices. It’s amazing! Although we had waited for quite a long time before we entered the hall, I think it is well worth seeing with your own eyes.

I like painting, so my parents took me to the France Pavilion to see the famous paintings. But the French cooks caught my eye. I saw them cook. My mom told me that they are some of the best cooks in the world. The dish they cooked looked like a painting. So, being a cook is my second dream job.

小题1: The 21st Century Kids seems to be _____.

A.a travel service

B.a newspaper

C.a group of tourists

D.four children小题2: What does the underlined word “electricity” mean in the passage?

A.电

B.汽油

C.灯光

D.指示小题3: What activity did the writer experience in the France Pavilion?

A.Using the “Wonder Camera”

B.Driving the green car “Leaf”.

C.To have delicious French dishes.

D.To see cooks painting.小题4: What made the ball in the Germany Pavilion swing?

A.Visitors’ smiling faces

B.CO2 from cars

C.Some secret power

D.Noise made by visitors小题5: The trip to the Shanghai World Expo made the four kids _____.

A.hardworking and polite

B.amazed and frightened

C.patient and cheerful

D.excited and curious

选择题