问题 选择题

如图所示,一个可看作质点的物体沿两个半径分别为R和r,相连接的半圆弧轨道从A点经B点运动到C点,A、B、C三点共线,则物体从A运动到C的位移大小和路程分别为(  )

A.2R,2πR

B.2R-2r,πR+πr

C.2R-2r,2R+2r

D.R-r,R+r

答案

位移由初位置指向末位置,质点沿半径为R的半圆运动后,接着沿半径为r的半圆运动到C点,当质点由A点运动到C点时,位移大小等于AC的长度,等于2R-2r,路程等于两个半圆的长度,即等于πR+πr.故B正确.A、C、D错误.

故选B.

解答题
阅读理解

阅读理解

     Remember how great exercise was when you were a little kid?Back then, racing around the

playground or skipping rope for hours, you weren't thinking fitness, you were thinking entertainment.

But in this age of high-tech home equipment and underused gym memberships, the simple joy of

jumping rope has been forgotten.Rediscovering it will give you a total-body exercise you can find.

     Although considered an excellent  form of exercise, jumping rope  has  never gained widespread

acceptance because of two fundamental reasons.First, most people recognize jumping rope as an

excellent form of cardiovascular (心血管的) exercise, but they also believe that it is simply too

difficult.In other words, they don't think they'll be able to continue jumping for the near 20 minutes

that it takes to achieve a beneficial physical outcome.Second, many view it as somewhat boring and

overly repetitive-not as something fun or enjoyable.

     As a matter of fact, jumping rope can be great fun if you find a proper way to practice it.Instead

of doing the usual two-foot bounce over and over again, people good at rope-jumping often change

their pattern every 10 or 20 jumps.A single bounce, a double-bounce, a skip, a knee-up, side swings

as well as a variety of other easy-to-learn free-style rope-jumping.

     Now researchers are learning that jumping rope also prepares the brain for learning.It is an exercise

allowing both brain hemispheres to perform in parallel to each other.In short, jumping rope can be a

life-long activity requiring little equipment, time and space, yet leading to a much healthier life.

1. From the first paragraph we learn that________.

A. jumping rope has faded from people's memories

B. people now have more advanced equipment

C. racing around the playground was preferred

D. people now like to have exercise in a gym

2. Rope jumping has not spread widely because________.

A. it benefits the cardiovascular system

B. it is neither easy nor enjoyable

C. it is considered boring and repetitive

D. it requires little equipment, time and space

3. The first sentence in the 3rd paragraph implies________.

A. there is only one proper way to follow

B. the usual way should not be used

C. the easiest way is always the best

D. there are many ways to follow

4. According to the researchers, jumping rope________.

A. only prepares the brain for learning

B. is suitable for students only

C. helps both brain hemispheres work together

D. can be dangerous for old people

5. What is the author's attitude towards rope jumping?

A. He is arguing against it.

B. He is in favor of it.

C. He is sitting on the fence of it.

D. He is not clear about it.