问题 多选题
如图所示,一水平传送带以速度v1向右匀速传动,某时刻有一物块以水平速度v2从右端滑上传送带,物块与传送带间的动摩擦因数为μ.则下列说法正确的是(  )

A.如果物块能从左端离开传送带,它在传送带上运动的时间一定比传送带不转动时运动的时间长

B.如果物块还从右端离开传送带,则整个过程中,传送带对物块所做的总功一定不会为正值

C.如果物块还从右端离开传送带,则物块的速度为零时,传送带上产生的滑痕长度达到最长

D.物块在离开传送带之前,可能做匀速直线运动

答案

BD

阅读理解

阅读理解。

     Planting is the most important part of beautifying the surroundings (美化环境), remaking nature

(改造自然) and developing the national economy (经济). It can make the country rich and the people

happy. Planting can stop the wind from blowing the earth and sand away. It conserves (贮存) water and

keeps water and soil. It can care the people's life and property. It is good for everything around us.

     Today the forests' area of our country is very little. Among one hundred and sixty countries all over

the world, it reaches the one hundred and twentieth place. Our forests' area is less than that of other

countries.

     In the recent years, because we don't do the work about planting well, we have met with lots of

natural calamities (灾害), like drought, floods, wind, sandstorms and so on. They bring much harm to

the people's health, life and property. From now on, we must plant trees and cover the mountains with

trees. Everyone should love trees, care trees and look after trees. We must plant trees everywhere and

make our country green. We must bring benefit (造福) to human beings.

1. We plant trees because

A. planting is very interesting

B. planting can only beautify our schoolyard

C. planting is good for the people's life, health and property

D. we can get lots if wood

2. Which of the following isn't true ?

A. Planting can keep the earth and sand away .

B. Planting can stop the wind from blowing the earth and sand away .

C. Our forests' area is at the back of one hundred and sixty countries .

D. Planting brings good wealth (财富) to mankind .

3. It reaches the one nundred and twentieth place . Here "it" means

A. the coverage rate

B. the forests' area

C. the world's average number

D. the countries all over the world

4. If we don't pay attention to planting ,what will we meet with ?

A. Drought

B. Sand storms

C. Floods

D. All of the above

问答题 简答题