问题 单项选择题

如图所示,一轻质弹簧其上端固定在升降机的天花板上,下端挂一小球,在升降机匀速竖直下降过程中,小球相对于升降机静止。若升降机突然停止运动,设空气阻力可忽略不计,弹簧始终在弹性限度内,且小球不会与升降机的内壁接触,则以地面为参照系,小球在继续下降的过程中()

A.速度逐渐减小,加速度逐渐增大

B.速度逐渐增大,加速度逐渐增大

C.速度逐渐减小,加速度逐渐减小

D.速度逐渐增大,加速度逐渐减小

答案

参考答案:A

解析:题意,原来降机匀速竖直下降,小球受到的弹簧的弹力与重力平衡;若升降机突然停止运动,由于惯性,小球继续下降,弹簧的弹力逐渐增大,小球的合力方向竖直向上,与速度方向相反,小球的速度逐渐减小,根据牛顿第二定律得知,其加速度逐渐增大;则选项B、C、D错误.故选A。

阅读理解

阅读理解。

     Imagine one day the water taps (水龙头) in your house stop running. You have to pay a lot of money.

to buy water from shops. And still there isn't enough water for everyone. Your mother has to save the

family's shower water to use for washing clothes. Would you be able to stand that kind of life?

     Probably not. But that is what kids in Yemen (也门) are experiencing (经历). According to a report,

the capital, Sana, will run out of drinking water as early as 2025.

     Because of the water shortage (短缺), the Yemen government often cuts the water supply (供应).

Hannan, an 18-year-girl, tells the Teens, "In a good week we'll have a water supply all week. But then

the next week there will be water only for a day or two."

     Hannan says only rich people have enough water to use. They can buy water from the shops or from

the water trucks. Private (私人的) companies own the trucks, They travel around the city every day to

sell water — at very high prices.

     "A lot of people can't pay for it," she says.

     The government is thinking of making use of seawater. But it will cost a lot and it may not happen

soon enough to help the people of Yemen.

1. How old is Hannan?

A. She is eight.

B. She is eighteen.

C. She is seventeen.

D. She is seven.

2. The underlined word "that" means _____.

A. buying water from shops

B. having enough drinking water

C. suffering (忍受) from water shortage

D. washing clothes with used water

3. From the passage, we know that _____.

A. the people of Yemen don't have to worry about water

B. people in Yemen have no water to use during a bad week

C. Sanna will have no drinking water in 10 years

D. private companies make much money by selling water

4. The Yemen government _____.

A. may try to make use of seawater

B. does nothing about the water problem

C. only cares about rich people's lives

D. may solve the problem soon

5. The writer wants to tell us _____.

A. not to use seawater

B. not to waste water

C. to drink less water

D. to sell water to Yemen

单项选择题 共用题干题