问题 多选题

如图所示,MPQO为有界的竖直向下的匀强电场,电场强度为E,ACB为光滑固定的半圆形轨道,圆轨道半径为R,AB为圆水平直径的两个端点,AC为

1
4
圆弧.一个质量为m电荷量为-q的带电小球,从A点正上方高为H处由静止释放,并从A点沿切线进入半圆轨道.不计空气阻力及一切能量损失,关于带电粒子的运动情况,下列说法正确的是(  )

A.小球一定能从B点离开轨道

B.小球在AC部分可能做匀速圆周运动

C.若小球能从B点离开,上升的高度一定小于H

D.小球到达C点的速度可能为零

答案

A、由于题中没有给出H与R、E的关系,所以小球不一定能从B点离开轨道,故A错误;

B、若重力大小等于电场力,小球在AC部分做匀速圆周运动,故B正确.

C、由于小球在AC部分运动时电场力做负功,所以若小球能从B点离开,上升的高度一定小于H,故C正确;

D、若小球到达C点的速度为零,则电场力大于重力,则小球不可能沿半圆轨道运动,所以小球到达C点的速度不可能为零.故D错误.

故选BC.

填空题
阅读理解

阅读理解。

     阅读下面短文并回答问题,然后将答案写到答题卡相应的位置上(请注意问题后的词

数要求)。

    [1] When you get in your car,you reach for it. When you're at work, you take a break to

have a moment alone with it. When you get into a lift, you play with it.

    [2] Cigarette? Cup of coffee? No, it's the third most addictive thing in modern life:               .

And experts say it is becoming more difficult for many people to control their longing to bug it

more tightly than most of their personal relationships.

    [3] The costs are becoming more and more evident,and I don't mean just the monthly bill.

Dr. Chris Knippers,a counselor at the Betty ford Center in Southern California,reports that the

overuse of cell phones has become a social problem not much different from other harmful addictions:

a barrier to one-on-one personal contact,and an escape from reality.

    [4] Sounds extreme,but we've all witnessed the evidence:The person at a restaurant who talks on

the phone through an entire meal,ignoring his kids around the table;the woman who talks on the

phone in the car, ignoring her husband;the teen who texts messages all the way home from school,

avoiding contact with kids all around him.

    [5] Jim Williams,an industrial sociologist based in Massachusetts, notes that cell-phone addiction

is part of a set of symptoms in a widening gulf of personal separation. He points to a study by Duke

University researchers that found one-quarter of Americans say they have no one to discuss their

most important personal business with. Despite the growing use of phones,e-mail and instant messaging,

in other words,Williams says studies show that we don't have as many friends as our parents."Just as

more information has led to less wisdom,more acquaintances via the Internet and cell phones have

produced fewer friends."he says.

1. Fill in the blank in Paragraph 2 with proper words. (no more than 3 words)

                                                                                    

2. Which sentence in the text is closest in meaning to the following one?

    It is getting increasingly apparent to me that cell phone users have more to sacrifice besides

    the regular payment for the calls and messages.

                                                                                    

3. What is the purpose of Paragraph 4? (no more than 5 words)

                                                                                    

4. What do experts say about cell-phone addiction?

                                                                                    

5. Translate the underlined sentence in Paragraph 5 into Chinese.