问题 选择题

如图所示,在绝缘平面上方存在着足够大的水平向右的匀强电场,带正电的小金属块以一定初速度从A点开始沿水平面向左做直线运动,经L长度到达B点,速度变为零.此过程中,金属块损失的动能有

2
3
EK转化为电势能.金属块继续运动到某点C(图中未标出)时的动能和A点时的动能相同,则金属块从A开始运动到C整个过程中经过的总路程为(  )

A.1.5L

B.2L

C.3L

D.4L

答案

小金属块从A运动到B过程,金属块损失的动能有

2
3
EK转化为电势能,则得克服电场力做功为 W=
2
3
EK=qEL

根据动能定理:外力所做的总功等于物体的动能变化,可知,金属块克服摩擦力做功为 Wf=

1
3
EK=fL

设小金属块从B运动到C经过的路程为s,则由动能定理得:qEs-fs=Ek

联立以上三式,得s=3L

所以金属块从A开始运动到C整个过程中经过的总路程为x=L+s=4L.

故选D

判断题
阅读理解

阅读理解。

    Cambodia was in many ways similar to Laos, although it has twice the population. At another inn, we

talked with a teacher who told us that half of the people in her country couldn't read or write. Her village

couldn't even afford to build a school, so she had to teach outside under a large tent. When we said goodbye,

we all felt very lucky to have studied in college. Back on the road, we passed between many hills and forests.

Then we came to the plains and entered Phnom Penh (金边), the capital of Cambodia. In many ways it looked

like Vientiane and Ho Chi Minh City; it also had wide streets with trees in rows and old French houses. Unlike

Vientiane, ships could travel the Mekong River here. In the centre of the city we visited the palace and saw a

beautiful white elephant. It can only be seen outside the palace on special days. We ate an early supper and

went to see a great temple with floors made of silver.

    The next morning our group slept late. We were very tired from the long bike ride the day before. Cycling

in the hills had been difficult. Now our cousins had the chance to make jokes about Wang Wei and me. Perhaps,

they said, they were the strong ones! We had lunch at a nice outdoor cafe. then rode out of the city.

    Two days later we crossed the border into Vietnam. We began to see many more people, but I wasn't

surprised. I read in an atlas before our trip that Vietnam has almost seven times the population of Cambodia.

We met a farmer who gave us directions and told us that he grows a new rice crop four times every year so

he can feed more people. He also told us that the northern part of his country has many mountains and it is

much cooler than here in the south, where it is flat. Although the flat delta made it easier for us to cycle, we

got warm very quickly. So we drank lots of water and ate lots of bananas. Soon the delta separated into nine

smaller rivers. Two days later, after we had passed thousands of rice fields, we came to the sea. We were

tired but also in high spirits: our dream to cycle along the Mekong River had finally come true.

1. Why did the author (作者) say they all felt very lucky to have studied in college? Because _____. [ ]

A. they studied a lot

B. they knew how to cycle down along Mekong River

C. they learnt that half of Cambodians couldn't read or write

D. the teacher they talked with told them

2. Which is the right order of the autor's travel route (路线) according to the passage?

    a. passed thousands of rice field, then came to the sea

    b. talked with a Cambodian teacher at an inn

    c. had lunch at a nice outdoor cafe

    d. entered Phnom Penh

    e. crossed the border into Vietnam

    f. visited the palace and saw a beautiful white elephant [ ]

A. dbfcea

B. dbfeca

C. bdfaec

D. bdfcea

3. Who gave the author directions when they were in Vietnam? [ ]

A. hunter.

B. A teacher.

C. A farmer.

D. A fisherman.

4. Suppose Laos has a population of 6.5 million, what's the population of Vietnam?[ ]

A. 13 million

B. 1.3 million

C. 91 million

D. 9.1 million

5. Which of the following statements about Phnom Penh is true according to the passage? [ ]

A. It's the capital of Vietnam.

B. It has twice the pobulation of Laos.

C. Ships can't travel the Mekong River here.

D. There is a great temple with floors made of silver.