问题 问答题

一匀强电场,场强方向是水平的(如图).一个质量为m的带正电的小球,从O点出发,初速度的大小为v0,在电场力与重力的作用下,恰能沿与场强的反方向成θ角的直线运动.求小球运动到最高点时其电势能与在O点的电势能之差.

答案

设电场强度为E,小球带电量为q,因小球做直线运动,它受的电场力qE和重力mg的合力必沿此直线,如图:

mg=qEtanθ  

由此可知,小球做匀减速运动的加速度大小为:a=

g
sinθ

设从O到最高点的路程为s,

由速度和位移的关系得:v02=2as

物体运动的水平距离为:l=scosθ  

两点的电势能之差:△W=qEl  

由以上各式得:△W=

1
2
m
v20
cos2θ

O点的电势能与在O点的电势能之差为

1
2
m
v20
cos2θ.

综合题
阅读理解

C

On May 8, last Thursday morning, the Olympic flame was lit on the top of Mount Qomolangma! It was a historic moment for China exactly three months ahead of the Beijing Games.

Tibetan female climber Cering Wangmo reached the summit with a special extreme-altitude Olympic torch. This event is what China has promised to the world as the high point of the torch relay, the longest and mostchallengingof all time.

Five torchbearers(火炬手) finished the highest relay ever—three of the climbers are Tibetan athletes.

Team members said “Beijing welcomes you!” in Chinese, English and Tibetan, as they stood at the summit celebrating, with the event broadcast on national television.

“I finally fulfilled the last dream of my husband,” said Gyigyi with tears in her eyes. She was the first torchbearer. Her husband lost his life in an attempt to climb all 14 mountains in China more than 8,000 meters three years ago.

“We are on top of the world! One world, one dream,” shouted Nima Cering, at the top of his voice. He said that although he had climbed the peak(峰顶)several times, this was the most significant ascent(登高). “As a Tibetan and a Chinese I pray for the success of Beijing 2008,” he added.

The special torch is different from the one currently touring the Chinese mainland, after an around-the-world relay. Officials organizing the trek up the peak had earlier set an April 26 target to begin the ascent, but high winds and heavy snow had delayed the mission.

The Xinhua News Agency said that the Qomolangma leg of the torch relay represents the Olympic motto, “higher, faster and stronger,” in practice and will promote unity among all 56 ethnic groups in China.

小题1:It was a historic moment because ______.

A.some Tibetan torchbearers took part in it

B.it was lit three months before the Olympic Games were held

C.it was the first time that the Olympic torch had been lit on the top of Mount Qomolangma

D.it was the longest attempt in history小题2:Which is RIGHT according to the whole passage?

A.All the five torchbearers are men.

B.Only the five torchbearers reached the summit.

C.Team members said “Beijing welcomes you!” in their native language.

D.At least two of the five torchbearers are females.小题3: What does the word “leg” underlined in the last paragraph refer to?

A.One part of a journey or race.

B.One of the long parts that connect the feet to the rest of the body.

C.The leg of an animal, especially the top part, cooked and eaten.

D.The part of a pair of trousers that covers the leg.小题4: What would be the best title for the text?

A.One world, one dream

B.Torch relay reaches its highest point

C.Qomolangma, the highest summit in the world

D.A special torch小题5: Where can you read about this passage?

A.In the English textbook.

B.In the newspaper.

C.In the science report.

D.On the street wall.