问题 问答题

如图所示,倾角为37°,长度为L的光滑绝缘斜面上,一个带正电荷的小物体质量为m,带电量为q,置于斜面顶端.当沿水平向右加一个如图所示的匀强电场时,木块恰好静止在斜面上.从某时开始,匀强电场的电场强度突然减小为原来的

1
2
,并保持下去.求(1)原来的电场强度;(2)带电物体滑到斜面底端时速率.(已知重力加速度为g,sin37°=0.6)

答案

(1)受力分析图,如图所示

  将电场力与重力合成,则有:

  mg=Ncos37°

  qE=Nsin37°

 所以

qE
mg
=tan37°=0.75

 得:E=

3mg
4q

(2)当电场强度减为一半时,根据动能定理,则有:

(mgsin37°-

E
2
qcos37°)L=
1
2
mv2

 解得:v=

0.6gL

答:原来的电场强度为

3mg
4q
;带电物体滑到斜面底端的速率为
0.6gL

阅读理解

阅读理解。

     Louis Armstrong had two famous nicknames (绰号). Some people called him Bagamo. They said his mouth

looked like a large bag, Musicians often called him Pops, as a sign of respect for his influence (影响) on the

world of music.

     Born in 1901 in New Orleans, be grew up poor, but lived among great musicians. Jazz was invented in the

city a few years before his birth. Armstrong often said, "Jazz and I grew up together."

     Armstrong showed a great talent (天赋) for music when he was taught to play the cornet (短号) at a boy's

home. In his late teens, Armstrong began to live the life of a musician. He played in parades, clubs, and on the

steamboats that traveled on the Mississippi River. At that time, New Orleans was famous for the new music of

jazz and was home to many great musicians. Armstrong learned from the older musicians and soon became

respected as their equal.

     In 1922 he went to Chicago. There, the tale of Louis Armstrong begins. From then until the end of his life,

Armstrong was celebrated and loved wherever he went. Armstrong had no equal when it came to playing the

American popular song.

     His cornet playing had a deep humanity (仁爱) and warmth that caused many listeners to say, "Listening to

Pops just makes you feel good all over." He was the father of the jazz style (风格) and also one of the best-

known and most-admired people in the world. His death, on July 6, 1971, was headline news around the world.

1. Armstrong was called Pops because he _____.

A. looked like a musician

B. was a musician of much influence

C. showed an interest in music

D. travelled to play modern music

2. The third paragraph is developed _____.

A. by space

B. by examples

C. by time

D. by comparison

3. Which statement about Armstrong is true?

A. His tale begins in New Orleans.

B. He was born before jazz was invented.

C. His music was popular with his listeners.

D. He learned popular music at a boy's home.

4. Which would be the best title for the text?

A. The Invention of the Jazz Music

B. The Father of the Jazz Style

C. The Making of a Musician

D. The Spread of Popular Music

单项选择题