问题 多选题

甲物体以速度v匀速通过直线上的A、B两点,用时为t.乙物体从A点静止出发,做加速度大小为a1的匀加速直线运动,到达某一最大速度vm,接着做加速度大小为a2的匀减速直线运动,到B点停止,用时也为t.则关于乙物体的运动,下列判断正确的是(  )

A.无论a1、a2为何值,最大速度vm=2v

B.最大速度vm可为许多值,与a1、a2的大小有关

C.加速度a1、a2值必须是一定的

D.加速度a1、a2必须满足

1
a1+
1
a2
=
t
2v

答案

A、当物体匀速通过A、B两点时,x=vt.当物体先匀加速后匀减速通过A、B两点时,根据平均速度公式,总位移x=

vm
2
t1+
vm
2
t2=
vm
2
t,解得vm=2v,与a1、a2无关.故A正确,B错误.

C、匀加速运动的时间和匀减速运动的时间之和t=

vm
a1
+
vm
a2
=
2v
a1
+
2v
a2
,整理得,
1
a1
+
1
a2
=
t
2v
.故C错误,D正确.

故选:AD.

阅读理解

阅读理解。

     In the United States, when you become rich you want people to know about it. And even if you do

not become very rich you want people to think that you are. That is what "keeping up with the Joneses"

is about. It is the story of someone who tried to look as rich as his neighbors.

     The expression was first used in 1913 by a young American called Arthur Momand. He told this story about himself. He began earning $125 a week at the age of 23. That was a lot of money in those days.

He got married and moved with his wife to a very wealthy neighborhood outside New York City. When

he saw that rich people rode horses, Momand went horseback riding every day. When he saw that rich

people had servants, Momand and his wife also hired a servant and gave big parties for their new

neighbors.

     It was like a race, but one could never finish this race because one was always trying to keep up. The

race ended for Momand and his wife when they could no longer pay for their new way of life. They

moved back to an apartment in New York City.

     Momand looked around him and noticed that many people do things just to keep up with the rich

lifestyle of their neighbors. He saw the funny side of it and started to write a series of short stories. He

called it "Keeping up with the Joneses" because "Jones" is a very common name in the United States.

"Keeping up with the Joneses" came to mean keeping up with the rich lifestyle of the people around you. Momand's series appeared in different newspapers across the country for over 28 years.

     People never seemed to get tired of keeping up with the Joneses. And there are "Joneses" in every

city of the world. But one must get tired of trying to keep up with the Joneses because no matter what

one does, Mr. Jones always seems to be ahead.

1. Some people want to keep up with the Joneses because they _______.

A. want to be as rich as their neighbors

B. want others to know or to think that they are rich

C. don't want others to know they are rich

D. want to be happy

2. It can be inferred from the story that rich people like to _______.

A. live outside New York City

B. live in New York City

C. live in apartmentsD. have many neighbors

3. Arthur Momand used the name "Jones" in his series of short stories because "Jones" is _______.

A. an important name

B. a popular name in the United States

C. his neighbor's name

D. not a good name

4. The best title for this passage probably is _______.

A. The secret lives of the rich

B. How New Yorkers behave

C. Why being rich is never enough

D. The funny side of being rich

单项选择题 A3/A4型题