问题 选择题

某人站在某一星球上,以速度v0竖直上抛一物体,经t 物体落回手中.已知该星球半径为R,现将此物体沿此行星表面抛出,欲使其不再落回到星球上,则抛出时速度至少为(  )

A.

v0
R

B.

2v0R
t

C.

v0R
t

D.

v0
Rt

答案

物体向上和向下的时间相等,均为

t
2
,则由速度公式可得:

v0=g

t
2

解得:g=

2v0
t

在星球表面

要使物体不落后星球,则有:

G

Mm
R2
=m
v2
R
=mg=m
2v0
t

解得v=

2v0R
t

故选B.

阅读理解

Our village carpenter (木匠) , John, came one day and made a dining table for my wife. He made it just the right size for the space between the two windows. When I got home that evening, John was drinking a cup of tea and writing out his bill for the job.

My wife said to me quietly,“That’s his ninth cup of tea today. ”But she said, in a loud voice,“It’s a beautiful table, dear, isn’t it?”

“I’ll decide about that when I see the bill. ”I said.

John laughed and gave me his bill for the work. I read:

One dining table                        10 November, 1998

Cost of wood                            $17. 00

Paint                                   $1. 50

Work, 8 hours ( $1 an hour )            $8. 00

Total                                   $36. 50

When I was looking at the bill, John said, “It’s been a nice day, hasn’t it?Quite sunny. ”

“Yes. ”I said. “ I’m glad it’s only the tenth of November. ”

“Me, too. ”said John. “You wait — it’ll be a lot colder by the end of the month. ”

“Yes,colder — and more expensive! A dining table will be $20 more expensive on November 30, won’t it, John?”

John looked hard at me for half a minute. Was there a little smile in his two blue eyes?I gave his bill back to him.

“If there isn’t too much trouble, John,” I said,“please add it up again. You can forget the date…”

I paid him $26. 50 and he was happy to get it.

小题1:Why did John talk about the weather when the writer was looking at the bill?

A.Because he didn’t want the writer to go through the bill carefully.

B.Because it was really a fine day.

C.Because he wanted the writer to check the bill carefully.

D.Because he wanted to tell the writer what the weather was like.小题2:The writer thought John would ask for _______ if he made a dining table on the last day of November.

A.$20. 00

B.$46. 50

C.$56. 50

D.$26. 50小题3:When the writer gave John the money, he was happy because _______.

A.he got what he should get for his work

B.he got much more money for his work

C.he got the money easily

D.he didn’t have to add up the costs again小题4:From the story we know that ______.

A.John made a mistake in the bill

B.John tried to fool the writer in order to get more money for his work

C.John had written out the bill before the writer got home

D.John still wanted to get $36.50 for his work in the end小题5:What do you think of the writer? He is ______.

A.honest

B.clever

C.foolish

D.afraid

多项选择题