问题 选择题

如图所示,A、B两球用细线悬挂于天花板上且静止不动,两球质量mA=2mB,两球间是一个轻质弹簧,如果突然剪断悬线,则在剪断悬线瞬间(  )

A.A球加速度为

3
2
g,B球加速度为g

B.A球加速度为

3
2
g,B球加速度为0

C.A球加速度为

3
2
g,B球加速度为0

D.A球加速度为

1
2g,B球加速度为g

答案

设B球质量为m,则A球质量为2m,

悬线剪断前,以B为研究对象可知:弹簧的弹力F=mg,以A、B整体为研究对象可知悬线的拉力为3mg;

剪断悬线瞬间,弹簧的弹力不变,F=mg,根据牛顿第二定律得

    对A:2mg+F=2maA,又F=mg,得aA=

3
2
g,

    对B:F-mg=maB,F=mg,得aB=0

故选B

阅读理解

Top lists are lecturing people on everything from “100 places to visit” to “100 books to read ”.

Aren't you just tired of being told what to do with your time?

Now you have a list to end all lists!

Take a look at the following two examples from the list of “101 things not to do”:

Swim with Dolphins(海豚)?

Swimming with dolphins is one of the world’s most profitable tourist activities. However, every dolphin will welcome having their busy, tiring day interrupted by tourists screaming and pushing around them in the water. Worse yet, when dolphins get too near to the boats loaded with tourists, they could get caught up in ropes and killed by propellers(螺旋桨).

Here’s a little secret. Dolphins look like smiling at you, but actually they’re just opening their mouths.

Go to See the Mona Lisa?

There must be something about the mysterious(神秘的)smile. The 6 million people who visit the lady in the Louvre every year can’t all be wrong, after all. But they can be quite annoying, standing in front of you, holding up their cameras to prevent you from seeing anything. In fact, it is hard for you to see the painting clearly because you have to stay away from it for security reasons. After queuing for hours, many tourists can remain in front of the painting only for 15 seconds at most.

If the mysterious lady in the picture knew her fate, she wouldn’t just be smiling, she’d be laughing.

So, still long to see the Mona Lisa? If you want to find out more about the list, read 101 Things NOT to Do Before You Die. Visit www.not2dobeforeidie.co.uk and buy the book at a 20% discount.

小题1:According to the passage, swimming with dolphins________.

A.is the world’s most popular tourist activity

B.gives fun to both tourists and dolphins

C.will make tourists busy and tired

D.can cause danger to dolphins小题2:What does the author think about going to see the Mona Lisa?

A.It is wrong to go and see the mysterious smile.

B.It is not as satisfying as expected.

C.Fifteen seconds in front of the painting is enough.

D.Queuing for hours is worthwhile.小题3:The list of “101 things not to do” is made most probably because its author_______.

A.thinks it boring to do the things suggested by other lists

B.believes other lists are not humorous enough

C.intends to persuade people to read more lists

D.wants to provide a list different from other lists小题4:What is the main purpose of the passage?

A.To advertise a book

B.To introduce a website

C.To comment on popular lists

D.To recommend tourist activities.

单项选择题