问题 选择题

光滑水平桌面上有一个静止的木块,枪沿水平方向先后发射两颗质量和速度都相同的子弹,两子弹分别从不同位置穿过木块.假设两子弹穿过木块时受到的阻力大小相同,忽略重力和空气阻力的影响,那么在两颗子弹先后穿过木块的过程中(  )

A.两颗子弹损失的动能相同

B.木块每次增加的动能相同

C.因摩擦而产生的热量相同

D.木块每次移动的距离不相同

答案

A:木块在第一颗子弹的作用下木块已经获得一个速度,所以第二颗子弹以与第一颗子弹相同的速度进入木块时,其相对与木块的速度就比第一颗子弹的要小.

因为子弹在木块内受的阻力相同即加速度相同,相对木块的位移也一样,因此,第二颗子弹穿过木块需要的时间就比第一颗子弹需要的时间长.

而木块在子弹作用下的加速度两次是一样的,所以时间长的,其作用下的位移就大.

又由于两颗子弹对木块的作用力一样,因此位移大的做的功就多.

由动能定理合外力做的功等于动能变化,合外力做的功多,其动能增加就多,所以第二颗子弹穿过的过程中木块动能增加多,因此A错

B:由A的分析可知,木块的阻力对第二颗子弹做的负功更多,所以其动能损失也就更多,因此B错

C:产生的热量等于阻力在相对位移上做的功,阻力一样,相对位移也一样都是木块的长度.所以产生的热量也就一样.因此C错

D:由冲量I=Ft可知力相同的情况下,时间长的,冲量大,由A的分析知道,第二颗子弹作用时间长,所以其所受的冲量就大,故D正确

故选D

阅读理解

阅读理解。

     We can achieve knowledge either actively or passively (被动地). We achieve it actively by direct

experience, by testing and proving an idea, or by reasoning.

     We achieve knowledge passively by being told by someone else. Most of the learning that takes place

in the classroom and the kind that happens when we watch TV or read newspapers or magazines is

passive. If we achieve knowledge by passive learning, it's not surprising that we depend on it in our

everyday communication with friends and co-workers.

     Unfortunately, passive learning has a serious problem. It makes us tend to accept what we are told

even when it is little more than hearsay and rumor (谣言).

     Did you ever play the game Rumor? It begins when one person writes down a message but doesn't

show it to anyone. Then the person whispers it, word for word, to another person. That person, in turn,

whispers it to still another, and so on, through all the people playing the game. The last person writes

down the message word for word as he or she hears it. Then the two written statements are compared.

Typically, the original message has changed.

     That's what happens in daily life. The simple fact that people repeat a story in their own words

changes the story. Then, too, most people listen imperfectly. And many enjoy adding their own creative

touch to a story, trying to improve on it, stamping (打上标记) it with their own personal style. Yet those

who hear it think they know.

     This process is also found among scholars and writers: A statement of opinion by one writer may be

re-stated as fact by another, who may in turn be quoted by yet another; and this process may continue,

unless it happens to someone to question the facts.

1. According to the passage, passive learning may occur in _______.

A. doing a medical experiment

B. solving a math problem

C. visiting an exhibition

D. doing scientific reasoning

2. The underlined word "it" in Paragraph 2 refers to _____.

A. active learning

B. knowledge

C. communication

D. passive learning

3. The writer mentions the game Rumor to show that _____.

A. a message may be changed when being passed on

B. a message should be delivered in different ways

C. people may have problems with their sense of hearing

D. people tend not to believe in what they know as rumor

4. What can we infer from the passage?

A. Active learning is less important.

B. Passive learning may not be reliable.

C. Active learning happens more often.

D. Passive learning is not found among writers.

单项选择题