问题 填空题

在“验证机械能守恒定律”的一次实验中,质量m=1kg的重物自由下落,在纸带上打出一系列的点,P为第一个点,A、B、C为从合适位置开始选取的三个连续点(其他点未画出)如图所示.(相邻记数点时间间隔为0.02s),那么:

(1)纸带的______(用字母“P”或“C”表示)端与重物相连;

(2)打点计时器打下计数点B时,物体的速度vB=______;

(3)从起点P到打下计数点B的过程中物体的重力势能减少量△EP=______,此过程中物体动能的增加量△Ek=______;(g取9.8m/s2)(结果保留2位有效数字)

(4)通过计算,数值上△EP______△Ek(填“<”、“>”或“=”),这是因为:______.

答案

(1)重物在下落的过程中,速度越来越大,在相等时间内的位移越来越大,知纸带的P端与重物相连.

(2)B点的速度等于等于AC段的平均速度,则vB=

xAC
2T
=
(7.06-3.14)×10-2
2×0.02
m/s=0.98m/s.

(3)从起点P到打下计数点B的过程中物体的重力势能减少量△EP=mgh=1×9.8×0.0501J≈0.49J.

此过程中动能的增加量Ek=

1
2
mvB2=
1
2
×1×0.982≈0.48J.

(4)通过计算,数值上△Ep>△Ek,因为克服阻力做功,使得机械能损失.

故答案为:(1)P(2)0.98m/s(3)0.49J,0.48J(4)“>”,由于克服阻力做功使机械能损失

阅读理解

阅读理解

       You have been working hard to learn English. Do you know American kids are trying to learn

Chinese? More than 60,000 American students now study Chinese.

      “Because of China's fast development, Chinese has become more useful, and more and more

Americans begin to learn it," said Scott McGinnis, a Washington language expert (专家).

       For example, in 1999, only three schools in Chicago had Chinese lessons. This year more than

30 schools there have Chinese during school days. Last year China gave them 3,000 textbooks

because they had done a good job.

      Although Chinese is becoming more and more popular, it is one of the most difficult languages to

learn. Usually, an English speaker takes about 1,320 hours to become good at Chinese. He needs

only 480 hours for French, Spanish or Italian.

1. According to Scott McGinnis, more and more Americans begin to learn Chinese  _____.  

A. because of China's fast development  

B. because of China's interesting culture  

C. because they have more free time

D. because Chinese is easy to learn    

2. This year, _____ schools in Chicago give students Chinese lessons.

A. only three         

B. thirteen

C. over thirty      

D. less than thirty  

3.The students at those schools in Chicago take Chinese lessons   _____.

A. after school

B. at weekends    

C. on school nights    

D. on school days

4. Last year China gave _____  textbooks to those schools in Chicago.

A. thirty      

B. three hundred

C. three thousand      

D. thirty thousand

5. From the last two sentences we know that  _____.

A. Chinese is easier to learn than French, Spanish or Italian

B. Chinese is more difficult to learn than French, Spanish or Italian  

C. English speakers like to learn French, Spanish or Italian more than Chinese

D. English speakers like to learn Chinese more than French, Spanish or Italian

单项选择题