问题 计算题

如图所示,1和2是放在水平地面上的两个小物块(可视为质点),与地面的滑动摩擦因数相同,两物块间的距离d=170 m ,它们的质量分别为m1=2 kg、m2=3 kg。现令它们分别以初速度v1=10 m/s和v2=2 m/s相向运动,经过时间t=20 s,两物块相碰,碰撞时间极短,碰后两者粘在一起运动。求:

(1)两物块相碰前瞬间的速度大小;

(2)从刚碰后到停止运动过程中损失的机械能。

答案

解:(1)因为两物块始终作减速运动,有可能出现在相碰前有物块停止运动,故应进行讨论。先假定在时间t内,都未停下。以a表示此加速度的大小,现分别以s1和s2表示它们走的路程,则有

 ①, ②,

解①②③三式并代入有关数据得 ④   

经过时间t,两物块的速度分别为 ⑤, ⑥  

V2'为负值是不合理的,因为物块是在摩擦力作用下作减速运动,当速度减少至零时,摩擦力消失,加速度不复存在,v2'不可为负。V2'为负,表明物块2经历的时间小于t时已经停止运动。在时间t内,物块2停止运动前滑行的路程应是 ⑦   

解①③⑦式,代入有关数据得 ⑧   

由⑤⑥式求得刚要发生碰撞时物块1的速度 ⑨   

而物块2的速度

(2)设v为两物块碰撞后的速度,由动量守恒定律有

刚碰后到停止运动过程中损失的机械能

并代入数据得

阅读理解

阅读理解。

     The Chinese New Year is called the Spring Festival which is the biggest festival in China.

     It usually comes in January or February. Where there are Chinese, there is the Spring Festival. Each

Chinese year has a name. We may call it the year tiger, the year of pig or the year of snake. And the year

of 2003 is called the year of sheep. The year after it is the monkey year.

     People always spend a month before the festival in preparing (准备) for it. People are busy shopping

and cleaning their houses. On the Eve(除夕夜) there is a big family dinner. All the family stay up late to

welcome the New Year, watching the CCTV programs or playing majiang. On the first day of the

Chinese New Year, people put on their new clothes and go to visit their relatives(亲戚) and friends,

saying "Happy New Year", "Good luck" and some other greetings to each other. It often lasts to the

Lantern Festival, which is also an important Chinese festival and people usually have sweet dumplings

(yuanxiao). People always have a very good time, especially children. Children can usually get some

money for the new year from their parents and the elder relatives.

1. We call the Chinese New Year_____.

A. the biggest festival                  

B. the Lantern Festival

C. the Spring Festival                  

D. the year of snake

2. The year of 2004 is the year of _____.

A. tiger      

B. pig    

C. sheep      

D. monkey

3. The sentence "Where there are Chinese, there is the Spring Festival"means that _____.

A. the Spring Festival is celebrated only by Chinese people in China

B. the Chinese people celebrate the Spring Festival, even when they are in foreign countries.

C. People celebrate the Spring Festival all over the world

D. People who can speak Chinese celebrate the Spring Festival in the world

4. What do people say when they meet on the Chinese New Year's Day?

A. They say "Merry Christmas" and "Happy New Year".

B. They say "Good luck" and Good night" to each other.

C. They say "Happy New Year" and "Good luck" only to their friends and relatives.

D. They say many greetings, such as "Good luck" to each other.

单项选择题