问题 问答题

如图所示,质量为M=3kg、长度为L=1.2m的木板静止在光滑水平面上,其左端的壁上有自由长度为L0=0.6m的轻弹簧,右端放置一质量为m=1kg的小物块,小物块与木块间的动摩擦因数为μ=0.4,今对小物块施加一个水平向左的瞬时冲量I0=4N•s,小物块相对于木板向左运动而压缩弹簧使弹性势能增大为最大值Emax,接着小物块又相对于木板向右运动,最终恰好相对静止于木板的最右端,设弹簧未超出弹性限度,并取重力加速度为g=10m/s2.求:

(1)当弹簧弹性势能最大时小物块速度v;

(2)弹性势能的最大值Emax及小物块相对于木板向左运动的最大距离Lmax

答案

(1)由动量定理得I0=mv0

弹簧弹性势能最大时物块与木板的速度相同,则由动量守恒定律得 mv0=(m+M)v 

于是可解得:v=1m/s.

(2)由动量守恒定律和功能关系得

mv0=(m+M)u

物块相对于木板向左运动过程:

1
2
mv
20
=
1
2
(m+M)v2+μmgLmax+Emax      

物块相对于木板向右运动过程:

1
2
mv
20
=
1
2
(m+M)u2+2μmgLmax

可解得:Emax=3J,Lmax=0.75m.

答:

(1)当弹簧弹性势能最大时小物块速度v是1m/s;

(2)弹性势能的最大值Emax为3J,小物块相对于木板向左运动的最大距离Lmax是0.75m.

阅读理解

阅读理解

     The producer of a new Indonesian movie about President Obama's childhood years in Jakarta(雅加达) is hoping that interested Americans will flock to see the film, Little Obama, in theatres across the

country.

     Little Obama is the creation of Damien Dematra, an Indonesian writer and artist who has worked

on the project since November when he was struck with the idea of writing about the time a young

Obama lived in Jakarta with his mother and stepfather in the late 1960s and early 1970s.

     The film is based on a novel published in March Obama Anak Menteng, and focuses on the president's childhood friendships in Indonesia.

     The novel and the film are fictionalized(把-编成小说) accounts of the president's time in Jakarta and

are based on interviews with people who are childhood friends and neighbours of Mr. Obama. Many

Indonesians share a deep pride about a man who spent part of his shaping years in the country and went

on to become U.S. president.

     The film star is a 14-year-old Hasan Faruq Ali, an American who has lived in Indonesia with his family

for about 10 years. Like the president, he is the son of a white mother and a black father. Since filming

began, he has been under the shine of media attention. "My whole life changed overnight," he says.

     The film, which was made on a budget of $1 million, won't get its first public screening until

Wednesday, but ads for the movie on YouTube have got a few thousand hits. It isn't clear whether

Little Obama will get a U.S. release(发行), but the film's producer says he is in talks with a U.S.

distributor(发行人).

1.     What is the film Damien Dematra produced mainly about?     

A. The friendship between America and Indonesia.

B. The political views of Obama from his childhood.

C. The determination and hard work of young Obama.

D. Mr. Obama's friends when he was young in Indonesia.

2.     Which is NOT true about Obama and Hasan Faruq Ali?      

A. Their nationality is American.

B. They shoot a film when they are young.

C. they have the experience of living in Indonesia.

D. Hasan has a white mother and a black father.

3.     What can we learn from the last paragraph?

A. The future of Little Obama is unknown.

B. Little Obama will be popular in America.

C. Little Obama will be popular in Indonesia.

D. Ads for Little Obama proved unsuccessful.

单项选择题