问题 计算题

(12分)如图所示,质量为M的平板车P高h,质量为m的小物块Q的大小不计,位于平板车的左端,系统原来静止在光滑水平面地面上.一不可伸长的轻质细绳长为R,一端悬于Q正上方高为R处,另一端系一质量也为m的小球(大小不计).今将小球拉至悬线与竖直位置成60°角,由静止释放,小球到达最低点时与Q的碰撞时间极短,且无能量损失,已知Q离开平板车时速度大小是平板车速度的两倍,Q与P之间的动摩擦因数为μ,Mm=4:1,重力加速度为g.求:

(1)小物块Q离开平板车时速度为多大?

(2)平板车P的长度为多少?

(3)小物块Q落地时距小球的水平距离为多少?

答案

(1) 

(2)              

(3)    

(1)小球由静止摆到最低点的过程中,有

                          (1分)

小球与物块Q相撞时,没有能量损失,动量守恒,机械能守恒,

二者交换速度,即小球静止下来,而           (1分)

Q在平板车上滑行的过程中,有

小物块Q离开平板车时,速度为                     (2分)

(2)由能的转化和守恒定律,知

     解得,                 (4分)

(3)小物块Q在平板车上滑行过程中,对地位移为s,则

  解得,          (2分)

平抛时间

水平距离                (1分)

Q落地点距小球的水平距离为      (1分)

阅读理解
阅读理解。
    China celebrated the opening of the 2010 World Expo in Shanghai with an
evening of fireworks (烟火) and music on April 30. 
    Just like 2008 Beijing Olympic Games, Shanghai World Expo has given China
a chance to show its great progress.
     The theme (主题) of Exp0 2010, "Better City, Better Life," represents the common wish of the whole mankind
for a better living in the future.
     The emblem (象徽) stands for the big family of mankind, like three people-you, me,
him/her holding hands together, in the shape of the Chinese character "世" (the world).
     The mascot (吉祥物) of Expo 2010, Haibao, is a cartoon figure in the shape of the
Chinese character "人" (people).
      
     Volunteer logo (标志) includes the Chinese character "心" for heart, the letter "V" and
a dove (鸽) of peace carrying an olive branch (橄榄枝).
     The volunteer slogan (口号) for Shanghai World Expo is "At Your Service At Expo". It has three sub-slogans
(副口号)-"My Will, My Help, My Pleasure" "2010, We're Together As One" and "Our City, Your Joy."
1. The opening of the 2010 World Expo was celebrated in _____ on the _____ of April 30.
A. Beijing; morning
B. Shanghai; morning
C. Beijing; evening
D. Shanghai; evening
 2. The theme of Expo 2010 is "_____".
A. Better City, Better Life
B. Our City, Your Joy
C. My Will, My Help, My Pleasure
D. 2010, We're Together As One
3. The Expo emblem is in the shape of the Chinese Character "_____".
A. 世
B. 人
C. 心
D. 鸽
4. The volunteer slogan for the Expo is "At Your Service At Expo" with _____ sub-slogans.
A. two
B. three
C. four
D. five
判断题