伽利略利用铜球沿斜面运动的实验，实现了对重力的“冲淡”，从而完成了对自由落体运动的研究，关于这次实验下列说法正确的是（   ）

如图所示，xOy是位于足够大的绝缘光滑水平桌面内的平面直角坐标系，虚线MN是∠xOy的角平分线．在MN的左侧区域，存在着沿x轴负方向、场强为E的匀强电场；在MN的右侧区域，存在着方向竖直向下，磁感应强度为B的匀强磁场．现有一带负电的小球a从y轴上的P（0，l）点，在电场力作用下由静止开始运动，a球到达虚线MN上的Q点时与另一个不带电的静止小球b发生碰撞，碰后两小球粘合在一起进入磁场，它们穿出磁场的位置恰好在O点．若a、b两小球的质量相等且均可视为质点，a、b碰撞过程中无电荷量损失．求：

  “Cool" is a word with many meanings．Its old meaning is used to express a temperature that is a little bit cold．As the world has changed，the word has had many different meanings．
  “Cool"can be used to express feelings of interest in almost everything When you see a famous car in the street，maybe you will say，“It's cool." You may think，“He's so cool," when you see your favorite footballer．
    We all maximize（扩大） the meaning of “cool". You can use it instead of many words such as “new" or "surprising"．Here's an interesting story we can use to show the way the word is used．A teacher asked her students to write about the waterfall（瀑布）they had visited．On one student's paper was just the one sentence, “It's so cool．Maybe he thought it was the best way to show what he saw and felt. But the story also shows a lack of words．Without “cool”，some people have no words to show the same meaning．So it is quite important to keep some credibility（可信性）．Can you think of many other words that make your life as colorful as the word "cool"？ I can．And I think they are also very cool．