质量为m，带电量为+q的小球从距离地面高为h处以一定的初速度水平抛出，在距抛出点水平距离L处，有一根管口比小球直径略大的竖直细管，管的上口距离地面h/2．为使小球能无碰撞的通过管子，可以在管口上方的整个区域内加上水平方向的匀强电场，如图，则下列说法中正确的是
A．所加电场方向应该水平向左
B．小球平抛的水平初速度大小为
C．所加电场的电场强度为
D．小球落地的动能为mgh+
AC
题目分析：据题意，要使小球与管子无碰撞，需要使小球到达上管口时水平速度刚好为0，则电场水平向左，使小球在水平方向做匀减速直线运动，有：
，而
，竖直方向做自由落体运动，有：
，可求：
，故A、C选项正确；小球在水平方向做末速度为0的匀减速直线运动，则有：
，所以初速度为
，故B选项错误；小球从抛出到落地过程中据动能定理有：
，则小球落地动能为：
，故D选项错误。
| Brave Type: 3D cartoon Comedy Running time: 95 minutes Merida, a princess, refused to marry a person that she dislikes. This causes trouble for her family and the whole country. Merida turns to a witch(女巫) for help. In doing so, her family and the country are in danger. Merida must save her family and the country before it is too late. X K b1.C om |
| The Grey Type: Action Running time: 117 minutes The Grey is based on the short story “Ghost Walker” written by Ian Mackenzie Jeffers. It’s about a number of oil men who are in trouble in Alaska after a plane crash(空难). They have to fight against grey wolves that threaten their lives. |
| Mirror Mirror Type: Running time: 106 minutes You must know the story of Snow White. Would you like to see this story again? Come to see Mirror Mirror. It’s a comedy about Snow White. Lily Collins and Julia Roberts are the main actresses in this interesting movie. |
| Chimpanzee Type: Documentary Running time: 78 minutes This movie is directed by Alastair Fothergill and Mark Linfield. It’s a 2012 nature documentary film about a young chimpanzee named Oscar. He is alone in the African forests until he is raised by another chimpanzee. The chimpanzee takes care of Oscar like his own child. |
小题1: How many documentaries are mentioned in the passage?
A．One.
B．Two.
C．Three.
D．Four.小题2:Which movie lasts the longest time?
A．Brave.
B．The Grey.
C．Chimpanzee.
D．Mirror Mirror.小题3:Which statement about the movies is true?
A．Mirror Mirror is directed by Lily Collins and Julia Roberts.
B．The Grey directed by Ian Mackenzie Jeffers is based on a short story.
C．You can’t see many beautiful nature sights in Chimpanzee.
D．Brave is about a princess who obeys her heart.小题4:Who starred in(主演) one of the movies?
A．Lily Collins and Julia Roberts.
B．Alastair Fothergill and Mark Linfield.
C．Ian Mackenzie Jeffers.
D．Merida.小题5:Where can you probably read this passage?
A．In a nature magazine.
B．In a storybook.
C．In an entertainment newspaper.
D．In a science book.