(2011·黄冈模拟)如图所示，空间存在正交的匀强电场和匀强磁场，匀强电场方向竖直向上，匀强磁场的方向垂直纸面向里.有一内壁光滑、底部有带正电小球的试管.在水平拉力F
作用下，试管向右匀速运动，带电小球能从试管口处飞出.已知小球质量为m，带电量为q，场强大小为关于带电小球及其在离开试管前的运动，下列说法中正确的是( )
A．洛伦兹力对小球不做功
B．洛伦兹力对小球做正功
C．小球的运动轨迹是一条抛物线
D．维持试管匀速运动的拉力F应逐渐增大
选A、C、D.
洛伦兹力方向始终与小球运动速度方向垂直，不做功，故A正确B错误；小球在竖直方向受向上的电场力与向下的重力，二者大小相等，试管向右匀速运动，小球的水平速度保持不变，则竖直向上的洛伦兹力分量大小不变，小球竖直向上做加速运动，即小球做类平抛运动，故C正确；小球竖直分速度增大，受水平向左的洛伦兹力分量增大，为维持试管匀速运动拉力F应逐渐增大，D正确.
| 阅读理解。 | |||
| Students learn their lessons in class. In class teachers teach them. Students sit in the classroom listening to the teacher. This is a way of learning. Is this the only way for students to learn something? Of course not. There is another way to learn. That is students can teach themselves. For example, if you can not remember something when you are doing your homework, you can look at the book to find the answer. This is a way to teach yourself. And it is not a difficult thing. We can do this at any place and at any time. How do you teach yourself? The first thing you must do is reading. Read something you are interested in, or you have to read. The second is that you must ask yourself questions. The question is something you do not understand, or you want to know about.You can write down these questions. A clever student is usually good at asking questions. The third is to ask the question yourself by thinking hard, by reading the text or other books, and sometimes by asking other people. This is the way of teaching yourself. And you must do this all by yourself. If you keep doing like this for a long time, you are sure to succeed in your study. | |||
任务型阅读: 根据短文内容,填写下列表格。
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