问题 计算题

如图所示,在长为2L、宽为L的ABCD区域内有一半的空间存在场强为E、方向平行于BC边的匀强电场,现有一个质量为m,电量为e的电子,以平行于AB边的速度v0从区域的左上角A点射入该区域,不计电子所受的重力,则:

(1)当无电场的区域位于左侧时(如图甲),求电子射出ABCD区域时的动能;

(2)当无电场区域的左边界距AD的距离为x时(如图乙),要使这个电子能从区域的右下角的C点射出,电子的初速度v0应满足什么条件。

答案

解:(1)电子先做匀速运动,进入电场后做类平抛运动,设电子恰好从C点射出电场

电子在电场中运动的加速度a=

设电子在电场中的运动时间为t,则有L=v0t,L=at2

解得:v0

①当v0时,电子从电场的下边界射出电场

射出电场时的动能Ekmv02+eEL

②当v0时,电子从电场的右边界射出电场

射出电场时沿电场方向的位移y=at2

射出电场时的动能Ekmv02+eEymv02

(2)电子先做类平抛运动,接着做匀速直线运动,最后做类斜下抛运动

设电子整个运动时间为t1,则有

2L=v0t1

解得:v0

阅读理解

Kids' health: Four steps for fighting stress

Everybody gets stressed time to time. 小题1:    Some ways of dealing with stress 一like screaming or hitting someone一don't solve (解决) much. But other ways, like talking to someone you trust, can lead you to solving your problem or at least feeling better.

Try taking these four steps the next time you are stressed:

(1) Get support. When you need help, reach out to the people who care about you. Talk to a trusted adult, such as a parent or other relatives.   小题2:    They might have had similar problems, such as dealing with a test, or the death of a beloved pet.

(2) Don't take it out on yourself. Sometimes when kids are stressed and upset they take it out on themselves. Oh, dear, that's good idea. Remember that there are always people to help you. Don't take it out on yourself.    小题3:   

(3) Try to solve the problem. After you're calm and you have support from adults and  friends, it's time to get down to business.        小题4:    Even if you can't solve it all, you can solve a piece of it.

(4) Be positive. Most stress is temporary(暂时的).Remember stress does go away,especially when you figure out the problem and start working on solving it.

These steps aren't magic, but they do work. And if you can stay positive as you make your way through a tough time, you'll help yourself feel better even faster.      小题5:   

A.Ask for a helping hand to get you through the tough situation.

B.Notice your friends' feelings and find a way to help them.

C.Different people feel stress in different ways

D.Ah, it feels so good when the stress is gone.E. You need to figure out what the problem is.

F. And don't forget about your friends.

G. Then, find a way to calm down。

单项选择题 B1型题