问题 选择题

如图所示,在光滑绝缘水平面上的M、N两点各放有一个电荷量分别为+q和+2q的完全相同的金属球A、B.在某时刻,使A、B以相等的初动能E开始沿同一直线相向运动(这时它们的动量大小为P),若它们在碰撞过程中无机械能损失,碰后又各自返回.它们返回M、N两点时的动能分别为E1和E2,动量大小分别为P1和P2,则下列结论正确的是(  )

A..E1=E2>E,P1=P2>P

B..E1=E2=E,P1=P2=P

C..碰撞一定发生在M、N连线中点的左侧

D..两球不可能同时返回到M、N两点

答案

A、由动量观点看,系统动量守恒,两球的速度始终等值反向,也可得出结论:两球必将同时返回各自的出发点,且两球末动量大小和末动能一定相等.

从能量观点看,两球接触后的电荷量都变为1.5q,在相同距离上的库仑斥力增大,返回过程中电场力做的正功大于接近过程中克服电场力做的功,系统机械能必然增大,即末动能增大.故A正确,B错误.

C、由牛顿定律的观点看,两球的加速度大小始终相同,相同时间内的位移大小一定相同,必然在连线中点相遇,又同时返回出发点.故C错误,D错误.

故选A.

阅读理解

Learning English online is an effective way to improve your English. But with any type of learning you need to be prepared. This passage will give you advice and tips on how to get the most out of your online lessons.

  小题1:     

This is an obvious point but also the most important one. A computer, a good Internet connection and a headset are what you will need to take classes online. A web camera is also pretty important. Make sure that your computer is turned on, that everything is plugged in correctly and that you have tested the connection. Doing this before the class means that you will be ready when the lesson starts

Take the classes somewhere quiet.

      小题2:    Trying to listen and participate(参与) in English classes is very difficult when there is noise around you. So find a quiet room and make sure that no one disturbs you while you are taking your lesson.

      小题3:     

When grammar or anything else is being taught, make sure that you understand it. You won’t need to practice it at this time; just make sure that everything is clear. If you don’t understand what is being taught, ask your teacher to explain it again. Your teacher will be more than happy to deal with whatever puzzles you.

Practice speaking English.

Every class is an opportunity to practice your English speaking with a native and qualified teacher. Try to allow the conversation to flow by listening and talking in a natural way.   小题4:   The teacher should note the mistakes and correct them at a later time.

Relax.

Come to class feeling relaxed and excited.   小题5:    Come into the class excited about learning English. You will be able to get more out of the class this way.

A.Review after the lesson

B.Make sure everything is ready.

C.Understand what is being taught.

D.Get away from everything that may be going on in your homeE. Language learning should be enjoyable especially in online classes.

F. You will meet many problems when you are learning English online.

G. This is your chance to speak English without worrying about too many mistakes.

填空题