问题 选择题

如图所示,条形磁铁静止放在桌面上,电流方向垂直纸面向里的通电直导线由S极的上端平移到N极上端的过程中,导线始终保持与磁铁垂直,则这个过程中磁铁受到的摩擦力(  )

A.为零

B.方向由向左变为向右

C.方向保持不变

D.方向由向右变为向左

答案

如图所示,导线在S极上端时导线所受安培力方向斜向左上方,由牛顿第三定律可知,磁铁受到的磁场力斜向右下方,磁铁有向右的运动趋势,则磁铁受到的摩擦力水平向左;当导线在N极上端时,导线所受安培力方向斜向右上方,由牛顿第三定律可知,磁铁受到的磁场力斜向左下方,磁铁有向左的运动趋势,则磁铁受到的摩擦力水平向右;由以上分析可知,磁铁受到的摩擦力先向左后向右.故ACD错误,B正确;

故选:B.

选择题
阅读理解

It is sometimes reported that strange flying objects have been seen high up in the sky. People call these strange objects Unidentified(不明的)Flying Objects(UFOs). These UFOs caused a lot of interest. Some of the reports about them are difficult to believe.Some have been explained in scientific ways. Others have never been explained.

Here is an account of UFOs from a report.

Dennis Bardens,who wrote an article,tells us that he once saw a UFO himself.He was having dinner one night with a friend near Tripoli(的黎波里),Libya(利比亚),and after dinner the two men walked across a yard to his room to look at some papers.Bardens noticed that the moon looked strange that night.

“It wasn't the moon,”was the reply.

They watched the light and saw that it was moving.It seemed to be an object rather like a big plate.In some ways it looked like smoke with a light on it,but it wasn't smoke because it didn't change its shape.It was moving in a regular way.That is to say,it didn't go suddenly faster or more slowly,and it didn't go higher or lower.So the two men decided that there must be men in it.

When it came nearer to Libya,Bardens said that he felt eyes looking at him.He was being watched in the darkness.

When it was nearer,they noticed that the bottom parts were brighter than the top.The bottom was yellow and the top was red.Then the object suddenly turned away and left Libya,moving very fast.

On a later day Bardens talked to other people about it,and he found some who had seen it.They all described(描述)it in the same way.Could they all the mistaken?

60.The UFOs are______.

A.flying high up in the sky       B.strange flying objects

C.difficult to believe            D.very interested

61.A UFO was seen one night______.

A.when they were walking across a yard to Bardens' room

B.while they were looking at the light

C.when Bardens noticed the moon appearing strange

D.while they were having dinner

62.The UFO looked like______.

A.smoke                                   B.light

C.a moving object                         D.a large plate

63.When the UFO came nearer to Libya,______.

A.it disappeared all at once

B.it suddenly turned away and left

C.it was watched in the darkness

D.Bardens was looking at it