问题 多选题

如图所示的两物块,甲带正电,乙是不带电的绝缘体.甲、乙叠放在一起置于粗糙的水平地板上,地板上方空间有垂直纸面向里的匀强磁场.现用一水平拉力F拉乙物块,使甲、乙无相对滑动一起向左做匀加速运动,在此运动过程中(  )

A.甲、乙两物块间的摩擦力不断增大

B.甲、乙两物块间的摩擦力保持不变

C.水平拉力F保持不变

D.水平拉力F不断增大

答案

A、甲带正电,在向左加速运动的过程中,受到的洛伦兹力的方向向下,所以对乙的压力变大,乙与地面之间的为滑动摩擦力,压力变大,所以滑动摩擦力也变大;甲、乙两物块间没有相对的滑动,是静摩擦力,由于整体的加速度不变,所以对于甲来说,静摩擦力作为合力产生加速度,所以甲、乙两物块间的摩擦力不变,所以A错误,B正确;

C、由上分析可知,因洛伦兹力作用,导致乙对地面的压力增大,由于加速度不变,根据牛顿第二定律,则有水平拉力增大,故C错误,D正确;

故选:BD.

阅读理解

I was very disappointed not to be able to go to the jazz concert last Friday. The announcement in the paper said that you could buy tickets at the theatre box office in Richland Hills any day between 10:30 a.m. and 4:00 p.m. Since I work from 9:00 a.m. to 5:30 p.m., the only time I could go to the theatre was during my 45-minute lunch break. Unluckily, the theatre is on the other side of the town, and the bus service between my office and Richland Hills is not very good. But if you are lucky, you can make the round trip in 45 minutes. Last Monday, I stood at the bus stop for 15 minutes waiting for a bus. By the time I saw one come around the corner, there was not enough time left to make the trip. So I gave up and went back to the office.

The same thing happened on Tuesday, and again on Wednesday. On Thursday, my luck changed. I got on a bus right away and arrived at the theatre in exactly 20 minutes. When I got there, however, I found a long line of people at the box office. I heard one man say he had been waiting in line for over an hour. Realizing I would not have enough time to wait in line, I caught the next bus and went back across the town. By Friday, I realized that my only hope was to make the trip by taxi. It was expensive, but I felt it would be worth to hear the concert. The trip by taxi only took 10 minutes, but it felt like an hour to me. When I got to the theatre, I was relieved to see that nobody was waiting in line. The reason, I quickly discovered, was that they had already sold out all the tickets.

小题1:The writer could go and buy the ticket __________.

A.any day before work hours

B.both before and after work hours

C.only during lunch time

D.on Saturday and Sunday小题2:The writer tried to go to the theatre every day, but was only successful______.

A.twice

B.three times

C.four times

D.five times小题3:He failed to get the ticket on Friday because _________.

A.it was impossible to make the round trip to the theatre in 45 minutes

B.the concert wasn’t on Friday

C.it was expensive to take a taxi

D.all the tickets had been sold out小题4:The word "______" can be used instead of the underlined(划线的)word "relieved".

A.nervous

B.terrified

C.pleased

D.sad

选择题