问题 选择题

在水平地面上方有正交的匀强电场和匀强磁场,匀强电场方向竖直向下,匀强磁场方向水平向里。现将一个带正电的金属小球从M点以初速度v0水平抛出,小球着地时的速度为v1,在空中的飞行时间为t1。若将磁场撤除,其它条件均不变,那么小球着地时的速度为v2,在空中飞行的时间为t2。小球所受空气阻力可忽略不计,则关于v1和v2、t1和t2的大小比较,以下判断正确的是

 

A.v1>v2,t1>t2

B.v1<v2,t1<t2

C.v1=v2,t1<t2

D.v1=v2,t1>t2

答案

答案:D

如上题所述,因为洛伦兹力对粒子永远不做功,

则根据动能定理,磁场存在与否小球着地时的速度都应该是相等的,故此可以首先排除选项A、B;这里需要注意的是存在磁场时,小球就要受到向右上方的洛伦兹力,使得小球在竖直方向的加速度小于重力加速度,故在空中飞行的时间要更长些.

单项选择题
完形填空

There once was a master who went to India. He had never been there before. When he got there, he saw a lot of f 小题1: . In India they have plenty of fruit to sell, but much of it is expensive b 小题2:  they can’t grow much as a result of the water situation. He saw a big basket of some very red, long fruit. And it was the c 小题3:  in the shop, not expensive at all.

He went up and asked,“how much per kilo?”“Two rupees.”Two rupees in India is almost

n 小题4: ; it’s like dirt (尘土). So he bought a whole kilogram of it and started eating it. But

a 小题5:  he ate some of it: Oh, my god! His eyes watered, his mouth watered and burned, his eyes were burning, his head was burning and his face became red. He jumped up and down, saying,“Ah! Ah! Ah!”

But he s 小题6:  continued to eat! Some people looking at him shook their heads and said,“You’re crazy. Those are chilies (辣椒)! You can’t eat them like that. They’re not fruit!” However, the master said, “No, I can’t stop! I paid money for them, and now I’ll eat them. It’s my money!”

You may think that master was s 小题7: , right? Similarly, we sometimes do a lot of things

l 小题8:  that. We devote money, time or effort to a relationship. Even though the suffering experience tells us it won’t w 小题9: , and we know there’s no hope things will change in the future—we still continue just because we’ve devoted money, time, effort and love to it. Just like the man who ate the chilies and suffered so much but couldn’t stop.

So even if you’ve lost something, let it g 小题10:  and move on. That’s better than continuing to lose.