问题 选择题

如图所示,A、B为平行金属板,两板相距为d,分别与电源两极相连,两板的中央各有一小孔M和N,今有一带电质点,自A板上方相距d的P点由静止自由下落(P、M、N在同一直线上),空气阻力忽略不计,到达N孔时速度恰好为零,然后沿原路返回,若保持两极间的电压不变,则:     ( )

①把A板向上平移一小段距离,质点自P点自由下落后仍能返回。

②把A板向下平移一小段距离,质点自P点自由下落后将穿过N孔继续下落。

③把B板向上平移一小段距离,质点自P点自由下落后仍能返回。

④把B板向下平移一小段距离,质点自P点自由下落后将穿过N孔继续下落。

以上判断正确的是(  )

A、①②③④          B、①②④          C、②③④           D、①③④

答案

答案:D

题目分析:由题设条件知,知电场力做功等于重力做功的大小.把A板向上平移一小段距离,质点自P点自由下落,根据动能定理知,,小球到达N点速度为零然后返回.故①正确

将A板向下移动一小段距离,根据动能定理知,,小球到达N点速度为零然后返回.故②错误.

把B板向上平移一小段距离,根据动能定理知,,知小球未到达N点速度已减为零,然后返回.故③正确.

把B板向下平移一小段距离后,根据动能定理知,,知小球到达N点速度不为零,小球会穿过N孔继续下落.故④正确.

故选D.

点评:对于本题选择全过程为研究过程,运用动能定理进行求解比较方便,在运用动能定理时,抓住电势差不变,电场力做功不变,比较电场力做功与重力做功的关系进行分析.

阅读理解

One bitterly cold day, a snowstorm blew into our area. Needing firewood, I quickly set out with my best friend, Bruce, to look for a tree to cut down. I was glad to have Bruce along. Cutting down a tree in a snowstorm can be dangerous. So it was important to have a friend who could warn me of dangers.

When I chose a big 23-meter-tall tree and prepared to cut it down, my best friend suddenly warned me, “Don’t cut down that tree! It’s too close to the power line!” I wasn’t sure about it. So I decided to disregard his warning. I wanted to finish the job quickly and go home. So I began cutting down the tree. When the tree fell, there was no longer any doubt that my friend was right.

The tree caught the power line, bringing it to the ground. I considered cutting the tree off the line. After all, electricity can’t travel through wood, so I could safely remove the tree. As I reached out my finger to touch the tree, pain ran up my arm and through my head. I had been shocked!

After I returned home and told my mother what had happened, she quickly called the power company. Workers from the power company soon arrived on the scene. One of them asked if I had touched the tree, and when I told him I had, his face turned pale.

“You should have been killed,” he said.

So why am I still alive? It was my boots that saved my life.

Within two hours, the workers removed the tree. Soon the snowstorm calmed down—but not my mother.

Even though she was glad I wasn’t hurt, my mother was well serious with me. After all, I shouldn’t have brushed off my friend’s warning.

Through this experience, I learned that it’s important to listen to people who offer a different perspective(观点). Taking the time to listen might actually save a lot of time and trouble. It certainly would have kept me from getting the “shock of my life”.

小题1:The underlined word “disregard ” in the second paragraph means “________”.

A.pay no attention to

B.take notice of

C.consider

D.follow小题2:In spite of Bruce’s warning, the author still cut the tree because ________.

A.he was sure there would be nothing to happen

B.he wasn’t sure whether Bruce was right and was anxious to go home

C.he didn’t hear his friend’s warning at all because of the blowing wind

D.he was angry with his friend and didn’t want to listen to him小题3:The author wrote the passage mainly to _______.

A.give an account of one of his terrible experiences

B.make us know it’s important to listen to people who offer a different perspective (观点) through his experience

C.tell us not to cut trees down any more

D.remind us to be careful while working小题4: Which of the following statements is NOT true according to the passage?

A.But for his boots, the writer would have died.

B.The writer regretted having brushed off his friend’s warning.

C.The writer knew nothing about electricity.

D.When the tree was cut down, the writer realized his friend was right

单项选择题