问题 多选题

如图所示,一个由绝缘材料制成的轻弹簧水平放置,一端固定于竖直墙上,另一端与一带负电的小球相连,小球置于光滑的绝缘水平面上.当整个装置处于水平向左的匀强电场中时,振子在O点处于平衡状态,在B、C两点间振动.假定在振动过程中小球的电量保持不变,则(  )

A.小球在由B到O的过程中,弹性势能和电势能减少,动能增加

B.小球在由O到C的过程中,弹性势能增加,电势能和动能减少

C.小球在由B经O到C的过程中,电势能的变化量和弹性势能的变化量大小相等

D.小球在由C到O的过程中,电势能的变化量和弹性势能的变化量大小相等

答案

A、小球做简谐运动,O点是平衡位置,此处弹簧伸长,则知小球所受的电场力方向向右,则小球在由B到O的过程中,电场力做功负功,电势能增加,弹性减小,平衡位置动能最大,则知小球的动能减小.故A错误.

B、小球在由O到C的过程中,弹簧伸长量增大,弹性势能增加,电场力做正功,电势能减小,合力向左,做负功,则动能减小.故B正确.

C、小球在由B经O到C的过程中,动能的变化量为零,根据能量守恒得知,电势能的变化量和弹性势能的变化量大小相等.故C正确.

D、小球在由C到O的过程中,动能减小,弹性势能增加,电势能减小,由能量守恒得知,电势能的变化量大小小于弹性势能的变化量大小.故D错误.

故选BC

阅读理解

阅读理解。

     Mrs. White has a son. His name is Sam White. The young man works in a post office in a city. He asks

his mother to visit the city, and the old woman is happy and catches (赶上) a train. Soon she gets to the city,

but she doesn't know where the post office is. She begins to look for it. but she can't find it. At a bus stop she

asks all old mall, "Excuse me, sir, how can I get to the post office?"

     "We have about forty post offices in our city," says the old man."Which one do you want to go?"

     The old woman gives Sam's letter to him and the old man says,"Walk along this street and take the fourth

turning on the left. His post office is just there."

      Mrs. White thanks the old man and soon finds her son. They're very happy. A week later the woman goes

to the zoo. She sees all kinds of animals but after that she can't find the way to the post office again. At the

same bus stop she meets the same old man and she asks the same question again.

      "Oh dear!" the old man calls out."Are you still looking for your son?"

根据短文内容, 回答下列问题。

1. Where does Sam White work?

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2. How does Mrs. White go to the city?

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3. Does the old woman know how to get to her son's post office?

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4. How many post offices are there in this city?

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5. Does the old man know Mrs. White finds her son?

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单项选择题