问题 多选题

如图所示,匀强磁场的方向竖直向下.磁场中有光滑的水平桌面,在桌面上平放着内壁光滑、底部有带电小球的试管.试管在水平拉力F作用下向右匀速运动,带电小球能从管口处飞出.关于带电小球及其在离开试管前的运动,下列说法中正确的是(  )

A.小球带负电

B.洛伦兹力对小球做正功

C.小球运动的轨迹是一条抛物线

D.维持试管匀速运动的拉力F应增大

答案

A、小球能从管口处飞出,说明小球受到指向管口洛伦兹力,根据左手定则判断,小球带正电.故A错误.

B、洛伦兹力总是与速度垂直,不做功.故B错误.

C、设管子运动速度为v1,小球垂直于管子向右的分运动是匀速直线运动.小球沿管子方向受到洛伦兹力的分力F1=qv1B,q、v1、B均不变,F1不变,则小球沿管子做匀加速直线运动.与平抛运动类似,小球运动的轨迹是一条抛物线.故C正确.

D、设小球沿管子的分速度大小为v2,则小球受到垂直管子向左的洛伦兹力的分力F2=qv2B,v2增大,则F2增大,而拉力F=F2,则F逐渐增大.故D正确.

故选CD.

阅读理解

阅读理解。

     After Mom died, I began visiting Dad every morning before I went to work. He was frail and moved

slowly, but he always had a glass of freshly squeezed orange juice on the kitchen table for me, along with

an unsigned note reading, "Drink your juice." Such a gesture, I knew, was as far as Dad had ever been able

to go in expressing his love. In fact, I remember, as a kid I had questioned Mom "Why doesn't Dad love me?"

Mom frowned, "Who said he doesn't love you?" "Well, he never tells me," I complained. "He never tells me

either," she said, smiling. "But look how hard he works to take care of us, to buy us food and clothes, and

to pay for this house. That's how your father tells us he loves us."

     I nodded slowly. I understood in my head, but not in my heart. I still wanted my father to put his arms

around me and tell me he loed me. Dad owned and operated a small scrap (片) metal business, and after school

I often hung around while he worked. Dad hand fed scrap steel into a device that chopped as cleanly as a

butcher chops a rack of ribs. The machine looked like a giant pair of scissors. with blades thicker than my

father's body. If he didn't feed those terrifying blades just right, he risked serious injury. "Why don't you hire

someone to do that for you?" Mom asked Dad one night as she bent over him and rubbed his aching shoulders

with a strong smelling liniment. "Why don't you hire a cook?" Dad asked, giving her one of his rare smiles.

     Many years later, during my first daily visit, after drinking the juice my father had squeezed for me, I

walked over, hugged him and said, "I love you, Dad." From then on I did this every morning. My father never

told me how he felt about my hugs, and there was never any expression on his face when I gave them.

1. What would be the best title for the passage?

A. I just couldn't understand my father

B. My father never loved me

C. Silent fatherly love

D. My hard-working father

2. The author's father always prepared a glass of freshly squeezed orange juice for him because ____.

A. that was the author's favorite

B. he was sure the author would be thirsty

C. the author was always complaining

D. that was a gesture of love

3. The author's father didn't hire a helper because ____.

A. his job was too dangerous

B. his job required high skills

C. he wanted to save money

D. he was not good at communicating with others

4. We may infer from the passage that ____.

A. the author's father lacked a sense of humor

B. the author quite understood his father as time went on

C. the author's father didn't love him very much

D. the author's father was too strict with him

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