问题 选择题

下列关于做圆周运动的物体的说法中,正确的是(  )

A.物体所受的合外力提供向心力

B.向心力是一个恒力

C.向心加速度的方向始终与速度的方向垂直

D.向心加速度是恒定的

答案

A、做匀速圆周运动的物体所受到的合外力提供向心力,非匀速圆周运动向心力是物体受到指向圆心的分力,而不是合外力,故A错误;

B、物体做圆周运动时,受到的向心力方向时刻改变,是变力,故B错误.

C、向心加速度方向始终指向圆心,线速度方向沿轨迹的切线方向,则两者相互垂直.故C正确.

D、向心加速度方向始终指向圆心,故方向时刻改变,故向心加速度是变化的,故D错误.

故选:C.

阅读理解

Bill was a boy of 15 in Grade 9. His parents went gambling(赌博) day and night. Nobody cared about him. He always failed in his exams. So his teachers didn’t like him, nor did his classmates play with him. Bill often slept in class because his heart was not in school. He almost gave himself up.

One day, Miss Jones, a new teacher, came to Bill’s class. She was a smiling, beautiful woman. Bill was in love! For the first time in his young life, he couldn’t take his eyes off his teacher; yet, still he failed.

It was Miss Jones’ class. She was asking a question. “I’d like the smartest boy to answer my question.” Then she said loudly, “Bill, please!” Bill was surprised. And he stood up without a word. “Just try it! I believe you can!” After school, Miss Jones had a long talk with Bill. She told him that she cared about him and believed in him.

Bill thought a lot that evening. From then on he arrived at school on time every day. Miss Jones helped him with his homework in the afternoon. She even cooked delicious food for him. She told Bill, “Nothing is impossible if you put your heart in it.” Later, Miss Jones gave another exam. Bill did it with all his heart. He was very excited to find that he had passed the exam. Now Bill discovered that he also could learn well. A change took place in Bill’s heart.

You see, one teacher saved one boy, it’s simple, really.

小题1:How old was Bill?

A.9

B.14

C.15

D.16小题2: Bill always failed in the exams at first because he____________.

A.went gambling

B.talked in class

C.was silly

D.gave himself up小题3:Miss Jones changed Bill by ______________.

A.doing homework for him

B.encouraging and helping him

C.giving Bill another exam

D.cooking delicious food for him小题4:From the passage we can learn _____________.

A.Miss Jones was a loving and careful teacher

B.Bill’s parents cared about him very much

C.Miss Jones loved to answer Bill’s questions

D.Bill didn’t like to play with his classmates小题5:Which may be the best title for the passage?

A.Parents’ Day

B.A Lazy Boy

C.A Boy’s Change

D.A Strict Teacher

单项选择题