问题 选择题

如图所示,作用于O点的三个力平衡,设其中一个力的大小为F1,沿-y方向.大小未知的力F2与+x方向夹角θ,下列说法正确的是(  )

A.F3的最小值为F1cosθ

B.力F3只可能在第二象限

C.力F3可能在第三象限的任意区

D.力F3与F2夹角越小,则力F3与F2越小

答案

A、三力平衡时,三个力中任意两个力的合力与第三个力等值、反向、共线;

通过作图可以知道,当F1、F2的合力F与F2垂直时合力F最小,大小等于F1cosθ,根据平衡条件的推论可知,F3与F大小相等,则F3的最小值也为F1cosθ.故A正确;

B、当F1、F2的合力F在第一象限时,力F3在第三象限,故B错误;

C、通过作图可知,当F1、F2的合力F可以在F1与F2之间的任意方向,而三力平衡时,三个力中任意两个力的合力与第三个力等值、反向、共线,故力F3只能在F1与F2之间的某个方向的反方向上,故C错误;

D、由于三力平衡,F2与F3的合力始终等于F1,保持不变.故D错误;

故选A

阅读理解

阅读理解。

     When David left college, he went to Australia. When he returned to England for a visit twenty years

 later, he decided to go back to his old college.  David was very glad when he found out that his old

 college teacher was still teaching there. He went to visit the old man,  and after they had an interesting

 talk,  the teacher went out to get something. While he was away, David saw an exam paper on the

 desk. He looked at the date on it and saw that it was given to the student a week ago. David picked the

 paper up and read it.

     When the teacher came back, David said to him, "Mr. Smith, I'm sure that these are exactly the same

 questions that you asked us in our exams twenty years ago! How is that possible?"

     "Yes, that's right," agreed the teacher. The questions are the same,   but the answers have changed."

1. David went to _____  after he left college.

    A. England                

    B. Australia

    C. another college        

    D. his teacher's office

2. David felt _____ when he found out that his college teacher was still teaching there.

    A. worried                

    B. sorry

    C. amazed                  

    D. happy

3. The students took the same exam _____.

    A. as David did twenty years ago

    B. when David' s teacher was away

    C. as David did a week ago

    D. when David read the paper

4. Can you guess why the teacher still used the same exam paper twenty years later?

    A. Times have changed, answers should change, too.

    B. Different students have different answers.

    C. The teacher tried to find an excuse for his being lazy.

    D. The exam paper was done by David and his classmates 20 years ago.

判断题