问题 选择题

某质点在F1、F2和F3三个共点力作用下处于平衡状态,各力的方向所在直线如图所示,各力的矢量起点均在O点,终点未画出.若sin37°=0.6,sin53°=0.8,则各力的大小关系是(  )

A.Fl:F2:F3=3:4:5

B.Fl:F2:F3=4:3:5

C.Fl:F2:F3=5:3:4

D.Fl:F2:F3=5:4:3

答案

将三个力沿F1方向和垂直于F1方向分解,在垂直F1方向上有:F2sin37°=F3sin53°,所以

F2
F3
=
sin53°
sin37°
=
4
3
,在沿F1方向上有:F1=F2cos37°+F3cos53°,解得
F1
F2
=
5
4
.所以Fl:F2:F3=5:4:3.故D正确,A、B、C错误.

故选D.

阅读理解

阅读理解。

     These days, more and more young people wish to be more beautiful, So they turn to plastic surgery (整形

手术) for help. They spend thousands of yuan on these painful surgeries to fix (修整) their nose, to change

their eyelids (眼皮) or to take off weight from their stomachs.

    In fact, most people are not pleased with the way they look. Even the most beautiful actresses often wish

they could change something about their looks. Should we live our whole lives believing we are ugly? Why

can't we raise our head high and believe that we are as beautiful as others? A smile is beautiful on any face,

whether fat, oily or hairy. And a kind heart is much more beautiful than a pretty look.

    So, let's feel grateful (感激的) that we all have beautiful and healthy bodies that do not need surgeries!

Today, I call on you to make an easy change:hold your head high! smile and be kind to others. If you do these,

you will be full of confidence (自信), beauty, and warmth everywhere you go.

1. _______  are more and more interested in plastic surgeries.

A. Old people

B. Young people

C. Men

D. Women

2. What do people think they can do to make their dreams come true?

A. Sleep more.

B. Eat less.

C. Do sports.

D. Have surgery.

3. How can we feel great about ourselves according to the writer?

A. Hold your head up.

B. Smile.

C. Be kind.

D. A, B and C.

4. The writer suggests we should believe we are as _______ as others.

A. confident

B. good

C. beautiful

D. clever

5. Which is Not true according to the passage?

A. Actresses never worry about their looks.

B. It costs a lot of money to have plastic surgeries.

C. We only need to make a small change to feel beautiful enough.

D. A good look is less important than a kind heart.

填空题