问题 问答题

某轻质弹簧的弹力F与弹簧长度L的关系如图所示.一木箱重200N,静止放在水平面上.用该弹簧沿水平方向拉木块,当弹簧的伸长量为8cm时,木箱刚好被拉动.已知木箱与地面间的最大静摩擦力和滑动摩擦力相等.求:

(1)该弹簧的原长L0以及劲度系数k;

(2)木箱与水平面间的动摩擦因数;

(3)当弹簧的伸长量为4cm时,求木箱受到的摩擦力的大小.

答案

(1)弹簧的弹力为零时,弹簧处于原长,为10cm;

劲度系数:k=

△F
△x
=
200-0
0.20-0.10
=2000N/m;

(2)当弹簧的伸长量为8cm时,木箱刚好被拉动,拉力为:

F=kx=2000×0.08=160N

木箱刚好被拉动,故最大静摩擦力等于拉力,为160N;

木箱与地面间的最大静摩擦力和滑动摩擦力相等,故滑动摩擦力为160N;

根据f=μmg,有:

μ=

f
mg
=
160N
200N
=0.8

(3)当弹簧的伸长量为4cm时,拉力为:

F=kx=2000×0.04=80N

由于拉力小于最大静摩擦力,故拉力不动,静摩擦力为80N;

答:(1)该弹簧的原长为10cm,劲度系数k为2000N/m;

(2)木箱与水平面间的动摩擦因数为0.8;

(3)当弹簧的伸长量为4cm时,木箱受到的摩擦力的大小为80N.

阅读理解

         To be a good teacher, you need some of the gifts of a good actor; you must be able to hold the attention and interest of your audience; you must be a clear speaker, with a good, strong, pleasing voice which is fully under your control; and you must be able to act what you are teaching, in order to make its meaning clear.

Watch a good teacher, and you will see that he does not sit still before his class: he stands the whole time he is teaching; he walks about, using his arms, hands and fingers to help him in his explanations, and his face to express feelings. Listen to him, and you will hear the loudness, the quality (音色) and the musical note of his voice always changing according to what he is talking about.

The fact that a good teacher has some of the gifts of a good actor doesn’t mean that he will indeed be able to act well on the stage, for there are very important differences between the teacher’s work and the actor’s. The actor has to speak words which he has learnt by heart; he has to repeat exactly the same words each time he plays a certain part, even his movements and the ways in which he uses his voice are usually fixed beforehand (预先). What he has to do is to make all these carefully learnt words and actions seem natural on the stage.

A good teacher works in quite a different way. His audience takes an active part in his play: they ask and answer questions, they obey orders, and if they don’t understand something, they say so. The teacher therefore has to meet the needs of his audience, which is his class. He cannot learn his part by heart, but must invent it as he goes along.

I have known many teachers who were fine actors in class but were unable to take part in a stage-play because they could not keep strictly to what another had written.

51. What is the text about ?

A. How to become a good teacher.

B. What a good teacher should do outside the classroom.

C. What teachers and actors could learn from each other.

D. The similarities and differences between a teacher’s work and an actor’s.

52. The word “audience” in the fourth paragraph means ____ .

A. students                      B. people who watch a play

C. people who not on the stage     D. people who listen to something

53. A good teacher ____ .

A. knows how to hold the interest of his students

B. must have a good voice

C. knows how to act on the stage 

D. stands or sits still while teaching

54. In what way is a teacher’s work different from an actor’s ?

A. The teacher must learn everything by heart .

B. He knows how to control his voice better than an actor .

C. He has to deal with unexpected situations .

D. He has to use more facial expressions .

55. The main difference between students in class and a theatre audience is that ____.

A. students can move around in the classroom

B. students must keep silent while theatre audience needn’t

C. no memory work is needed for the students

D. the students must take part in their teachers’ plays

单项选择题