问题 选择题

质量为mA和mB的小球与劲度系数均为k的轻弹簧L1和L2连接如图,静止时,两弹簧伸长量分别为x1和x2,则(  )

A.只要mA=mB,有x1=x2

B.只要mA>mB,有x1<x2

C.只要mA<mB,有x1<x2

D.只要不超出弹性限度,始终有x1>x2

答案

对小球B进行受力分析有,小球B受重力弹簧L2的弹力作用处于平衡,则有弹簧L2中的弹力F2=mBg,再对小球A进行受力分析有,小球A受重力和L2对它向下的弹力F2和L1对它向上的弹力F1作用,根据平衡有弹簧L1的弹力F1=mAg+F2=mAg+mBg.因为F1恒大于F2,根据胡克定律有弹簧的形变量x=

F
k
有弹簧的形变量x1>x2

故选D.

单项选择题

Generally speaking, a British is widely regarded as a quiet, shy and conservative person who is (1) only among those with whom he is acquainted. When a stranger is at present, he often seems nervous, even (2) . You have to take a commuter train any morning or evening to (3) the truth of this.

Serious-looking businessmen and women sit reading their newspapers or dozing in a corner; hardly anybody talks, since to do so would be considered quite (4) .

(5) , there is an unwritten but clearly understood code of behavior which, once broken, makes the offender immediately the object of (6) .

It has been known as a fact that a British has a (7) for the discussion of their weather and that, if given a chance, he will talk about it (8) .

Some people argue that it is because the British weather seldom (9) forecast and hence becomes a source of interest and (10) to everyone.

This may be so. (11) a British cannot have much (12) in the weathermen, who, after promising fine, sunny weather for the following day, are often proved wrong (13) a cloud over the Atlantic brings rainy weather to all districts! The man in the street seems to be as accurate — or as inaccurate — as the weathermen in his (14) .

Foreigners may be surprised at the number of references (15) weather that the British (16) to each other in the course of a single day. Very often conversational greetings are (17) by comments on the weather. “Nice day, isn’t it” “Beautiful!” may well be heard instead of “Good morning, how are you” Although the foreigner may consider this exaggerated and comic, it is (18) .pointing out that it could be used to his advantage. If he wants to start a conversation with a British but is at a loss to know (19) to begin, he could do well to mention the state of the weather. It is a safe subject which will (20) an answer from even the most reserved of the British.

1()

A.relaxed

B.frustrated

C.amused

D.exhausted

单项选择题