问题 问答题

一轻质弹簧,两端各连质量均为m的滑块A和B.静止在光滑水平面上,滑块A被水平飞来的质量为、速度为v0的子弹击中且没有穿出,如图16-3-7所示,求:

图16-3-7

(1)击中瞬间,A和B的速度分别为多大?

(2)以后运动过程中弹簧的最大弹性势能为多少?

答案

(1) vB=0.

(2) Epm=

子弹击中A的瞬间,时间极短,弹簧弹力的冲量不计(弹簧压缩量不计),子弹和木块A组成的系统动量守恒,机械能不守恒(因子弹和木块间有摩擦力).

子弹射入木块后,木块A以获得的速度向右运动,弹簧被压缩,A、B分别受到方向相反的弹力作用,A做减速运动,B做加速运动,在两者速度达到相等前,因vA>vB,所以弹簧一直被压缩,在两者速度相等后,两木块受力方向仍不变,所以A机械能减速,B继续加速,则vA<vB,故在相等时间内A向右位移小于B向右位移,弹簧逐渐由被压缩变为被拉伸,所以速度再次相等时,弹簧形变量最大,弹性势能最大.在此过程中,子弹和A、B组成系统,所受合外力为零,系统内只有弹力做功,所以系统动量、机械能守恒.

(1)子弹和A组成的系统动量守恒v0=(+m)vA,所以vA=,因不考虑弹簧压缩量,故vB=0.

(2)子弹和A、B组成的系统动量守恒vA=(+m)vAB

系统的机械能守恒×mvA2= (+m)vAB2+Epm

联立以上两式可解得Epm=

阅读理解

British potato farmers were taking to the streets to call for the expression "couch potato" to be taken away from the dictionary on the grounds that it harms the vegetable's image.

The British Potato Council wants the Oxford English Dictionary (OED) to replace the expression with the term "couch slouch", with protests planned outside parliament in London and the offices of Oxford University Press. Kathryn Race, head of marketing at the Council, which represents some 4,000 growers and processors, said the group had complained in writing to the OED but had yet to receive a response.

"We are trying to get rid of the image that potatoes are bad to you," she said Monday.

"The potato has had its knocks in the past. Of course it is not the Oxford English Dictionary's fault but we want to use another term than couch potato because potatoes are naturally healthy."

The OED says "couch potato" originated as American slang, meaning "a person who spends leisure time passively or idly sitting around, especially watching television or video tapes."

John Simpson, chief editor of the Oxford English Dictionary, said the dictionary first included the term “couch potato” in 1993 and said "dictionaries just reflect the words that society uses."

Simpson said words were never taken out of the full-length dictionary, which includes some 650,000 words contained in 20 volumes. But little-used words can be removed from the smaller dictionaries to make way for newer ones.

“If society stops using words, then they are taken out of the smaller dictionaries”, he added. “The first known recorded use of the expression ‘couch potato’ was in a 1979 Log Angeles Times article,” Simpson said.

Nigel Evans, a member of parliament for the Ribble Valley in Lancashire, has made a motion in support of the campaign, highlighting the nutritional value of the British potato.

1.British potato farmers were taking to the streets _____.

A.to advertise their production

B.to call for a higher price for t heir potatoes

C.to remove the expression “couch potato” from the dictionary

D.to let people know how important the potato is in people’s lives

2.The farmers think that_________.

A.potatoes have had a bad imagine       

B.potatoes are good for people by nature

C.potatoes sometimes do harm to people

D.it's the dictionary's fault to use the expression "couch potato"

3.John Simpson thinks that _________.

A.the expression can be taken out of every kind of dictionary

B.dictionaries do not necessarily reflect the words the society uses

C.little used words can remain in the smaller dictionaries

D.it is impossible to take the word out of the dictionary

4.What is wrong with the expression "couch potato"?

A.It is connected with unfavorable meaning.

B.Potato should be used in the expression. 

C.It is borrowed from American English.

D.It refers to a kind of person.

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