问题 问答题

用细线悬挂一质量为M的木块,木块静止,如图8-5-4所示.现有一质量为m的子弹自左方水平地射穿此木块,穿透前后子弹的速度分别为v0和v.求木块能摆到的最大高度.(设子弹穿过木块的时间很短,可不计)

图8-5-4

答案

 

由子弹和木块组成的系统在水平方向上不受外力,满足动量守恒.即mv0=mv+Mv′,v′=(v0-v).木块以速度v′摆起过程中,只有重力做功,机械能守恒.即Mv′2=Mghm,hm=,所以hm=.

完形填空
Is money everything? At present some people think that        is everything. Some of them even        their lives for it. Money has its most useful        on the poor, but once a person has a rich life, a lot more money doesn't mean more       .
If money was       , all millionaires would have true love, true friendship, good health and a long life.       , this is not always true.
Nothing else is more       than the three words “I love you”. But can           be bought?I’m afraid not. Love means to give, not to take. To every person, health and long life are           the most valuable things. Well, can health and a long life be bought with money? The answer is “No”.
Of all the longest living people in the world,        of them are millionaires. True friendship can't be bought       . Many other things        be bought with money: knowledge, life and happiness and so on. Just think, if a person        these things at all, is the money still useful? No matter how much money you have, it is still not enough        you a happy person if you have no one to laugh      , no one to cry for. Now, do you think money is everything?
小题1:
A.time B.health C.friend D.money
小题2:
A.lose B.like C.save D.affect
小题3:
A.affect B.effect C.effort D.influence
小题4:
A.stress B.sadness C.happiness D.problems
小题5:
A.something B.anything C.nothing D.everything
小题6:
A.If B.But C.While D.However
小题7:
A.politeB.generous C.pleasantD.boring
小题8:
A.life B.love C.health D.friends
小题9:
A.probablyB.perhaps C.maybe D.impossible
小题10:
A.all B.few C.each D.none
小题11:
A.also B.too C.yet D.either
小题12:
A.cannotB.mustn’t C.needn’t D.shouldn’t
小题13:
A.has B.doesn’t have C.will have D.won’t have
小题14:
A.make B.makes C.to make D.making
小题15:
A.at B.to C.for D.with
填空题