题干

锤头松了,把锤柄的末端在物体上撞击几下,锤头就能紧套在锤柄上了,为什么?
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解:因为向下挥锤柄时,锤头随着锤柄一起向下运动,即锤柄和锤头在撞击地面前共同处于运动状态;锤柄撞击地面时,受到地面给它的一个向上的反作用力,运动状态由运动变为静止.而锤头由于惯性还要保持原来的运动状态,继续向下运动,所以锤头就就紧套在锤柄上了

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    If you have a watch,don't repair it!I know it1.Once I had a beautiful watch.And this watch2perfect time. But one night it happened that I forgot to3it up.Next morning I went to a watchmaker as I wanted my perfect watch to_4by the exact time.The watchmaker examined my watch and said:“The regulator(校准器)is to be pushed up5our watch is four minutes6.”

I tried to stop him,tried to7him understand that my watch kept perfect time,but he did not listen to me and pushed the regulator.

    My beautiful watch began to gain time.It8faster and faster day by day.By the end of the second month it 9all the clocks and watches of the town far behind.

    What did I have to do?To take it to another watchmaker to be regulated.I expected him to regulate the watch immediately10he asked me to come in a week's time.When at last I took my watch from him it began to11down. And I began to be late for trains,business appointments and even missed my dinners.

Now I went to12watchmaker.While I waited for him to repair my poor watch,he13it to pieces and said that he could finish this work14three or four days.I could do nothing but 15. That time my watch went for half a day and then stopped.

    So I kept16my watch from one watchmaker to another for a considerable period of time.And as a result of it the cleverest man in the world could not17the time by my watch.The thing was getting18.My watch had19 two hundred dollars originally but I paid for repairs more than two hundred.At last I decided to buy20watch,which I did.