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momentum

1994CE LQ_2

呢題既其中一part 講有粒ball 衝埋去一舊fixed 佐係地既plasticine, 兩舊痴埋佐唔郁, 根住就話 " In this collision, the total momentum of the ball and the plasticine is not conserved. Explain briefly. "

佢個sol. 話 " There is an external force acting on the plasticine. According to the law of conservation of momentum, no net force should be apply if the total momentum is to be conserved. "

1998CE LQ_2

呢題講有個炮彈射向個fixed佐係地既block, 插佐入去->停. 跟住話" A students thinks that as the ball and the block are both at rest after the impact, momentum has been lost. ... If you were a teacher, how could you answer his question ? " Sol 話 " the momentum is conserved as the mass of the earth is much larger than that of the block and the ball, the change in speed of the earth is so small that it is negeligible, thus the momentum is conserved. "

究竟邊個ans 對, 兩個分別講緊 有net force 同冇netforce, 但個case 可以話一樣~

Update:

回Audrey Hepburn : 94 年果題話"the total momentum of the ball and the plasticine is not conserved " , 咩唔係已介定個系統包埋舊plasticine ?

2 Answers

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  • 1 decade ago
    Favorite Answer

    基本上,兩個答案都是對的。但首要條件是要清楚介定系統內的物體。

    首先是94年那題,題目介定了系統只有那個ball。因此當它與plasticine碰撞,plasticine在只有ball的系統裏便作用了一外力external force。所以,total momentum of the ball and the plasticine is not conserved。

    但留意的是,若這題系統包括了ball和地球(或者plasticine),那麼系統的total momentum就是conserved了。那碰撞中的力是系統中的內力,根據牛頓第三定律,球也向plasticine作用了力。方向相反,量值一樣。

    相同道理,98題,系統包括了炮彈與地球。所以總體來說,total momentum是conserved的。但為何我們察覺不到地球在動呢?因為它的mass太大,所以change of velocity小得可忽略。

    2008-06-10 18:47:03 補充:

    不是,你誤會了問題的意思。題目是想講考慮ball為一系統。由於plasticine貼住地面,故不能運動,所以好似有個external force咁。

    Source(s): Myself~~~
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  • C C
    Lv 4
    1 decade ago

    怎樣分辨問題在問的系統包不包括地球?

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