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[領域]                             (題目相關領域)

近代物理

[來源]                             (課本習題、考古題、參考書...)

concept of MODERN PHYSICS, ARTHUR BEISER

[題目]

Example  2.6

[瓶頸]                             (寫寫自己的想法，方便大家為你解答)

綠色部分

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http://cph-theory.persiangig.com/1193-11.JPG

這題是近代物理課本內的例題  (pair annihilation)

有解答 可是我還是不懂orz...

    An elecrtron and a positron are moving side by side in the  +x direction

at 0.5c when they annihilate each other. Two photons are produced that move

along the x axis. (a) Do both photons move in the +x direction?



解答:

(a) In the center-of-mass (CM) system (which is the system moving with the

original particles), the photons move off in the opposite directions to

conserve momentum. They must also do so in the lab system because the speed

of the CM system is less than the speed c of the photons.

我想問的是綠色部分 (為什麼是相反方向?)

有請高手指教...

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