Clearly the heavier the atom the less affective it is because it's sort of like a billiardballcollision with a heavier atom.
很明显,原子越重,它就越有效,因为那有点像有一个重一点的原子,的撞球的撞击。
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Look at the billiardball alone — - forget the ping-pong ball — and try to see that the speed of the billiardball is practically unaffected by the collision.