but if you follow the center of mass,

但是如果你跟踪质心
in other words, at every instant

换个说法在每个瞬间
you take the m1·r1 + m2·r2 + m3·r3 and so on,

考察 m1·r1 + m2·r2 + m3·r3 等等
divided by the total mass,

除以总质量
that coordinate will simply be following

这个坐标就会沿着
the parabolic path of a body curving under gravity.

一个物体在重力作用下的抛物线变化
If at some point this complicated object fragments into two chunks,

如果在某个点这个复杂的物体碎成两块
one will land here and one will land there.

一块落到这里一块落到那里
But at every instant, if you follow the center of mass,

但是在每个时刻如果你考察质心的运动
it'll go as if nothing happened and it will land here.

它会像不受影响一下依然落在这里
The center of mass does not care about internal forces,

质心不受内力影响
only about external forces.

仅受外力作用