it had a velocity, which it maintained,

如果有速度它会保持
or it had no velocity, in which case it does not even move.

如果没速度它不会移动
See, if you apply F = m·a to a body and there are no forces,

如果你应用 F = m·a 这里没有力
you cannot say the body will be at rest.

你不能说物体会静止
You will say the body will maintain the velocity.

物体应该是保持速度
So, if it had a velocity, it'll go at the velocity,

如果它有速度它会以这个速度运行
if it was at rest it'll remain at rest;

如果它静止它会保持静止
the same goes for the center of mass.

质心的情况也一样
If the center of mass was moving, it'll preserve its momentum.

如果质心在运动它会保持它的动量
That really means the sum of the momenta

这是说每一个部分的动量总和
of the individual pieces will be preserved.

会保持不变
If it was at rest,

如果它静止