and move at a common speed.

然后以同样速度运动
The common speed should be such that the total momentum

这个速度将是系统的总动量
agrees with what you had before.

也就是你原先得到的
That's called "totally inelastic."

这就是 "完全非弹性碰撞"
The other category is called "totally elastic."

另一种情况叫做 "完全弹性碰撞"
In a totally elastic collision, the kinetic energy is conserved.

在完全弹性碰撞中动能是守恒的
That you can write as

那么可以写出
the following relation involving quadratic things,

下面的二次方程
You can, it turns out,

你可以显然
juggle this equation and that equation

可以用这个方程与这个方程
and solve for v1' and v2'.

联立解出 v'1 和 v'2
Well, I'll tell you what the answer is.

我将告诉你答案