Conservation of Momentum - Inelastic Collisions

[SkyrimKhajiit]

Hello, I recently posted a few conceptual questions regarding conservation of momentum. I'm processing the information more easily now, but I've hit yet another bump in the road. I've learned that in an inelastic collision in which "velocity is the same before and after the collision" the velocity can be determined by dividing the initial velocity by the ratio of m_new:m_old. In other words, mass and velocity are inversely proportional:

http://www.physicsclassroom.com/class/momentum/Lesson-2/Using-Equations-as-a-Guide-to-Thinking

(This is the animation): http://www.physicsclassroom.com/mmedia/momentum/fca.cfm

But then, how is velocity the same before and after if it's obviously changing? And when would this situation of being able to find the new velocity by a simple ratio not work?

Thanks, and I know this is going to be a very simple answer pointing out something I missed, but I'm just stumped :D.

 

[Doc Al]

Two objects undergoing an inelastic collision end up with the same final velocity. That velocity is different from their initial velocities.

 

[mfb]

I've learned that in an inelastic collision in which "velocity is the same before and after the collision"

That is not true, independent of the question which velocities you mean.

the velocity can be determined by dividing the initial velocity by the ratio of m_new:m_old

What are new and old? What exactly do you mean with "dividing [...] velocity by the ratio"? I think I know what you mean, this is a special case - a perfectly inelastic collision where the relative velocity between the collision partners after the collision is zero.

 

[haruspex]

There appears to be a mistake in the video. In the last frame it says the combined speed is 5km/hr, but if you note what it says in the preceding frame you can see that the last frame should say 4km/hr.