Hi

I am reading Microfluid Mechanics, principles and modeling by W.W. Liou and Y. Fang.

At page 12 they write the pre-collision velocities

in terms of the center of mass velocity and the relative velocity (image of text has been attached).we see that:

c1-cm=(m2/(m1+m2))cr

c2-cm=-(m1/(m1+m2))cr

These 2 relative velocity have positive and negative slop. How could they conclude they are parallel to each other? Is it misspelling or I missed something?

Best Regards