Poor, misunderstood, Sokarul. No-one understands his genius.
Let me give you an example of how this equation is supposed to work, dimwit.
Two objects of equal weight have a totally elastic collision. At what velocities/momentum do they bounce off at? (really simple example)
So, m1 = 5kg, m2 = 5kg, and v1 and v2 are their velocities.
m1v1 = -m2v2
5kg*3m/s = -5kg*-3m/s
15kg*m/s = 15kg*m/s
Ta-fucking-da. Opposite velocities, idiot, have opposing signs.