Here the skaters flying toward each other have equal masses
and speeds. The total momentum is initially zero, and it remains zero after the
collision, as they simply come to rest and stay that way.
Here the left skater initially has all the
system's momentum. After impact, since the total momentum is constant and they stay
together, the moving mass is twice what it was, and there fore the speed must be halved.
The two sail off at half the speed the left skater came in with.
Left skater comes flying towards another
skater, at twice right skater's speed. The total momentum is m(2v) + mv. After
the collision, the two skaters sail off together (2m) with a speed (3v/2) that conserves
the initial momentum (3mv).