A 100 g ball moving to the right at 4.0 m/s collides head-on with a 200 g ball that is moving to the left at 3.0 m/s.

Assuming that the collision is inelastic that is they are stuck together after the collision, momentum is conserved. We can determine the final velocity of the final mass as follows:

m1v1 + m2v2 = m3v3

100 (4) + (200) (-3) = (300) v3

v3 = - 0.6667 m/s

Therefore, the final mass moves at 0.6667 m/s to the left.