A 10-g metal ball is moving to the left with a velocity of magnitude 0.4 m/s has a head-on, elastic collision with a larger 30-g metal ball moving to the right with a velocity of magnitude 0.2 m/s.
conserve momentum and conserve energy
10*.4 - 30(.2) = 10 u + 30 v
.5(10)(.4^2)+.5(30)(.2)^2 =.5(100)u^2+.5(30)v^2