A 0.30-kg ball with a speed of 2.0 m/s in +x-direction has a head-on elastic collision with a stationary 0.70-kg ball. What are the velocities of the balls after collision? 

THINKING IT THROUGH. The incoming ball is less massive than the stationary one, so it might be expected that the objects separate in opposite directions after collision, with the less massive ball recoiling from the more massive one. Equations 6.14 and 6.15 can be used to find the velocities with v₂₀ = 0.