When you get a bit ahead in your studies, cut classes some afternoon and visit your local pool or billiards parlor to bone up on momentum conservation. Note that, no matter how complicated the collision of balls, the momentum along the line of action of the cue ball before impact is the same as the combined momentum of all the balls along this direction after impact, and that the components of momenta perpendicular to this line of action cancel to zero after impact, the same value as before impact in this direction. You€™ll see both the vector nature of momentum and its conservation more clearly when rotational skidding (€œEnglish€) is not imparted to the cue ball. When English is imparted by striking the cue ball off center, rotational momentum, which is also conserved, somewhat complicates analysis. But regardless of how the cue ball is struck, in the absence of external forces, both linear and rotational momentum are always conserved. Both pool and billiards offer a first-rate exhibition of momentum conservation in action.