15 balls are racked in a triangle. They are all alike and each one is touching its neighbors. Ignore friction and assume all collisions are elastic. You break by hitting the apex ball hard with the cue ball, as for a game of 8-ball. Assuming you know the velocity of the cue ball and where it contacts the apex ball, what else do you need to know to predict the resulting motions of the other balls? Conservation of momentum and energy are not enough by themselves, as would be the case if there were only one ball in the rack. My experience is that, provided the balls are well racked and I hit the apex ball full-on, the cue ball bounces back as if it had collided with a heavier object. But I'm not sure that the "effective mass" of this object is the mass of all 15 balls, because they do not all move together as a whole. I'm pretty sure that simplifying the problem to a triangle of 3 balls would capture most of the relevant aspects of the problem.