Mean free path regarding billiards Is there a generalization of the mean free path for macro-objects such as the distance the white billiard ball could travel before hitting an other billiard ball?
 A: The mean free path is defined as the average distance between collisions. There does not seem to be a valid reason as to why this cannot be applied to the case of billiard balls on a billiard table, only if we assume the motion of the balls is fairly random and not “too guided” by the players. Let’s assume the player who “breaks” hits the cue ball in a random direction very hard that when it hits the other balls they all keep moving indefinitely.
The mean free path in two dimensional space (surface of table) is mathematically defined by$^1$ $$\lambda = \frac{A}{\sqrt{8}Nd}$$
where $d$ is the diameter of the billiard balls, $N$ is the number of balls and $A=\text{length}\times \text{width}$ is the area of the billiard table.
Knowing the average area for a billiard table to be $3.4\ m^2$ (2.6m$\times$1.3m), and the number of balls to be 16 (15+1 cue ball) and the diameter of each ball to be $\approx  0.05\ m$ or 5cm, then $$\lambda\approx 1.5 \ m$$ which is the average distance a ball will travel before colliding with another.
$^1$ I have not included the derivation here for the sake of brevity, but will include it if asked to by people in comments.
