Throwing $\pi$ at a wall

We discuss a method for calculating $\pi$ using elastic collision between two masses $M$ and $m$, with $X=m\mathrm{/}M={10}^{2(1-d)}$ where $d$ is an integer, and a wall. The total number of collisions between $M$, $m$ and the wall corresponds to the first $d$ digits of $\pi$.