Mean free path in a vacuum chamber

You will find how far a molecule goes between collisions, and how low the pressure must be for a long free path.

The idea. $\lambda = \dfrac{k_BT}{\sqrt{2}\,\pi d^2 p}$. The number of collisions each second is $\bar v/\lambda$.

Your tasks.

1. In free_path, return $\lambda$ in m, for $p$ in Pa, $T$ in K and $d$ in m.
2. In pressure_for, return the pressure in Pa that gives a chosen mean free path.
3. In collisions_per_second, return how many collisions one molecule makes each second.