\magnification=\magstep1

$$
{\rm density\ } n \quad = \quad { {{\rm particles} } \over { {\rm meter}^3 }}
$$

$$
{\rm cross\ section\ } A \quad = \quad { {\rm area\ of\ particle\ } 
                                    ({\rm meter}^2) }
$$

$$
{\rm velocity\ } v \quad = \quad { {\rm meter} \over {\rm second} }
$$


$$
{\rm mean\ free\ path\ } L \quad = \quad  { {1}\over{n \times A} }
$$

$$
{\rm time\ between\ collisions\ } t \quad = \quad  { {L}\over{v} }
$$


\bye
