# Why is mean free path independent of mass?

In this page of Hyperphysics, I am puzzled as to why there isn't any mass dependence in the expressions for mean free path.

Here are the two equations as mentioned in the website:

When only one molecule is moving: $$l = \frac{1}{\pi d^2 n_v}$$

The mean free path when motion of all molecules is accounted for($$l'$$):

$$l' = \frac{l}{\sqrt{2}}$$

Is there any intuitive way to understand this aspect of the result?

Why I expect mass to be there in the expression: We know that collisions, the quantity always conserved is momentum and that is product of mass and velocity, since there is a mass factor in momentum (something which is related to collisions) I suspected that there must be some dependence on the mean free path as well.

• Can you clarify precisely why you'd expect there to be mass dependence? That might help focus the question somewhat. Commented Jun 1, 2021 at 21:25
• Note that the "size" of a particle seen by another particle changes with energy (really the cross-section interaction) and has little to do with rest mass. Some interactions do care about the type of particle, obviously, but not really the rest mass. Commented Jun 11, 2021 at 12:51