# Gravity on air molecule vs. diffusion into vacuum

1. Oct 22, 2015

### houlahound

OK the atmosphere by definition envelopes the earth.

The force keeping it here is solely gravity.

The "force" opposing this is diffusion across a pressure gradient into the vaccum of space.

Assuming that is correct how could I prove this other than looking out the window.

Newton's law of gravity will give the force earth exerts on a single molecule, the force of the vacuum??

2. Oct 22, 2015

### BvU

The force keeping it here is solely gravity. yes

Diffusion isn't really a force; it's more the consequence of a force. But I understand what you mean.

What kind of proof did you have in mind ? A derivation of the barometric law probably isn't what you mean, or is it ?

 made a quote of the quote

Last edited: Oct 22, 2015
3. Oct 22, 2015

### houlahound

A proof that is self evident and not too hard-core in the math.

4. Oct 22, 2015

### BvU

But what is it you want to prove ?

5. Oct 22, 2015

### houlahound

That the earths atmosphere sticks to the earth.

Other than just breathing the proof.

The question is motivated by many, many....many years ago as an undergrad I used to hang out in the postgrad e2e collision lab and saw the engineering involved to keep in gassing of the atmosphere screwing up the low vacuum chambers enclosing the experiments.

I figure if air can leak into those elaborate chambers to be such a measurable problem then whats the physics stopping same air leaking out of the wall-less chamber of the earth.

I get we exist therefore we have an atmosphere, I just want to get the physics of the opposing "forces".

Last edited: Oct 22, 2015