# Why doesn't the sky crush us all?

russ_watters
Mentor
jaredkipe said:
So air, unlike everything else has mass but when it is accelerated to the ground it bounces off and back up into the upper atmosphere? Can you detect this apparent movement of the air?
It doesn't bounce back up to the upper atmosphere - there are more collisions than you imply. Molecular movement due to kinetic energy is called Brownian motion and you can most certainly detect it. Watch smoke disperse.

I have to say I'm not quite happy with the Kinetic Theory of Gases.

If I can elaborate, the Kinetic Theory of Gases says that hydrogen molecules at room temperature and pressure are travelling at circa 4000mph. So if I were to suddenly decap a canister of hydrogen in a vacuum chamber, I should see molecules coming out at 4000mph. This seems like an explosive velocity that ought to blast the opposite wall of the chamber in the twinkling of an eye. It doesn't seem to square with my layman's experience. Can any more knowledgeable posters tell me about any experiments that prove the high velocity of the kinetic gas molecules?

Farsight said:
If I can elaborate, the Kinetic Theory of Gases says that hydrogen molecules at room temperature and pressure are travelling at circa 4000mph. So if I were to suddenly decap a canister of hydrogen in a vacuum chamber, I should see molecules coming out at 4000mph. This seems like an explosive velocity that ought to blast the opposite wall of the chamber in the twinkling of an eye. It doesn't seem to square with my layman's experience. Can any more knowledgeable posters tell me about any experiments that prove the high velocity of the kinetic gas molecules?
First off pressure doesn't have anything to do with how fast a molecule is moving. Secondly... circa...doesn't really mean that...

By equipartition, <E>=kT, where k is the Boltzmann factor and T is its temperature in degree's Kelvin.
<E>=.5m<v>^2=kT

so the average velocity of a molecule is related to its mass and temperature <v>=(2kT/m)^(1/2), I haven't plugged in numbers to see if you get your 4000mph, so I won't comment on that. But just because something is fast doesn't mean there is a lot of energy behind it.

As for an experiment, you could do what I just did in reverse. Specifically, suspend something that is very sensitive to pressure or impact. Like maybe an extreamly light film that you can measure its angle by bouncing a laser or something off of it. (obviously accounting for the momentum of the laser light) and pump on the chamber till there are few molecules of a specific mass. This should make it so that when one of the few molequles hits the film it is deflected, and you can calculate the energy of the collision. Thus deduce the velocity of the molecule.

Since it can happen at a variety of angles you should either reduce those angles, by making an aperture the molecule must go through to strike the film, or simply average over a lot of collisions.

Farsight said:
If I can elaborate, the Kinetic Theory of Gases says that hydrogen molecules at room temperature and pressure are travelling at circa 4000mph. So if I were to suddenly decap a canister of hydrogen in a vacuum chamber, I should see molecules coming out at 4000mph. This seems like an explosive velocity that ought to blast the opposite wall of the chamber in the twinkling of an eye. It doesn't seem to square with my layman's experience. Can any more knowledgeable posters tell me about any experiments that prove the high velocity of the kinetic gas molecules?
I once took a class on vacuum technology and we learned that hydrogen has an rms velocity (I think it's rms velocity) around 3973mph, helium 2808mph, and O2 960mph. The hydrogen molecules do come out of the container at about 3973mph, but since the mean free path is so small (about 3.7x10^-10m for N2 @ 22C), it keeps bouncing around the other molecules and takes quite a bit of time to evenly distribute the room.
Im not sure if this topic is kinetic theory, or statistical mechanics (im not even too sure what the difference is), but when i start studying for physics in the fall, im excited to learn about this stuff.

I guess I ought to start a new thread to ask my question.

It's a little off the Why Doesn't The Sky Crush Us All topic.

Just because there are "tons" of matter somewhere above you, doesn't mean it is exerting any force on you. Why does your roof not crush you? Because something is holding it up. The reason air doesn't crush you is because more air is holding the higher air up.

EDIT: Oh no, I thought you were asking it again, but its different.

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