Do you know elementary physics?

**Phrak** · Nov22-09, 10:45 PM

adsf. You might ask yourself why a cube of air doesn't fall to the floor. Or, what are the forces on a ball of air. When these makes sense, add some additional mass to either.

**FoxCommander** · Nov23-09, 12:46 AM

Originally Posted by adsf

Thanks for the quick replies! but...

@ sophiecentaur: I have in fact looked a little over kinetic theory of gases and I do understand there is something called the Maxwell distribution of speeds, but...

My question still stands: shouldn't the air pressure at the same point or line or plane (note all these have to be horizontal) from above be a little more than the air pressure from underneath since gravity tugs at the particles hitting the plane from underneath while gravity pulls and accelerates the particles hitting the plane from above?????

Like without getting too "physics - y", conceptually, please answer the question above. Imagine it like a true or false. The question is, which would you pick and why.

Can people just answer a yes or no (followed by an explanation)? I truly appreciated the responses I have received and hope many more will follow.

Ok so no the air pressure on top should not be greater.
Explination: I understand your question now. So what you are saying that esencially the molecules underneath the plane are being pulled down, which is correct since gravity tugs at everything. But what is the surrounding molecules doing, they are also trying to go down. Which means that when you put the plane in there you have moved the molecules away. So the molecules will be pushing back according to the height that they are at. So yes the molecules on top will be pushing back with the weight of the ones above them but the ones on the bottom will be pushing up with more force because they are being pushed up by the weight or the surrounding molecules, and since they are at a lower height then there is more molecules to push back with...... I hope this makes sense. If not let me know which part and i will try to explain

**sophiecentaur** · Nov23-09, 05:21 AM

ASDF
As I said /implied earlier, the effect of gravity on individual molecules is tiny, compared with the bashing they keep getting due to the thermal (kinetic) motion of their neighbours. Agreed, if you had a single molecule in a vacuum, it would be accelerated down towards the Earth and it would hit a top surface and not a bottom surface. But that's not the situation. A molecule does not fall far enough to 'gain speed' when it is constantly being bombarded by others. The result is, as was said above, an isotropic situation.
You seem reluctant to let go of your original model but you will have to, if you want to get to the bottom of this problem. We are not talking ball bearings bouncing on the top of a tin lid compared with ball bearings bouncing around underneath the tin - under those conditions, you might have a bit of a point but it's not like that.

**Gerenuk** · Nov23-09, 11:01 AM

I edit my answer:

This effect you mention is probably smaller than the statistical fluctuations of particle numbers. The molecules in the plane will be subject to gravity itself! So after all I guess there wouldn't be any additional effect due to gravity.

**FoxCommander** · Nov23-09, 10:27 PM

Originally Posted by sophiecentaur

ASDF
As I said /implied earlier, the effect of gravity on individual molecules is tiny, compared with the bashing they keep getting due to the thermal (kinetic) motion of their neighbours.

If this were true then the molecules would be bouncing out of the liquid, hence either boiling or evaporating, The reason why its a liquid is because the energy in each molecule isnt strong enough to overcome gravity or their molecular bonds for that matter