# If the earth constantly exerts a force on air molecules downwards, then why do they not fall to the ground?

I'm sorry if this seems like a really stupid question but it has really puzzled me...
The earth exerts gravitational force on all objects with mass, and gas has mass. I know that this force results in the varying density of the atmosphere, and gas is more concentrated close to the earth due to this force. But, over a period of time, all the gas molecules should settle down at the surface, similar to any other object falling towards the earth. So why does this not happen?
I imagined thousands of tennis balls randomly dropping from the sky at high speeds as a substitute for gas molecules, but intuitively I could only come to the conclusion that they will eventually fall on the surface and stay there. On searching other websites, the explanation was that the molecules exert pressure on each other and closer to the earth, this pressure is enough to keep them above the surface. Yes, but wouldn't such a significant and constant force be enough to eliminate this pressure at some point and bring the air to a complete stop(like the tennis balls)?
I'm clearly missing something here or have not understood a key concept. I will appreciate any help in this question :)

• The tennis balls stop moving because of their not perfectly collision with the ground, losing kinetic energy with each bounce. How do you think the gas molecules should lose energy here? Mar 14, 2021 at 16:13
• >You could ask the same question about the water molecules of the sea. Why are they not all on the bottom of the ocean? Because of interacting with molecules below them .and scattered back. Mar 14, 2021 at 16:20
• @ACuriousMind oh so they just keep bouncing around and have perfectly elastic collisions, hmm... kindof makes sense Mar 14, 2021 at 16:22
• They do fall to the ground. If not, we would not have an atmosphere. Mar 14, 2021 at 17:17
• BTW, at 20° C, the speed of air molecules is around 460 m/s. See en.wikipedia.org/wiki/… & physics.stackexchange.com/q/492013/123208 Mar 14, 2021 at 17:25

Here is the reason.

A volume of gas molecules are, for all temperatures above absolute zero, possessed of a distribution of speeds and therefore a distribution of momenta. When they collide with the walls of a container, they therefore exert a pressure against the walls and the higher the temperature, the higher that pressure.

Absent the walls (as for example on the surface of the earth), that pressure persists, and it resists the tendency of the gas molecules to all fall down and form a liquid or frozen coating on the earth's surface.

One way to think of this is to realize that if the kinetic energy of their motion is a significant fraction of the potential energy difference between the surface and some characteristic height scale above the surface, then gravity will have a hard time pulling all of them down below that height to the surface.

On a cold mini-planet like Pluto, those gas molecules lack the kinetic energy to resist gravity and so they do fall to the surface and condense into ices there.

The ground is also a bunch of molecules, that vibrate according to the heat input from the sun. So we can not imagine air molecules simply falling down and staying there. They are forced due to collisions to move around.

Without that soil thermal energy, I believe that the kinetic energy of the air molecules would decrease, tending in the limit to simply fall down completely.

• That's a nice insight too, thanks Mar 14, 2021 at 18:44