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Viewing Single Post From: The giant space ship example
gbaikie

I was looking for reference for velocity of atmospheric gas:

"Air molecules are never at rest. They undergo constant random thermal motions of a variety of types. The simplest type is that of uniform translation. The mean speed is about 500 m/s which is greater than the speed of sound (340 m/s). Each molecule has three degrees of translational motion: up and down, left and right, and backward and forward.
..
Air molecules take up only 0.1% of the volume they occupy. Thus air is a very sparse gas in which 99.9% of the atmosphere is vacuum. However there are 2.7 x 1019 molecules in every cubic centimeter of air. This high number density coupled with the large translational speeds implies that the air molecules are constantly colliding with each other. The mean time between collisions is 0.2 x 10-9 s. Thus an average molecule undergoes 5 collisions every nanosecond. (One nanosecond corresponds to the time it takes light to travel 30 centimeters in a vacuum.) The mean free path is the average distance traveled by a molecule before it collides with another molecule in the gas. Typically this distance is about 10-5 cm or about 500 to 1000 molecular radii. "
http://www.ems.psu.edu/~bannon/moledyn.html

There is more collisions occurring then I thought.
Not sure what to make of "99.9% of the atmosphere is vacuum"
what is meant by atmosphere- beyond the stratosphere? Surface level?
Venus "proves" one could have 92 sea level atmospheres.
And solid matter is said mostly open space- such as a wall, a brick, steel plating, etc.

One could say it isn't so much a molecule hits another molecule, but rather there so many molecules that they hit the molecule. Or a faster molecule hits less often.
It seems if molecules are traveling with mean velocity of 500 m/s in the higher atmosphere of dwarf, then large quantities of them would reach escape velocity.
I want find reference of molecule speed relative to air temperature. And whether any gases we could use travel at faster velocity at given temperature. I think mono-atomic gases- noble gases like argon do travel faster. Due not having paired rotational energy.

Add:
mean free path

The higher the density of gas, the smaller the mean free path (more likelyhood of a collision). The larger the molecules, the smaller the mean free path. The mean free path depends on the number density of the gas molecules and their size --- and nothing else
At sea level the mean free path of atmospheric gases is about 60 nm
At 100 km altitude, the atmosphere is less dense than where we live at the surface of the earth, and the mean free path is about 0.1 m (about 1 million times longer than at sea level)
http://itl.chem.ufl.edu/2041_u00/lectures/lec_d.html
Above also has graph showing gas velocity, but I don't temperature of gas on graph.

Oh here is calculator for gas velocity:
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html
mean velocity dry air at 260 K is 435.68 m/s
220 K: 400.7 m/s
160 K: 341.7 m/s
Edited by gbaikie, Dec 13 2011, 05:30 AM.
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The giant space ship example · Physical theory for climate