Momentum transport in gases in 2d

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SUMMARY

The discussion focuses on the momentum transport of gas molecules in a two-dimensional space, specifically addressing the assumption that half of the molecules move in the positive y-direction while the other half move in the negative direction. This assumption is based on the premise that the overall gas mass remains static, leading to an average speed of zero. The conversation highlights the need for clarity on this assumption and explores the implications of analyzing gas behavior under different conditions, such as an upward accelerating gas mass, as seen in rocket dynamics.

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Mohankpvk
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I was trying to understand the momentum transport between gas molecules in 2d.In the image below, it is stated that half of the molecules move up(positive velocity in y direction) and half negative.But the author didnt explain why he assumed it.
IMG_20180914_172950.jpeg
 

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Because the whole gas mass is assumed to be static, not moving up, down, left, right.

You could do a different analysis for an upward accelerating gas mass, such as on a rocket, but the answers would change.
 
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anorlunda said:
Because the whole gas mass is assumed to be static, not moving up, down, left, right.

You could do a different analysis for an upward accelerating gas mass, such as on a rocket, but the answers would change.
But why half if the gas is static?Please explain.I referred a few books for momentum transport but all of them used tensors.Is there any book(or other source) in which transport is explained only in 2d?
 
Mohankpvk said:
But why half if the gas is static?

No, all the particles are moving, some + some -, the average speed of all is zero.
 
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