# Momentum transport in gases in 2d

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. #### Attachments

anorlunda
Staff Emeritus
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.

• Mohankpvk
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 refered 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?

anorlunda
Staff Emeritus
But why half if the gas is static?

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

• Mohankpvk