Chronology Current Month Current Thread Current Date [Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] conserved momentum thru gas...

• From: John Denker <jsd@av8n.com>
• Date: Fri, 23 Dec 2022 22:19:09 -0700

On 12/23/22 7:26 PM, O A via Phys-l wrote:

My original question (actual mechanism of transferring momentum
through, say, 100 kms of gas molecules) is still something I see as

Several people have offered a one-word answer: Viscosity.
A slightly better answer would be: Pressure and viscosity.

*) Are you not happy with the conventional definition of viscosity?
*) Are you not happy with the idea of "sliding friction" transferring
momentum across the boundary between two parcels that are moving
relative to each other?
*) Viscosity is a property of the bulk continuous fluid. Are you
looking for an explanation of where viscosity comes from in terms
of the submicroscopic particles that make up the fluid?
*) Are you not happy with the conventional definition of pressure?
*) Are you not happy with the idea of pressure transferring
momentum across the boundary between two parcels?
*) Pressure is a property of the bulk continuous fluid. Are you
looking for an explanation of where pressure comes from in terms
of the submicroscopic particles that make up the fluid?

Note that shear and pressure go together:
-- pressure means z-momentum is being transported in the z-direction.
also x-momentum transported in the x-direction
also y-momentum transported in the y-direction
(3 possibilities total)

-- Shear stress means x-momentum is being transported in the z-direction
and so on for the other directions (6 possibilities total)

The 9 components can be collected and represented by a tensor:
https://en.wikipedia.org/wiki/Viscous_stress_tensor

Again I say: This thread has focused on momentum transfer, which is
good.
Third law of motion ≡ conservation of momentum.

If you are a masochist you could restate everything in terms of forces,
but I don't recommend it. This applies super-especially to fluids, but
it also applies to other topics. A couple of the most error-prone
problems on the FCI become absolutely trivial when reformulated in
terms of momentum transfer.