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Re: [Phys-L] ? FCI --> momentum flow



On 11/01/2013 01:36 PM, LaMontagne, Bob wrote:

Informative response

:-)

I should have been a little more precise and said "momentum flow".

OK.

"momentum flow" [....] "upward"

That's trouble; see below.

what can be said about the momentum flow?
Can that be anything but upward (upward flow of upward momentum)?

That's much better. That is: Please say "upward flow of upward
momentum" rather than just "upward flow of momentum".

And yes, the table imparts an upward force on the book, which means
that upward momentum is flowing upward across the table/book boundary

BTW, note that flow of upward momentum is exactly synonymous with
downward flow of downward momentum. This takes some getting used
to. The key is to realize that momentum-flow is *not* a vector.
It is a symmetric second-rank tensor.

Momentum is a vector.

Momentum-flow is not a vector; it is a tensor.

If you rotate a vector 180° (in a plane containing the vector),
you get the opposite vector.

If you rotate a string 180° (in a plane containing the
string) you get an equivalent situation with the *same*
tension in the string.

I went to a lot of trouble to invent symbols for momentum flow. The
symbols use *two* arrows: The red part indicates what is flowing,
while the black part indicates the direction of flow. See e.g.
tension:
http://www.av8n.com/physics/force-intro.htm#fig-force-oring-3s-left
pressure:
http://www.av8n.com/physics/force-intro.htm#fig-ballistic-momentum
shear:
http://www.av8n.com/physics/force-intro.htm#fig-block-sliding


It takes a nontrivial investment to figure this stuff out ... but
there is a tremendous return-on-investment.