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



On 10/22/2013 07:43 AM, Jeffrey Schnick wrote:

I think the non-local conservation option mentioned above is none
other than the action-at-a-distance model.

Well, either way, there's no advantage. The laws of physics
don't permit action-at-a-distance, and they don't permit
non-local conservation. In particular, neither one is
consistent with special relativity. A process that exhibits
non-local conservation in one frame will exhibit outright
non-conservation in another.

I think we can also get around the issue by saying that there is no
upward flow of downward momentum to the book but rather that the
downward flow of downward momentum, say from the top book to the
middle book, corresponds to an unbalanced ever-increasing amount of
upward momentum in the top book which we don't see because we are in
an accelerated reference frame.

I don't think that works either. That's looks like an
improper mixture of ideas from two different reference
frames.

Allowed possibilities include:
a) In the lab frame, the book is not accelerating. Its
velocity is not changing and its momentum is not changing.
b) In a local freely-falling frame, there is no gravitational
acceleration. The book is steadily accumulating upward
momentum due to the unbalanced forces, namely the forces
associated with contact with the table, the other books,
et cetera. The momentum is changing (in this reference
frame). The velocity is changing (relative to this
reference frame).

You can't mix half of (a) with half of (b). You can't say
there is an accumulation of momentum when mv is not changing.

I think it is complicated

I'm not convinced. The right answer looks pretty simple to
me. The confusing scenarios suggested above do not appear
relevant to the real physics.