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



On 10/23/2013 09:20 AM, Jeffrey Schnick wrote:
Harry is viewing the situation from an inertial reference frame that
was commoving with the lab frame at time 0.

OK.

He's the one that tells
us that there is no gravitational force.

OK, if properly interpreted.

He's the one that tells us
that the reason we don't notice the book gaining momentum and
velocity is because we are riding along right beside the book--our
velocity and momentum are changing too.

That's unnecessarily confusing. In the introductory course,
the simplest thing is to pick one reference frame and stick
to it. If you are going to compare results from one frame
to another, it pays to be super-fastidious.
-- I assume "we" are in the lab frame.
-- Our velocity and momentum /relative to the lab frame/
are not changing. This is the frame we would naturally
use.

He claims that there is no
gravitational force (and hence we shouldn't concern ourselves with
upward flow of downward momentum)

Well, *IF* we were in the freely-falling frame we would
not need to concern ourselves with the gravitational upward
flow of downward momentum ... but we're not, so his claim
is misleading at best.

and advises us that if we want to
determine whether or not some apparent force is an actual force in
accord with Newton's third law, we have to look at the situation from
an inertial reference frame.

It is straightforward to get the right answer in either
frame ... so long as you pick a frame and stick to it.

This is a perennial introductory physics issue, having
no particular association with momentum-flow. As a
familiar example, a baseball bouncing off a bat gains
zero energy in the frame of the bat, but gains quite
a lot in the frame of an ordinary batter or bystander.

There are equations for transforming one frame's results
into another frame, but the transformations are nontrivial;
you can't just Frankenstein the top of one calculation
onto the bottom of another.