[Phys-L] Re: A Third law question

[rtarara at SAINTMARYS.EDU (Rick Tarara)]

Thanks to all for your suggestions.  Many of these were part of the 30
minute dialog with the student, but there are (for me) some new ideas here
as well.  I intend to pursue the deformation tact some more--by going
through a series of 'platforms' that will first clearly show deformation
when 'asked' to support a weight, then working up to where the deformation
is less visible, until it becomes invisible.  I will try the laser and
mirror trick here hopefully to show that there was some deformation even
though it was not visible to the naked eye.

I must confess that I do not clearly see how the conservation of momentum
helps with this student's question, but then I haven't had time to think it
through.   My first response, however,  would be that both the momentum of
the hand and the disk change in this action (lifting the disk and
accelerating it upwards).  This requires an outside influence.  The hand is
the influence on the disk, the arm on the hand, the body on the arm,
etc.---but how does all that relate to N3 between the hand and disk?
Momentum conservation, IMO, gets just as tricky in 'real-world' situations
as N3 and forces because it _seems_ to get lost when the earth (or at least
large diffuse structures) get involved (here giving the disk upwards
momentum must given the earth some downwards momentum).  Sure if we are
throwing the disk horizontally, standing on a frictionless surface, then
delta-p = 0 leads to an N3 pair, but that is not the physical situation in
question, and extrapolating from one to the other is by no means trivial.

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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----- Original Message -----

> Rick Tarara wrote:
>
> > .....'Where does the disk get the
> > extra force when I push up with more force than its weight?' was her
> > repeated question.
> >
> > Any suggestions here?