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[Phys-L] Re: A Third law question



Regarding Rick's Student's question:

In the end though, she was still having trouble. She could
'understand' how she increased the force of her hand on the disk but
couldn't really accept the inertia arguments about how the disk
increased its force back on the hand. '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? How would you try to deal with this question?

Rick

Maybe it would help to reiterate that forces are not substantive
things to be possessed by objects. Objects don't have to go looking
for forces to exert in response to other forces exerted on them any
more than a hot object cooling off, as it heats a colder object,
needs to acquire some coldness from the cooler object to make itself
colder. In the heat transfer case thermal energy is transferred
from the hotter object to the colder one and the increased thermal
energy in the colder object causes its temperature to rise, and the
decrease in internal thermal energy in the hotter object causes its
temperature to fall (assuming for now that both objects have
positive heat capacities and are not exotic things like black holes
that have negative heat capacities). In the other case the hand
transfers some vector momentum to the disk with the force
exerted by the hand on the disk *being* the rate at which the hand
is transferring that particular momentum vector. Since momentum is
conserved, when the disk acquires some momentum from the hand the
hand looses a corresponding amount of that vector quantity. The
rate of loss of the momentum from the hand to the disk corresponds
to the force exerted by the disk on the hand. The rate of momentum
gain by the disk is necessarily accompanied by a corresponding rate
of momentum loss by the hand.

How the hand and the disk each change their individual total momenta
(and consequently individually accelerate in space) is determined by
accounting for *all* the individual momentum fluxes into and out of
them from all interactions in which they each participate.

David Bowman