Re: Apparent weight

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

Regarding A. R. Marlow's puzzlement about my quip about hydroelectricity:

> On David Bowman's query about stockholders buying power from falling
> water: Where is there a problem there?  There seems to be no room for a
> fictitious force.  The water hits say a turbine blade with a very real
> force, and the turbine blade exerts an equal and oposite force on the
> water.  No one debates that these are real forces.  A problem would arise
> if one were to introduce a third "force" with no third law counterpart, as
> people do when they invent an outward force in a centrifuge pressing them
> against the wall.

My point had nothing to do with the details of turbine operation per se.
You claimed that 'fictitious' forces do no work.  (I was trying to show my
disagreement with this point as long as the fictitious forces in question
are not due to the Coriolis effect).  You also, properly, placed
gravitational forces in the same catagory as other 'fictitious' forces (to
the extent that large-scale tidal inhomogeneities of the gravitational
field can be safely ignored in a given context).  My point was a
counterexample that hydroelectric utilites are in the business of selling
electric power (i.e. work) produced by the action of falling water which
is pushed downward by the earth's gravitational field (mostly usual
Newtonian gravity with a small negative contribution from the centrifugal
force field from the earth's rotation) that *exists* (locally) by virtue of
our choice of a frame in which the earth's surface is taken to be at rest
rather than using a freely falling frame.  Just because a force exists in a
given frame by virtue of the state of acceleration of that frame wrt a
freely falling frame is no reason to consider that force nonexistent, nor to
claim that such a force can do no work in that frame.  I will concede that a
Coriolis force does no work on a mass (just as a magnetic field does no work
on a charge), but centrifugal forces, Newtonian gravitational forces, and
forces due to other translational accelerations of a frame's origin wrt a
freely falling frame certainly *can and do* do work in the frame in which
they appear.  (BTW, neither forces nor work transform as scalars under
transformations among accelerated frames, but that is no reason to claim
their nonexistence in any frame in which they do not vanish.)

David Bowman
dbowman@gtc.georgetown.ky.us