Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Apparent weight



On Thu, 19 Feb 1998, David Bowman wrote:

...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.


Now we are at the nub of our difficulty: I say only accelerations come
and go by virtue of our choice of frame, while you say that forces also
come and go in the same way. To me, the choice of a frame for describing
motion and the accelerations that can be measured relative to a chosen
frame are matters of kinematics. For you, apparently, it is also a matter
of dynamics, and you are willing to say by definition that a force comes
into existence every time I start measuring acceleration relative to some
noninertial frame. Definitions are free, so of course you can do that if
you wish, but you will pardon me I hope if to me it seems madness.

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,

Forces do not exist relative to frames; accelerations exist relative to
frames. The choice of a frame (inertial or not) to describe motion is a
purely kinematic affair; it has absolutely no effect on what forces exist
or do not exist. Unless, of course, you *define* forces to coexist with
accelerations relative to noninertial frames, and that is apparently what
you are chosing to do and where I do not choose to follow.

The mathematical choice of whether to write an acceleration on the left or
the right side of an equation, for purposes of more easily solving the
equation is of no physical consequence, and is not a problem. Declaring a
force to exist everytime I move an acceleration term over to the force
side of an equation is a more serious matter. Again, you are perfectly
free to do so, but I do not choose to follow.

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 magnetic fields do work on massive objects. If Coriolis force is real
(as opposed to Coriolis acceleration which is certainly real) but does no
work on masses, what *does* Coriolis force work on?

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.)


Again, the notion that forces can appear and disappear at the whim of the
frame choser seems to be at the nub of our disagreement. There is no
dispute about forces not transforming as scalars (has anyone made a claim
otherwise?) -- I claim that if a force is zero (tensor) in one frame it
will be zero in all frames. You apparently claim otherwise, and validate
your claim by decreeing arbitrary accelerations as forces. That is
certainly your right to do, but it makes communication, and I think,
teaching very difficult.

A. R. Marlow E-MAIL: marlow@loyno.edu
Department of Physics, Box 124 PHONE: (504) 865 3647 (Office)
Loyola University 865 2245 (Home)
New Orleans, LA 70118 FAX: (504) 865 2453