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Re: [Phys-l] pseudo-force



On 10/27/2006 01:22 PM, John Mallinckrodt wrote:
.... The gravitational tidal effect near the surface of
Earth is too small to feel, but it's easy to set up a very noticeable
centrifugal tidal effect by simply spinning your body about a
vertical axis like an ice skater.

That's an excellent point.

1) Remark: The term "centrifugal tidal" takes for granted the
analogy between gravitational fields and centrifugal fields,
which is commendable IMHO.



2) Note that while the local gravitational acceleration can be
made to disappear -- locally, to first order -- by a suitable
choice of reference frame, the tidal effects do not disappear.

By way of analogy, the surface of a sphere is locally flat,
to first order ... but nevertheless the sphere is curved.
The laws of physics are not restricted to talking about
local, first-order effects.

This points out a limitation to Einstein's principle of
equivalence. The gravitational field is a /field/, which
means it attributes an acceleration to each and every point.
The case of a uniform field is a special case, by no means
the only case of interest. Similar words apply to the
centrifugal field, and indeed it is very common to find
significant non-uniformity in the centrifugal field,
although OTOH it is easier to make a purely centrifugal
field disappear globally.

This cuts to the core of the subject of this thread. Are we
going to say that a uniform field gives rise to pseudo forces,
while the corresponding non-uniform field gives rise to
non-pseudo forces? (That sounds ugly to me.)

I'm optimistic that we can sort out this mess. After all,
motion is lawful, even motion in accelerated frames. We
know the equations of motion; we're just fussing with how
to interpret and/or explain the various terms in the
equations.

I've never thought much about the "pseudo force" issue. So
far, the more I think about it, the less I like it.