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At 08:32 -0500 11/19/06, Herb Gottlieb wrote:
1) Why "g " is not an acceleration .
Doesn't "g" refer to something that changes velocity as it travels?
No. g refers to the strength of the local gravitational field (and in
the case of the earth's surface, it has a small correction normally
incorporated for latitude and the earth's rotation, but this is small
enough that it can be neglected for most situations).
2) What is the "irrelevant accident" of physics that you cite here?
It is the fact that inertial mass and gravitational mass appear to be
equal. I know of no principle of physics that requires this to be
true. Because of this fact, the proper field units for g, N/kg,
reduce to m/s^2. But g is used in many cases where there is no
acceleration occurring, such as in determining the force of gravity
on an object (sometimes called its weight), mg. Calling g an
acceleration is one of the things that I think makes the whole
concept of g very confusing to introductory students.
3) Why isn't the same true of electricity?Yes, but it accelerates in proportion to both its charge and mass.
Won't an isolated electron in outer space be accelerated similarly when
it happens to be in an electric field?
The units of the electric field, in analogy with those of the
gravitational field are N/C, which works out to m kg/C s^2. so an
elctron and an anti-proton, both having the same charge, but
different inertial masses will accelerate differently in an isolated
electric field. The same particles, however, will accelerate
identically in an isolated gravitational field, because the field
quantity that they carry in this case, is the same as their inertial
mass. If we think of the gravitational field unit as m
kg-inertial/kg-grav s^2, then the symmetry with the electric field
units becomes obvious. Because of the equivalence of inertial and
gravitational mass all objects accelerate the same in a uniform,
isolated gravitational field, but because of the non-equivalence of
"electrical mass" and inertial mass, the same is not true of objects
in an electric field.
Hugh