Re: Weight and reference frame

["Carl E. Mungan" <mungan@USNA.EDU>]

> If ALL books refrained from identifying the gravitational force
> on an object (mg) as weight, then John's camp would have fewer problems.

My current view, which was advocated particulary clearly by Leigh
Palmer previously on this list is:

weight = spring-scale-reading = mg

So I don't see these two as either/or.

It's just that g = GM/R^2 only for an ideal (nonrotating, isolated,
spherical, homogeneous, airless) planet.

If we now turn on the rotation, g_rotating = g_ideal - a_c where a_c
= centripetal acceleration of reference frame. (This is actually a
vector equation and can be written generally as g_noninertial =
g_inertial + a where a = acceleration of noninertial frame relative
to an inertial frame. This is nothing but the derivative of the
Galilean law of relative velocity if we interpret each "g" as the
negative acceleration of its frame a la Einstein.)

This g_rotating is after all what you ACTUALLY measure for a
projectile (on a rotating but otherwise ideal) planet.

So I see the weight=spring-scale pros and cons as follows:
PROS: no need to ever use the word "apparent"
      corresponds to everyday definition of weight
      automatically gives the right value for g
CONS: g is now frame-dependent
      doesn't correspond to very many textbooks

I get around the second CON by letting g with no subscript be the
actual g we measure on the real earth and say that its value can be
approximated as 9.8 m/s^2 (when we do projectile motion) and as 9.8
N/kg (when we do dynamics).

I am more worried about the first CON. My question is: Is the
electric field dependent on the acceleration of the frame in which it
is measured? I would like to be as symmetric as possible in my
teaching of gravitational and electromagnetic fields.

I would like to hear the list's wisdom. Carl
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/