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Re: [Phys-l] Weightlessness in 4 easy steps




|
| ----- Original Message -----
| From: "John Denker" <jsd@av8n.com>
| >
| > Bottom line:
| > -- You are free to define "weight" and "gravity" however
| you like ...
| > but if you choose wacky definitions, it's a disservice to
| your students.
| > -- You are free to define "weight" and "gravity" however
| you like ...
| > but if you want your definition to be *consistent* with practical
| > applications, including universally-accepted weighing procedures
| > and universally-accepted notions of horizontal and vertical, then
| > your choices are more limited.
| >
|
| Once again, I think there is another alternative here, maybe
| 'wacky' in JDs mind, but one that has a considerable following.

While preferring either John M's or John D's version of things I believe
that the below is or can be made consistant and is *not* wacky. The
lack of wackiness is evident from the widespread use of the ideology in
intro courses and intro texts.

The quid pro quo is that one is now forced to distinguish "weight" from
"apparent weight"; and what one does when *carefully* weighing something
on a spring scale is much closer to measuring "apparent weight" than
"weight".

My preference is to forgo making that distinction; I suppose at the
price of making a distinction between F_E and F_N (if I introduce
GMm/r^2).

|
| Using g_N, and mg = W (although some may choose g_N to
| include the corrections for the rotation of the earth, but
| only that)

The parenthetical comment might be deserving of "wacky", ;-)

|then the term Apparent Weight or Perceived Weight
| is used to accommodate all those other factors built into
| g_E. While it can certainly be argued that this is a
| complication that needn't be introduced, a reason for doing
| so is that in intro classes we tend to remain in inertial
| reference frames for our analyses. We talk about fictitious
| forces and explain them by looking at the situation always
| from the non-accelerating frame to see how the illusion of a
| force may arise for those in the accelerating frame. When we
| look at the astronauts from the earth frame we see only that
| one force acting on them, they are accelerating towards the
| earth, their g_N is not zero, so from the earth frame they
| can't be weightless. But they certainly feel and act
| weightless in their frame. BUT...we specifically try to stay
| out of those frames where the fictitious forces become real.
| So, how to deal with this---apparent weight.
|
| Again, this may not be to many people's liking, but it works
| well in such classes to explain what they feel on carnival
| rides, in an elevator, when diving. I must confess I didn't
| follow JDs dismissal of my skydiver counter example for the
| 'weightless is weightless' view point. I guess I may not
| understand how to calculate his g_E in that example. I would
| say the acceleration of the reference frame attached to the
| diver keeps changing as the air-resistance increases.
| Doesn't that make the weight change? If we look from the
| earth frame, then g (however defined) doesn't change
| (much--sure R changes a very little).
|
| Also, what exactly changes by .5% over the 35 feet? I get a
| laser level to be off by only .00035 inches due to curvature .
|
| Rick
|
| ***************************
| Richard W. Tarara
| Professor of Physics
| Saint Mary's College
| Notre Dame, IN
| rtarara@saintmarys.edu
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