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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.

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) 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

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN
rtarara@saintmarys.edu
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