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Re: [Phys-l] definitions ... purely operational, or not

If you want to use a definition based on what a body feels, then by all means look at things from that frame of reference. We move from inertial frames of reference to accelerated frames of reference and back all the time. But as long as we analyze something from a given frame, we should not accommodate our definitions to account for what's going on in the other frame. That, it seems, would be like telling people centrifugal forces exist when we're not in the rotating frame of reference.

Bill


William C. Robertson, Ph.D.
Bill Robertson Science, Inc.
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On Nov 8, 2010, at 3:34 PM, LaMontagne, Bob wrote:

The problem is that a person's body does not feel gravitational pull directly. He/She feels the force on their feet. If the person jumps from a significant height, the feel they are floating - i.e., as if they lost their "weight". I prefer a definition of weight that is more in tune with what the body can sense.

When flying a small airplane in turbulence one feels alternately very "heavy" to almost "weightless" Same on a roller coaster ride.

An astronaut feels "heavy" returning to earth because of the force on the astronaut's feet. The pull of gravity is basically the same at the surface and at 300 miles up - so the astronaut is obviously not sensing gravity itself.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Rauber, Joel
Sent: Monday, November 08, 2010 9:35 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] definitions ... purely operational, or not

Interestingly, the current issue of TPT (The Physics Teacher) has a
short article by Al Bartlett who weighs in on this after a long
distinguished career in physics. The simple one sentence definition of
weight that he proposes is:

"Then the weight of a mass M in a specified frame of
reference is M times the free-fall acceleration in that specified
frame of reference."

This is not contrary to several versions proposed on this list and
their authors websites. Bartlett precedes the above definition with
the following admonition: "All of this awkwardness can be avoided if we
always
replace the terms 'acceleration of gravity' and 'acceleration
due to gravity' with the more accurate term "free-fall acceleration."
Read the article for details.

http://tpt.aapt.org/ if you have access.

______________
On the personal opinion level, I prefer the above definition. I am
reasonably comfortable with the operational definitions that John D.
Eschewed and have argued for the "what the scale weighs" definition
several incarnations ago. I don't think it's a bad way to go, but does
suffer from the defects that John D. mentions and leaves a lot unsaid
or assumed such as, properly calibrated scales, removal of buoyant
effects, etc. It really is operational only in an "ideal" sense of the
word. And the Bartlett et. al. definition above avoids a lot of the
assumptions by replacing it with the "simple" assumption of
understanding what "free-fall acceleration" means.

Joel Rauber


-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of William Robertson
Sent: Sunday, November 07, 2010 12:24 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] definitions ... purely operational, or not

You are as always free to define terms however you choose,
but please be aware that other may choose differently.


In particular, if you want to define some sort of _net weight_
or _effective weight_ and explain in a footnote that it includes
a buoyancy term (as Michael H. did), then I am 100% OK with
that. The rule here is simple: Say what you mean and mean
what you say.

If you are talking among physicists or physics educators, or
relatively advanced physics students, then I would agree with you.
Such an audience can understand an effective weight. But since this
is
a physics education listserv, I would think we would want to use
terms
that one would use in the average classroom. Even though we expect
people in this current discussion should be able to follow (with
careful explanation of the "new" definition) any non-standard
definition, I see the very real possibility of many taking something
explained here directly to the classroom. Coming up with one's own
definition of weight (operational or otherwise) cannot help but
confuse the average student. For the classroom, I believe one should
use the simple definitions provided in classic texts by classic
educators. And no, I'm not completely sure what a classic educator
is. ;o)



Bill


William C. Robertson, Ph.D.


On Nov 7, 2010, at 6:34 AM, John Denker wrote:


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