Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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

But that's not how we analyze problems in physics. We choose a reference frame and then work accordingly. You have described a situation in which a person wishes to ascribe the term "weight" to what he reads on the scale. Fine for that person, but when helping people understand physics, we should stick to clear definitions and let people know we analyze situations from a given frame of reference. If we choose to analyze something from the freely-falling frame of reference, then there is no gravity and the reading on a scale corresponds to other things happening besides the Earth exerting a force on something. I don't recommend we confuse students by switching reference frames without telling them. Again, this is very much like the issue of centrifugal force. It exists in one frame of reference but not in another. Because all of us have experienced a centrifugal force in a rotating frame of reference, do you suggest we go ahead and include that force when we're analyzing problems from the non-rotating frame of reference? For that matter, we could really confuse them by telling them that when they move in a circle, they have a horizontal weight that depends on how fast they're rotating. After all, we can put a scale on the wall of a rotating cylinder.

With respect to riding in an elevator all day long, to turn that into something we could analyze from the outside we don't need to confuse the issue by a changing weight. As I said in the beginning, why can we not just stick with a basic definition that is used in all the classic textbooks? Using that, we can interpret all the scale readings we want. I don't think I'd want NASA scientists to do their calculations while inside an elevator whose acceleration they're not sure of.

Bill


William C. Robertson, Ph.D.
Bill Robertson Science, Inc.
Stop Faking It! Finally Understanding Science So You Can Teach It.
wrobert9@ix.netcom.com
1340 Telemark Drive
Woodland Park, CO 80863
719-686-1609

On Nov 8, 2010, at 4:10 PM, LaMontagne, Bob wrote:

If you have been riding up and down an elevator all day to the point that you have no idea what the actual acceleration of the elevator is at a given time any more, and you then step on a scale and it reads 150 lbs - you have no choice but to call that your 'weight". It not a matter of comparing reference frames - you only make measurements in the frame you are in.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of William Robertson
Sent: Monday, November 08, 2010 5:54 PM
To: Forum for Physics Educators
Subject: 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.
Stop Faking It! Finally Understanding Science So You Can Teach It.
wrobert9@ix.netcom.com
1340 Telemark Drive
Woodland Park, CO 80863
719-686-1609

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:


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l