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



The post below did not come through as hoped. I could use advice on how to make sure different colors and font sizes actually come through on the listserv.

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 9:31 PM, William Robertson wrote:

I hope this comes through with my color changes so who's saying what
is clear. I'll change font size also in case that doesn't happen. My
comments previously and now are hopefully in larger font, and this is
in response to John Denker's comments. And here I didn't want to get
involved in this discussion!

... We choose a reference frame and then work accordingly.

As always, the rule is that you are free to choose whatever
reference frame(s) you like ... but other folks are free to
choose differently.

I never claimed that people are not free to choose other reference
frames.

I don't recommend we confuse students by switching reference frames
without telling them.

That's true as stated. But it doesn't mean we should avoid
switching entirely; it just means we have to tell them.

Exactly. I didn't say we should avoid switching frames of reference.

Frame-switching *does* happen. The astronauts *are* going to
switch to the space-station frame, and in that frame they *are*
weightless ... even though meanwhile in the terrestrial lab
frame they are not weightless.

Yes, it happens. I believe I was arguing that when we are not clear
about reference frames, then we confuse the issue. My argument was
that we should not change our definition of weight because someone in
a different frame of reference experiences a certain sensation. In
another post, I discuss the fact that we teach students at a young age
that the difference between mass and weight is that one changes and
the other doesn't when we travel to the moon. That assumes we are not
in the freely-falling frame of reference. I am all for students
understanding what happens when we change the frame of reference, but
I am not for a definition of weight that completely confuses the issue.

Again, this is very much like the issue of centrifugal force. It
exists in one frame of reference but not in another.

Exactly so.

People in cars and in airplanes and on merry-go-rounds *are* sometimes
going to use comoving reference frames, and in such a frame they *are*
going to experience centrifugal fields during a turn. Physics teachers
are free to decide such situations are beyond the scope of the course,
but they are not free to say such situations do not exist.

We agree here, I believe. Drives me nuts when physics teachers tell
their students that there is no such thing as a centrifugal force.

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.

That's not correct as stated. The centrifugal field does *not*
depend on the state of motion of the object being weighed. It
depends on the choice of reference frame. They might or might
not be moving in a circle ... what matters is whether they choose
a rotating reference frame. A pilot may sometimes use a ground-
based reference frame, and a ground-based observer may sometimes
use a frame comoving with the aircraft.

I guess I wasn't clear, and I see how my wording was not precise. If
you view this situation from the outside, no one would call the change
in the scale reading a change in weight. But from the definition of
weight as the reading on a scale, then why not call this change in
scale reading as a change in weight? I'm trying to illustrate the
problem with defining weight as the reading on a scale.

It's not hopeless. It's not even ambiguous. It's just frame-
dependent. Unambiguously frame-dependent.

Complete agreement here.

Bill


William C. Robertson, Ph.D.




_______________________________________________
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