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

[William Robertson <wrobert9@ix.netcom.com>]

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.

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