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



On 11/08/2010 04:34 PM, William Robertson wrote:

... 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 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.

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.

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.

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.

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

you only make measurements in the frame you are in.

That's not true at all!

There is an idealized notion of an "observer" who is "in" a single
reference frame ... but those observers don't call the shots. In
my capacity as principal investigator, I can hire hordes of such
observers and assign them to many different reference frames.

Consider the aforementioned "scale on the wall of a rotating cylinder".
The scale reads some number, and all observers agree on the number,
but my *interpretation* of that number in verrry different, depending
on whether I am using a rotating reference frame or a non-rotating
reference frame ... or using both and switching back and forth. It
is very common to find that part of the problem is easier in one frame
and another part is easier in another frame, so I use measurements in
both frames and convert them as necessary. The conversion formulas
are well known.

For example; It is absolutely standard procedure, even in the most
introductory course, to solve part of a dynamics problem in the CM
frame and then to re-express the results in the lab frame.

Other examples abound.

On 11/08/2010 06:09 PM, Hugh Haskell wrote:

So let me add my voice to the hopeless cause of getting the ambiguity
out of our understanding of gravity and weight.

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