Re: What Flows?

["Dewey Dykstra, Jr." <dykstrad@varney.idbsu.edu>]

Dykstra said:
> > But, Leigh, physical quantities are not merely magnitudes.  They have
> > dimension.  Isn't this what distinguishes one physical quantity from
> > another as in velocity as compared to acceleration?
Palmer replied:
> The magnitude of a torque is a physical quantity, just as speed is.

Now Dykstra replies:
It appears that there is a confusion over the term, 'physical quantity.'
In the context of talking about magnitudes, vectors, scalars, units,
problem-solving, I've always seen the expression, 'physical quantity,' used
to refer to things like:  5.2 m/s, due East, in other words a specific
value, measured or hypothetical, for a velocity to be used in a problem or
example.  For some reason, I thought you were referring to 5.2 as magnitude
and not 5.2 m/s.  I can go along with the statement that the magnitude of a
torque is a physical quantity.

Palmer said:
> > > The bottom line is this: energy and torque have the same
> > > dimensions. They can be given the same units, too, but
> > > there is probably a lot to be said for distinguishing
> > > them by using cosmetically different units.
Dykstra replied:
> > So you're telling me that you can refer to the physical quantity, 5 Joules,
> > and this could be either a torque or an energy?  What happened to all the
> > precision in language?

Palmer replies:
> "5 joules" is not a physical quantity. Work is a physical quantity,
> torque is a physical quantity, and energy is a physical quantity.

Now Dykstra replies:
It is getting strange here.  Above you say, the "magnitude of torque is a
physical quantity."  You have been insisting that torque can have the same
units as energy.  And now you say:  ""5 joules" is not a physical quantity."

So, how can the magnitude of a torque (vector) which might be for example,
(5 Nm, up), not be 5 Joules by what you said originally?  It seems to me
you have switched from magnitude of a torque being a physical quantity to
torque being a physical quantity.

I don't know what term you would use.  So, instead I say that I have
measured something and found it's value to be 5 Joules.  You're saying that
this could be the magnitude(?) of a torque or an energy or that I might
have 'measured' either torque or energy to get this quantity?  Again, all
the insistence on precision in language is lost on me if something in
Joules can be either torque or energy.

> Perhaps I'm being abrupt, but I really don't think I have
> understood the question you are asking. As a result I'm reduced to
> simple reiteration, though of course I don't believe strongly in
> Lewis Carroll's "What I tell you three times is true".

Tell me about it ;^)...  It seems to me that in order to get results which
do not require an ad hoc adjustment as to which name we give the units, we
either need to not drop angle from our dimensional and unit consideration
or we need not to reduce our operations to mere arithmetic or scalar
multiplication when we consider dimensional analysis.

But then that's just my opinion, I could be wrong ;^)

Dewey


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Dewey I. Dykstra, Jr.                   Phone: (208)385-3105
Professor of Physics                    Dept:  (208)385-3775
Department of Physics/SN318             Fax: (208)385-4330
Boise State University                  dykstrad@varney.idbsu.edu
1910 University Drive                   Boise Highlanders
Boise, ID 83725-1570                       novice piper
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